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Title: A Brief Account of Radio-activity
Author: Venable, Francis Preston, 1856-1934
Language: English
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A BRIEF ACCOUNT OF
RADIO-ACTIVITY
BY
FRANCIS P. VENABLE, PH.D., D.SC., LL.D.
PROFESSOR OF CHEMISTRY, UNIVERSITY OF NORTH CAROLINA
AUTHOR OF
"A SHORT HISTORY OF CHEMISTRY,"
"PERIODIC LAW," ETC.
D. C. HEATH & CO., PUBLISHERS
BOSTON NEW YORK CHICAGO
COPYRIGHT, 1917,
BY D. C. HEATH & CO.
IA7
PREFACE
I have gathered the material for this little book because I have found
it a necessary filling out of the course for my class in general
chemistry. Such a course dealing with the composition and structure of
matter is left unfinished and in the air, as it were, unless the
marvellous facts and deductions from the study of radio-activity are
presented and discussed. The usual page or two given in the present
text-books are too condensed in their treatment to afford any
intelligent grasp of the subject, so I have put in book form the
lectures which I have hitherto felt forced to give.
Perhaps the book may prove useful also to busy men in other branches
of science who wish to know something of radio-activity and have scant
leisure in which to read the larger treatises.
It is needless to say that there is nothing original in the book
unless it be in part the grouping of facts and order of their
treatment. I have made free use of the writings of Rutherford, Soddy,
and J. J. Thomson, and would here express my debt to them--just a part
of that indebtedness which we all feel to these masters. I wish also
to acknowledge my obligations to Professor Bertram B. Boltwood for his
helpful suggestions in connection with this work.
CONTENTS
CHAPTER I
DISCOVERY OF RADIO-ACTIVITY PAGE
The beginning--Radio-active bodies--An atomic
property--Discovery of new radio-active bodies--Discovery
of Polonium--Discovery of Radium--Other radio-active
bodies found 1
CHAPTER II
PROPERTIES OF THE RADIATIONS
Ionization of Gases--Experimental confirmation--Application
of electric field--Size and nature of ions--Photographing
the track of the ray--Action of radiations on photographic
plates--Discharge of electrified bodies--Scintillations
on phosphorescent bodies--Penetrating power--Magnetic
deflection--Three types of rays--Alpha rays--Beta rays--Gamma
rays--Measurement of radiations--Identifications of the rays 7
CHAPTER III
CHANGES IN RADIO-ACTIVE BODIES
Radio-activity a permanent property--Induced
activity--Discovery of Uranium X--Conclusions drawn--Search
for new radio-active bodies--Methods of investigation--Nature
of the radiations--Life-periods--Equilibrium series 17
CHAPTER IV
NATURE OF THE ALPHA PARTICLE
Disintegrating of the elements--Identification of the
rays--The alpha rays--Alpha rays consist of solid
particles--Electrical charge--Helium formed from alpha
particles--Discovery of Helium--Characteristics of
Helium--Table of constants 25
CHAPTER V
THE STRUCTURE OF THE ATOM
Properties of Radium--Energy evolved by radium--Necessity for
a disintegration theory--Disintegration theory--Constitution
of the atom--Rutherford's atom--Scattering of alpha
particles--Stopping power of substances 32
CHAPTER VI
RADIO-ACTIVITY AND CHEMICAL THEORY
Influence upon chemical theory--The periodic system--Basis
of the periodic system--Influence of positive
nucleus--Determination of the atomic number--Use of X-ray
spectra--Changes caused by ray-emission--Atomic weight
losses--Lead the end product--Changes of position
in the periodic system--Changes from loss of beta
particles--Isotopes--Radio-activity in nature--Radio-active
products in the earth's crust--Presence in air and soil
waters--Cosmical radio-activity 41
INDEX 53
A BRIEF ACCOUNT OF RADIO-ACTIVITY
CHAPTER I
DISCOVERY OF RADIO-ACTIVITY
The object of this brief treatise is to give a simple account of the
development of our knowledge of radio-activity and its bearing on
chemical and physical science. Mathematical processes will be omitted,
as it is sufficient to give the assured results from calculations
which are likely to be beyond the training of the reader. Experimental
evidence will be given in detail wherever it is fundamental and
necessary to a confident grasp of some of the marvelous deductions in
this new branch of science. Theories cannot be avoided, but the facts
remain while theories grow old and are discarded for others more in
accord with the facts.
The Beginning
As so often happens in the history of science, the opening up of this
new field with its fascinating disclosures was due to an investigation
undertaken for another purpose but painstakingly carried out with a
mind open to the truth wherever it might lead.
In 1895, Röntgen modestly announced his discovery of the _X_ rays.
This attracted immediate and intense interest. Among those who
undertook to follow up these phenomena was Becquerel, who, because of
the apparent connection with phosphorescence, tried the action of a
number of phosphorescent substances upon the photographic plate, the
most striking characteristic of the _X_ rays being their effect upon
such sensitive plates. In these experiments he obtained no results
until he tried salts of uranium, recalling previous observations of
his as to their phosphorescence. Distinct action was noted.
Furthermore, he proved that this had no connection with the phenomenon
of phosphorescence, as both uranic and uranous salts were active and
the latter show no phosphorescence. Becquerel announced his
discoveries in 1896 and this was the beginning of the new science of
radio-activity.
Radio-active bodies
The rays given off by uranium and its salts were found to differ from
the _X_ rays. They showed no appreciable variation in intensity, no
previous exposure of the substance to light was necessary, and neither
changes of temperature nor any other physical or chemical agency
affected them.
At first uranium and its compounds were the only known source of these
new radiations, but many other substances were examined and two years
later thorium and its compounds were added to the list. In general the
discharging action seemed about the same. Other elements and ordinary
substances show a minute activity. Only potassium and rubidium have a
greater activity than this, and theirs is only about one-thousandth
that of uranium.
An Atomic Property
In the examination of uranium and thorium compounds it was found that
the activity was determined by the uranium and thorium present; it was
proportto the amount ofional these elements present and independent of
the nature of the other elements composing the compound. The
conclusion was, therefore, that the activity was an inherent property
of the atoms of uranium and thorium, that is, an atomic property. This
was a long step forward and introduced into science the conception of
a new property of matter, or at least of certain forms of matter.
Discovery of New Radio-active Bodies
In examining a large number of minerals containing uranium and
thorium, Mme. Curie made the important observation that many of these
were more active than the elements themselves. In measuring the
activity she made use of the electrical method which will be described
later. In the following table giving her results for uranium minerals
the numbers under _i_ give the maximum current in amperes. They serve
simply for comparison.
_i_
Pitchblende from Joachimsthal 7.0 × 10^{-11}
Clevite 1.4 × 10^{-11}
Chalcolite 5.2 × 10^{-11}
Autunite 2.7 × 10^{-11}
Carnotite 6.2 × 10^{-11}
Uranium 2.3 × 10^{-11}
Uranium and potassium sulphate 0.7 × 10^{-11}
Uranium and copper phosphate 0.9 × 10^{-11}
The last three are pure uranium and compounds of that element given
for comparison with the first five, which are naturally occurring
minerals. The last compound has the same composition as chalcolite and
is simply the artificially prepared mineral. It has the activity which
would be calculated from the proportion of uranium present, the copper
and phosphoric acid contributing no activity.
Since the activity is not dependent upon the composition but upon the
amount of uranium present, the activity in all of the minerals should
be less than that of uranium. On the contrary, it is several times
greater. Natural and artificial chalcolite also show a marked
difference in favor of the former. The supposition was a natural one,
therefore, that these minerals contained small quantities of an
element, or elements, undetected by ordinary analysis and having a
much greater activity than uranium. Similar results were obtained in
the examination of thorium minerals and thorium salts.
Discovery of Polonium
Following up this supposition, M. and Mme. Curie set themselves the
task of separating this unknown substance. Starting with pitchblende,
a systematic chemical examination was made. This is an exceedingly
complex mineral, containing many elements. The processes were
laborious and demanded much time and minute care. They need not be
described here. It is sufficient to say that along with bismuth a very
active substance was separated, to which Mme. Curie gave the name of
polonium for Poland, her native land. Its complete isolation is very
difficult and sufficient quantities of the pure substance have not
been obtained to determine its atomic weight and other properties, but
some of the lines of its spectrum have been determined. Chemically it
is very closely analogous to bismuth.
Discovery of Radium
In a similar manner a barium precipitate was obtained from pitchblende
which contained a highly active substance. The pure chloride of this
body and barium can be prepared together and then separated by
fractional crystallization. To the new body thus found the name of
radium was given. It is similar in chemical properties to barium. Its
atomic weight has been determined by several careful investigators and
is accepted as 226. Its spectrum has been mapped and its general
properties are known. It is a silvery white, oxidizable metal. In one
ton of pitchblende about 0.2 gram of radium is present; this is about
5000 times greater than the amount of polonium present. The activity
of the products was depended upon as the guide in these separations.
The radium found is relatively enormously more active than the
pitchblende or uranium.
Other Radio-active Bodies Found
In the above separations use was made of relationships to bismuth and
barium. Similarly, by taking advantage of chemical relationship to the
iron group of elements, another body was partially separated by
Debierne, to which he gave the name actinium. Boltwood discovered in
uranium minerals the presence of a body which he named ionium, and
which is so similar to thorium that it cannot be separated from it.
It, however, far exceeds thorium in activity.
The lead which is present in uranium and thorium minerals--apparently
in fairly definite ratio to the amount of uranium and thorium--is
found, on separation and purification, to possess radio-active
properties. This activity is due to the presence of a very small
proportion of an active constituent called radio-lead, which has
chemical properties identical with those of ordinary lead. The bulk of
the lead obtained from radio-active minerals differs in atomic weight
from ordinary lead and appears also to be different according to
whether its source is a thorium or a uranium mineral.
A large number of other radio-active substances have been separated
and some of their properties determined, but these were found by
different means and will be noted in their proper place. They number
in all more than thirty. The sources or parents of these are the
original uranium or thorium, and the products form regular series with
distinctive properties for each member.
CHAPTER II
PROPERTIES OF THE RADIATIONS
The activity of these radio-active bodies consists in the emission of
certain radiations which may be separated into rays and studied
through the phenomena which they cause.
Ionization of Gases
One of these phenomena is the power of forming ions or carriers of
electricity by the passage of the rays through a gas, thus ionizing
the gas. The details of an experiment will serve to make the meaning
of this ionization clear.
[Illustration: FIG. 1.--IONIZATION OF GASES.]
When this apparatus is set up a minute current will be observed
without the introduction of any radio-active matter. This, as
Rutherford says, has been found due mainly to a slight natural
radio-activity of the matter composing the plates. If radio-active
matter is spread on plate _A_, which is connected with one pole of a
grounded battery, and if plate _B_ is connected with an electrometer
which is also connected with the earth, a current is caused which
increases rapidly with the difference of potential between the plates,
then more slowly until a value is reached that changes only slightly
with a larger increase in the voltage.
According to the theory of ionization, the radiation produces ions at
a constant rate. The ions carrying a positive charge are attracted to
plate _B_, while those negatively charged are attracted to plate _A_,
thus causing a current. These ions will recombine and neutralize their
charges if the opportunity is given. The number, therefore, increases
to a point at which the ions produced balance the number recombining.
When an electric field is produced between the plates, the velocity of
the ions between the plates is increased in proportion to the strength
of the electric field. In a weak field the ions travel so slowly that
most of them recombine on the way and consequently the observed
current is very small. On increasing the voltage the speed of the ions
is increased, fewer recombine, the current increases, and, when the
condition for recombination is practically removed, it will have a
maximum value. This maximum current is called the saturation current
and the value of the potential difference required to give this
maximum current is called the saturation P.D. or saturation voltage.
The picture, then, is this. The radiations separate the components of
the gas into ions, or carriers of electricity, half of which are
charged negatively and half positively. In the electric field those
negatively charged seek the positive plate and those positively
charged seek the negative plate. If time is given, these ions meet and
recombine, their charges are neutralized, and there is no current.
Experimental Confirmation
This theory of the ionization of gases has been most interestingly
confirmed by direct experiment. For instance, the ions may form nuclei
for the condensation of water, and in this way the existence of the
separate ions in the gas may be shown and the number present actually
counted.
When air saturated with water vapor is allowed to expand suddenly, the
water present forms a mist of small globules. There are always small
dust particles in air and around these as nuclei the drops are formed.
These drops will settle and thus by repeated small expansions all dust
nuclei may be removed and no mist or cloud will be formed by further
expansions.
If now the radiation from a radio-active body be introduced into the
condensation vessel, a new cloud is produced in which the water drops
are finer and more numerous according to the intensity of the rays. On
passing a strong beam of light through the condensation chamber, the
drops can readily be seen. These drops form on the ions produced by
the radiation.
Application of Electric Field
If the condensation chamber has two parallel plates for the
application of an electric field like that already described, the
ions will be carried at once to the electrodes and disappear. The
rapidity of this action depends upon the strength of the electric
field and experiment shows that the stronger the field the smaller the
number of condensation drops formed. If there is no electric field, a
cloud can be produced some time after the shutting off of the source
of radiation, showing that time is required for the recombination of
the ions.
Size and Nature of Ions
If the drops are counted (there being special methods for this) and
the total current carried accurately measured, then the charge carried
by each ion may be calculated. This has been determined. The mass of
an ion compared with the mass of the molecules of gas in which it was
produced can also be approximately estimated. In the study of these
ions the view has been held that the charged ion attracted to itself a
cluster of molecules which surrounded the charged nucleus and traveled
with it. It is roughly estimated that about thirty molecules of the
gas cluster around each charged ion.
Photographing the Track of the Ray
Utilizing the fact that these ions with their clusters of molecules
form nuclei for the condensation of water vapor, C. T. R. Wilson has
by instantaneous photography been able to photograph the track of an
ionizing ray through air. The number of the ions produced, and hence
the number of drops, is so great that the trail is shown as a
continuous line. In the copy of this photograph it will be seen that
at some distance from its source the straight trail is slightly but
abruptly bent. Near the end of its course there is another abrupt and
much sharper bend. These bends show where the ionizing ray, in this
case an alpha particle, has been deflected by more or less direct
collision with an atom. These collisions and the final disappearance
of the ray will be discussed later.
[Illustration: FIG. 2.--PHOTOGRAPH OF THE TRACK OF AN
IONIZING RAY.]
Action of Radiations on Photographic Plates
Taking up now other means of examining these radiations, it is well to
consider their action upon a photographic or sensitive plate. It will
be recalled that this was the method by which their existence was
originally detected. To illustrate the method, the following account
of how one such photograph was taken may be given.
The plate was wrapped in two thicknesses of black paper. The objects
were placed upon this and the radio-active ore, separated by a board
one inch thick, was placed above. The exposure lasted five days. The
action is much less rapid and the result not so clearly defined as in
the case of photographs taken by _X_ rays. Of course, the removal of
the board and the use of more concentrated preparations of radium
would give quicker and better results. The method, however, on
account of time consumed and lack of definition is ill adapted to
accurate work.
[Illustration: FIG. 3--PHOTOGRAPH OF VARIOUS OBJECTS TAKEN
BY MEANS OF PITCHBLENDE]
Discharge of Electrified Bodies
The radiations from radio-active bodies can discharge both positively
and negatively electrified bodies by making the air surrounding them a
conductor of electricity. To demonstrate this, use is made of an
electroscope. If the hinged leaf of such an instrument be electrically
charged and a radio-active body be brought into its neighborhood, the
electricity will be discharged and the leaf return to its original
position. The rapidity of this discharge is used to measure the degree
of activity of the body giving off the radiation.
[Illustration: FIG. 4.--GOLD-LEAF ELECTROSCOPE.
The gold-leaf _L_ is attached to a flat rod _R_ and is
insulated inside the vessel by a piece of amber _S_ supported
from the rod _P_. The system is charged by a bent rod _CC'_
passing through an ebonite stopper. After charging, it is
removed from contact with the gold-leaf system. The rods _P_
and _C_ and the cylinder are then connected with the earth.]
Scintillations on Phosphorescent Bodies
It was found by Crookes that a screen covered with phosphorescent zinc
sulphide was brightly lighted up when exposed to the radiations. This
is due to the bombardment of the zinc sulphide by a type of ray called
the alpha ray. Under a magnifying glass this light is seen to be made
up of a number of scintillating points of light and is not continuous,
each scintillation being of very short duration. By proper subdivision
of the field under the lens, the number of scintillations can be
counted with close accuracy.
A simple form of apparatus called the spinthariscope has been devised
to show these scintillations. A zinc sulphide screen is fixed in one
end of a small tube and a plate carrying a trace of radium is placed
very close to it. The scintillations can be observed through an
adjustable lens at the other end of the tube. Outer light should be
cut off, as in a dark room. The screen then appears to be covered with
brilliant flashes of light. Other phosphorescent substances, such as
barium platino-cyanide, may be substituted for the zinc sulphide, but
they do not answer so well.
Penetrating Power
By penetrating power is meant the power exhibited by the rays of
passing through solids of different thicknesses and gases of various
depths. This power varies with different radiations and with the
nature of the solid or gas. For instance, a sheet of metallic foil may
be used and the effect of aluminum will differ from that of gold and
the different rays vary in penetrating power. In the case of gases
air will differ from hydrogen, and it is noticed that certain rays
disappear after penetrating a short distance, while others can
penetrate further before being lost.
Magnetic Deflection
If the radiations are subjected to the action of a strong magnetic
field, it is found that part of them are much deflected in the
magnetic field and describe circular orbits, part are only slightly
deflected and in the opposite direction from the first, and the
remaining rays are entirely unaffected.
[Illustration: FIG. 5.--SHOWING MAGNETIC DEFLECTION OF
[alpha], [beta], AND [gamma] RAYS.]
Three Types of Rays
By the use of these methods of investigation it is learned that the
radiations consist of three types of rays. These have been named the
alpha, beta, and gamma rays, respectively. Some radio-active bodies
emit all three types, some two, and some only one. The distinguishing
characteristic of these types of rays may be summed up as follows:
Alpha Rays
The alpha rays have a positive electrical charge and a comparatively
low penetrating power. They are slightly deflected in strong magnetic
and electric fields. They have a great ionizing power and a velocity
about one-fifteenth that of light.
Beta Rays
The beta rays are negatively charged and have a greater penetrating
power than the alpha rays. They show a strong deflection in magnetic
and electric fields, have less ionizing power than the alpha rays, and
a velocity of the same order as light.
Gamma Rays
The gamma rays are very penetrating and are not deflected in the
magnetic or electric fields. They have the least ionizing power and a
very great velocity.
The penetrating power of each type is complex and varies with the
source, so the statements given are but generalizations. The alpha
rays are projected particles which lose energy in penetrating matter.
As to the power of ionizing gases, if that for the [alpha] rays is
taken as 10,000, then the [beta] rays would be approximately 100 and
the [gamma] rays 1.
Measurement of Radiations
The rays are examined and measured in several ways:
1. By their action on the sensitive photographic plates. The use
of this method is laborious, consumes time, and for comparative
measurements of intensity is uncertain as to effect.
2. By electrical methods, using electroscopes, quadrant
electrometers, etc. These are the methods most used.
3. By exposure to magnetic and electric fields, noting extent and
direction of deflection.
4. By their relative absorption by solids and gases.
5. By the scintillations on a zinc sulphide screen.
Identification of the Rays
The alpha rays have been identified as similar to the so-called canal
rays. These were first observed in the study of the _X_ rays. When an
electrical discharge is passed through a vacuum tube with a cathode
having holes in it, luminous streams pass through the holes toward the
side away from the anode and the general direction of the stream. They
travel in straight lines and render certain substances phosphorescent.
These rays are slightly deflected by a magnetic field and in an
opposite direction from that taken by the cathode rays in their
deflection. The rays seem to be positive ions with masses never less
than that of the hydrogen atom. Their source is uncertain, but they
may be derived from the electrodes.
The beta rays are identical in type with the cathode rays and are
negative electrons.
The gamma rays are analogous to the _X_ rays and are of the order of
light. They are in general considerably more penetrating than _X_
rays. For example, the gamma rays sent out by 30 milligrams of radium
can be detected by an electroscope after passing through 30
centimeters of iron, a much greater thickness than can be penetrated
by the ordinary _X_ rays.
CHAPTER III
CHANGES IN RADIO-ACTIVE BODIES
Is Radio-activity a Permanent Property?
Is this power of emitting radiations a permanent property or is it
lost with the passage of time? The first investigations of the
activity of uranium and thorium showed no loss of intensity at the end
of several years, and radium also seemed to show no decrease in its
enormous activity. Polonium, however, was found to lose most of its
activity in a year, and later it appeared that some radio-active
substances lost most of their activity in the course of a few minutes
or hours.
Induced Activity
A phenomenon called induced or secondary radio-activity was also
observed. Thus a metal plate or wire exposed to the action of thorium
oxide for some hours became itself active. This induced activity was
not permanent but decreased to half its value in about eleven hours
and practically disappeared within a week. Similar phenomena were
observed when radium was substituted for thorium.
Discovery of Uranium X
In 1900 Crookes precipitated a solution of an active uranium salt with
ammonium carbonate. The precipitate was dissolved so far as possible
in an excess of the reagent, leaving an insoluble residue. This
residue was many hundred times more active, weight for weight, than
the original salt, and the solution containing the salt was
practically inactive. At the end of a year the uranium salt had
regained its activity while the residue had become inactive.
Another method of obtaining the same result is to dissolve
crystallized uranium nitrate in ether. Two layers of solution are
formed, one ether and the other water coming from the water of
crystallization. The aqueous layer is active, while the water layer is
inactive. Similarly, by adding barium chloride solution to a solution
of a salt of uranium and then precipitating the barium as sulphate,
the activity is transferred to this precipitate. These experiments
give proof of the formation and separation of a radio-active body by
ordinary chemical operations.
So, too, in the case of thorium salts a substance can be obtained by
means of ammonium hydroxide which is several thousand times more
active than an equal weight of the original salt. After standing a
month, the separated material has lost its activity and the thorium
salt has regained it. Here, again, there is the formation, separation,
and loss of a radio-active body.
Conclusions Drawn
Now, these are ordinary chemical processes for the separation of
distinct chemical individuals. The results, therefore, lead naturally
to the conclusions: (1) it would seem that uranium and thorium are
themselves inactive and the activity is due to some other substance
formed by these elements; (2) this active substance is produced by
some transformation in those elements, for on standing the activity
is regained. This latter conclusion is startling, for it indicates a
change in the atom which, up to the time of this discovery, was deemed
unchangeable under the influence of such physical and chemical changes
as were known to us.
Search for New Radio-active Bodies
The search for new radio-active bodies and the study of their
characteristics has been systematically and successfully carried on.
The bodies obtained in the above experiments were named uranium _X_
and thorium _X_, respectively. Further, it became clear from the
investigation of uranium minerals that radium, polonium, actinium, and
ionium originated from uranium. From thorium minerals a body was
separated called mesothorium, which was analogous to radium. Both
thorium and radium were found to give off a radio-active gas. The
first lost half of its activity in less than one minute. The second
was more stable and lost half of its activity in about four days. The
name radium emanation was given to the latter and it was found
chemically and physically to belong to the class of monatomic or noble
gases, such as helium, argon, neon, etc., which had been discovered by
Ramsay. In some cases the chemical action was determined and these new
bodies were found analogous to well-known elements, as radium to
barium, polonium to bismuth. The physical properties were investigated
and, where possible, spectra were mapped and atomic weights
determined.
It is clear, therefore, that these bodies are elemental in character
and as such are made up of distinct, similar atoms, just as the
commonly recognized elements are believed to be. In this way more than
thirty new elements have been added to the list. These new elements
are called radio-active elements, but it is an open question whether
all atoms do not possess this property in greater or less degree.
Certainly, it is possessed in varying degree by four of the old
elements widely separated in the Periodic System, namely, uranium,
thorium, rubidium, and potassium. The last two, while feebly active
themselves, do not form any secondary radio-active substance so far as
is known. Only two of the elements, then, can definitely be said to go
through these transformations. It is just possible that radio-activity
may be found to be a common property of all atoms and of all matter.
Methods of Investigation
It is important to know how these new bodies were discovered and
distinguished from one another. Two properties are relied upon. One is
the nature of the rays emitted and the other is the duration of the
activity. Of course, knowledge of the physical and chemical properties
is also of great importance whenever obtainable.
Nature of the Radiations
The nature of the radiation is a distinguishing characteristic, though
similarity here does not prove identity of substances. Some emit
[alpha] rays only, some emit [beta] rays, some emit two of the
possible rays, as for instance, [beta] and [gamma], and some emit all
three. The rays may also differ in the velocity with which they are
emitted by different radio-active substances. Thus, in the case of one
substance the [alpha] rays may have a slightly greater or less
penetrating power than those emitted by some other substance, and this
may be true also of the other rays.
Life Periods
The duration of the activity is called the life period. This is
absolutely fixed for each body and furnishes the most important mode
of differentiating among them. It measures the relative stability and
is the time which must elapse before their activity is lost and they,
changing into something else, entirely disappear. The measure usually
adopted is the half-value period. Two hypotheses are made use of:
1. That there is a constant production of fresh radio-active matter by
the radio-active body.
2. That the activity of the matter so formed decreases according to an
exponential law with the time from the moment of its formation.
These hypotheses agree with the experimental results. The decrease and
rise of activity, for example, of uranium and uranium _X_, and also of
thorium and thorium _X_, have been measured, plotted, and the
equations worked out.
Manifestly, a state of equilibrium will be reached when the rate of
loss of activity of the matter already produced is balanced by the
activity of the new matter produced. This equilibrium and the
knowledge of the rate of decrease in general will have little value if
this rate, like chemical changes, is subject to the influence of
chemical and physical conditions. The rate of decrease has been found
to be unaltered by any known chemical or physical agency. For
instance, neither the highest temperatures applicable nor the cold of
liquid air have any appreciable effect.
Equilibrium Series
In order to measure the disintegration of a radio-active body in units
of time so that the rate may be comparable with that of other
radio-active bodies, the relation between the amounts under
consideration must be a definite one. For this purpose equal weights
of the bodies are not taken, but use is made of the amounts which are
in equilibrium with a fixed amount of the parent substance.
One gram of radium has been settled upon as the standard for that
series and a unit known as the "curie" has been adopted to express the
equilibrium quantity of radium emanation. Thus, a curie of radium
emanation (or niton) is the weight (or, as this is a gas, the volume
at standard pressure and temperature) of the emanation in equilibrium
with one gram of radium. This, by calculation and experiment, is found
to be 0.63 cubic millimeter. When this amount has been produced by one
gram of radium, the formation and decay will exactly balance one
another. This is, therefore, one curie of emanation.
The measurement of the rate of decay is difficult but can be carried
out with great accuracy, even down to seconds, in the case of certain
short-lived bodies. Errors crept in at first from the failure to
completely separate the substances produced in the series, and
sometimes because of the simultaneous production of two substances.
As stated, the decay follows an exponential law. The time required for
the decay of activity to half-value does not mean, therefore, that
there will be total decay in twice that time. Thus the half-value
period for uranium _X_ is about 22 days. The period for complete decay
is about 160 days. This half-value period corresponds to the
half-value recovery period of uranium, which is also 22 days.
These were the earlier figures obtained for uranium _X_ and they
illustrate some of the difficulties surrounding such determinations.
It was found later that the body examined as uranium _X_ was really a
constant mixture and of course the decay and recovery periods were
also composite. It required later and very skilful work to separate
them into the bodies indicated in the disintegration series.
The half-value period for thorium _X_ is much shorter, namely, a
little over four days, and this is also the recovery period for
thorium _X_. The plotted decay and recovery curves will intersect at
this point.
The consecutive disintegration series, with the half-value periods,
for the uranium and thorium series as given by Soddy are seen in the
following tables. They are probably subject to some changes on further
and more accurate determination. The nature of the rays emitted is
also given.
[Illustration:
Uranium (8 × 10^9 years) 238.5 -> [alpha]
-> [alpha]
\/
Uranium X (35.5 days) (230.5) -> [beta]&[gamma]
-> ([beta])
\/
\/
\/
Ionium (5 × 10^4 to 10^6 years) (230.5) -> [alpha]
\/
Radium (2,500 years) 226.4 -> [alpha]
\/
Emanation (5.57 days) (222.4) -> [alpha]
\/
Radium A (4.3 minutes) (218.4) -> [alpha]
\/
Radium B (38.5 minutes) (214.4) -> ([beta])
\/
Radium C_{1} (28.1 minutes) { (214.4) -> [alpha]
{ -> [beta]&[gamma]
{ \/
Radium C_{2} (1.9 minutes) { (210.4) -> [beta]&[gamma]
\/
Radium D (24 years?) (210.4) -> ([beta])
\/
Radium E (7.25 days) (210.4) -> [beta]&[gamma]
\/
Radium F (Polonium 202 days) (210.4) -> [alpha]
\/
Radium G (probably lead) (206.8)
Actinium (?)
\/
Radio-Actinium (28.1 days) -> [alpha]
-> ([beta])
\/
Actinium X (15 days) -> [alpha]
\/
Emanation (5.6 seconds) -> [alpha]
\/
Actinium A (0.0029 second) -> [alpha]
\/
Actinium B (52.1 minutes) -> ([beta])
\/
Actinium C_{1} (3.10 mins.) { -> [alpha]
{ \/
Actinium C_{2} (?) { -> [alpha]
\/
Actinium D (7.4 minutes) -> [beta]&[gamma]
\/
Actinium E (unknown)
Thorium (4 × 10^{10} years?) 232.4 -> [alpha](?)
\/
Mesothorium_{1} (7.9 years)
\/
Mesothorium_{2} (8.9 hours) -> [beta]&[gamma]
\/
Radiothorium (2.91 years?) -> [alpha]
\/
Thorium X (5.35 days) -> [alpha]
\/
Emanation (76 seconds) -> [alpha]
\/
Thorium A (0.203 second) -> [alpha]
\/
Thorium B (15.3 hours) -> ([beta])
\/
Thorium C_{1} (79 minutes) { -> [alpha]
{ \/
Thorium C_{2} (?) { -> [alpha]
\/
Thorium D (4.5 minutes) -> [beta]&[gamma]
\/
Thorium E (unknown)
FIG. 6.--DISINTEGRATION SERIES FOR URANIUM, ACTINIUM, AND
THORIUM, AS GIVEN BY SODDY.]
CHAPTER IV
NATURE OF THE ALPHA PARTICLE
Disintegration of the Elements
The remarkable disintegrations related in the last chapter, in which
the heaviest known elementary atom--that of uranium (at. wt. 238)--is
by successive stages changed into others of lower atomic weight,
afford a clue to the nature of the atom and to that goal of the
chemist, the final constitution of matter. The composite nature of the
atom and some sort of interrelation of the elements had previously
been made apparent from a study of the Periodic System and data
gathered still earlier, but all attempts at working out a so-called
genesis of the elements had proved vague and unsatisfactory.
Identification of the Rays
To get an understanding of the disintegration occurring in
radio-active substances, the nature of the rays produced must be
known. These rays are the cause of the activity and their emission
accompanies the changes or disintegration. They have for the sake of
convenience been called the alpha, beta, and gamma rays. The gamma
rays have been identified with the _X_ rays discovered by Röntgen and
are a form of energy analogous to light. The beta rays are particles
of negative electricity or electrons. With these, then, we have some
degree of familiarity. But what are the alpha rays? An answer to this
question should make clearer the character of the changes taking
place, and should give some insight into the composition and mechanism
of the atom.
The Alpha Rays
It has already been stated that these alpha rays are similar or
analogous to the canal rays, but this advances the matter very little,
as the nature of these canal rays has not been fully determined. The
full identity with them, if proved, should have an important
theoretical bearing.
Alpha Rays Consist of Solid Particles
In the first place, these alpha rays have been found to be made up of
solid particles, that is, of what we are accustomed to call matter.
Since it has become more and more difficult to draw a clear
distinction between matter and energy, it would perhaps be better to
say that these particles appear to have some of the properties
hitherto attributed solely to matter. The best evidence that these
particles are of atomic mass is furnished by their deflection in
electric and magnetic fields.
Electrical Charge
It is not of first importance to discuss this or other proofs of the
material nature of these particles. That they carry a charge of
positive electricity is, however, a fact of very great import. The
value of this charge has been carefully determined by a number of
investigators working with different sources of the alpha particles
and has been found to be 9.3 × 10^{-10} electrostatic units
(.000,000,000,93 e.s.). From the consideration of the charge upon an
electron previously obtained by J. J. Thomson and others, it was
concluded that the alpha particle carried two unit positive charges;
the fundamental unit charge, therefore, is half this value, or
4.65 × 10^{-10} e.s.
Helium Formed from Alpha Particles
To determine the nature of the alpha particle a crucial experiment was
carried out by Rutherford and Royds, which was described as follows:
[Illustration: FIG. 7.--APPARATUS USED IN EXPERIMENT BY
RUTHERFORD AND ROYDS.]
A large quantity of radium emanation was compressed into a fine glass
tube _A_, about 1.5 cm. long. This tube, which was sealed to a larger
capillary tube _B_, was sufficiently thin to allow the alpha particles
from the emanation and its products to pass through, but sufficiently
thick to withstand atmospheric pressure. The thickness of the glass
wall was in most cases less than .01 mm. On introducing the emanation
into the tube, the escape of the alpha particles from the emanation
was clearly seen by the scintillations produced at some distance on a
zinc sulphide screen. After this test the glass tube _A_ was
surrounded by a glass tube _T_ and a small spectrum tube _V_ attached
to it. The tube _T_ was exhausted to a charcoal vacuum. By means of
the mercury column _H_, the gases in the tube _T_ could at any time be
compressed into the spectrum tube _V_ and the nature of the gases
which had been produced determined spectroscopically. It was found
that two days after the introduction of the emanation into _A_ the
spectrum showed the yellow line of helium, and after six days the
whole helium spectrum was observed. In order to be certain that the
helium, coming possibly from some other source, had not diffused
through the thin walls of the tube _A_, the emanation was pumped out
and helium substituted. No trace of helium could be observed in the
vacuum tube after several days, showing that the helium observed in
the first experiment must have originated from the alpha particles
which had been propelled through the thin glass tube into the outer
tube.
Most of the alpha particles are propelled with such force that they
penetrate some distance into the walls of the outer tube and some of
these gradually diffuse out into the exhausted space. The presence of
helium in the spectrum tube can be detected after a shorter interval
if a thin cylinder of lead is placed over the emanation tube, since
the particles fired into the lead diffuse out more rapidly than from
glass.
A still more definite proof of the identity of the alpha particle with
the helium atom was obtained by removing the outer glass tube _T_ and
placing a cylinder of lead over the emanation tube in the open air.
Helium was always detected in the lead after it had remained several
hours over the thin tube containing a large quantity of the emanation.
In order to test for the presence of helium in the lead, the gases
present were released by melting the lead in a closed vessel. There
can thus be no doubt that the alpha particle becomes a helium atom
when its positive charge is neutralized.
Thus the chemist was afforded the experience of the building up of at
least one element under his observation, and both the analysis and
synthesis of matter have been revealed through the discoveries of
radio-activity.
Discovery of Helium
It is of interest at this point to learn something of the history of
helium and its occurrence. In 1868 there was discovered by Janssen and
Lockyer a bright yellow line in the spectrum of the sun's
chromosphere. Because of its origin the name helium was given to the
supposed new element causing it. Later it was found in the spectra of
many of the stars, and because of its predominance in some of these
they were called helium stars. Its existence on our planet was not
detected for nearly thirty years.
In 1895, in connection with the discovery of argon in the atmosphere,
a search was made to see if the latter element could be obtained from
mineral sources. In analyzing certain uranium minerals Hillebrand had
found considerable quantities of a gas which he took to be a peculiar
form of nitrogen. Ramsay made a further examination of the gas coming
from these minerals and the spectroscope revealed the yellow line of
helium, thus at last proving the presence of this element on the
earth. It is known now to be present in thorium minerals, in the
waters of radio-active wells, and in minute amounts in the atmosphere.
Its occurrence in every case, in the light of the experiment described
above, would seem to be due to the presence of radio-active changes.
Characteristics of Helium
Helium, on account of its chemical inactivity and physical properties,
is classed along with argon, neon, krypton, and xenon in the zero
group of the Periodic System, and forms with them the monatomic, inert
gases. In this class are now placed also the three radio-active gases,
emanating respectively from radium, thorium, and actinium. These are
generally known as radium emanation, thorium emanation, and actinium
emanation. The first mentioned was once called niton. Emanium was the
name originally proposed by Giesel for the body now known as actinium.
The calculated rate of production of helium in the series in
equilibrium with one gram of radium is 158 cubic millimeters per year.
This corresponds quite well with the experimental results.
Table of Constants
Some of the more important atomic and radio-active constants are given
in the following table. They are recorded here to show how helpful the
study of radio-activity has been in working out the composition of
matter, and to give some idea of the magnitude of the numbers and the
minuteness of the quantities dealt with.
Electric charge carried by each H atom in
electrolysis 4.65 × 10^{-10} e.s.[1]
Electric charge carried by each [alpha]
particle 9.3 × 10^{-10} e.s.
Number of atoms in 1 gram of H 6.2 × 10^{23}
Mass of 1 atom of H 1.6 × 10^{-24} gram
Number of molecules per cc. of any gas at
standard pressure and temperature 2.72 × 10^{19}
Number of [alpha] particles expelled per
second per gram of radium itself 3.6 × 10^{10}
Number of [alpha] particles expelled per
second per gram of radium in
equilibrium with its products 14.3 × 10^{10}
[1] The expression 10^{-10} means multiplying by .000,000,000,1;
10^{10} means multiplying by 10,000,000,000.
CHAPTER V
THE STRUCTURE OF THE ATOM
Properties of Radium
A study of the properties of radium will aid in throwing light upon
the question as to the building up of the atom. First to be considered
are the usual properties which distinguish an elementary body.
Metallic radium has been prepared by a method similar to that used in
the preparation of barium. It is a pure white metal, melting at 700°,
and far more volatile than barium. It rapidly alters on exposure to
the air, probably forming a nitride. It energetically decomposes water
and the product dissolves in the water. Its atomic weight is 226.
Radium forms a series of salts analogous in appearance and chemical
action to those of barium. In the course of time they become colored,
especially if mixed barium salts. The radiations from radium produce
marked chemical effects in a number of substances. Carbon dioxide is
changed into carbon, oxygen, and carbon monoxide, and the latter is
changed into carbon and oxygen. Ammonia is dissociated into nitrogen
and hydrogen; hydrochloric acid into chlorine and hydrogen. Oxygen is
condensed into ozone. In general, the action upon gases appears to be
similar to that of the silent electric discharge. Water is decomposed
into hydrogen and oxygen. If moist radium chloride or a salt of radium
containing water of crystallization is sealed in a glass tube, the
gradual accumulation of hydrogen and oxygen will burst the tube.
The radiations rapidly decompose organic matter with the evolution of
gases. Thus grease from stopcocks of apparatus used with radium or
paraffin will give off carbon dioxide. Under an intense alpha
radiation paraffin or vaseline become hard and infusible. White
phosphorus is changed into red.
The action upon living tissue is most noteworthy, as its possible use
as a remedial agent is dependent upon this. A small amount of a radium
salt enclosed in a glass tube will cause a serious burn on flesh
exposed to it. It therefore has to be handled with care and undue
exposure to the radiations must be avoided. Cancer sacs shrivel up and
practically disappear under its action. Whether the destruction of
whatever causes the cancer is complete is at least open to serious
doubt.
The coagulating effect upon globulin is interesting. When two
solutions of globulin from ox serum are taken and acetic acid added to
one while ammonia is added to the other, the opalescence in drops of
the former is rapidly diminished on exposure to radium, showing a more
complete solution, whereas the latter solution rapidly turns to a
jelly and becomes opaque, indicating a greatly decreased solubility.
Energy Evolved by Radium
The greater part of the tremendous energy evolved by radium is due to
the emission of the alpha particles, and in comparison the beta and
gamma rays together supply only a small fraction. This energy may be
measured as heat. It was first observed that a radium compound
maintained a temperature several degrees higher than that of the air
around it. The rate of heat production was later measured by means of
an ice calorimeter and also by noting the strength of the current
required to raise a comparison tube of barium salt to the same
temperature. Both methods showed that the heat produced was at the
rate of about 135 gram calories per hour. As the emission is
continuous, one gram of radium would therefore emit about 1,180,000
gram calories in the course of a year. At the end of 2000 years it
would still emit 590,000 gram calories per year. Such a production of
energy so far surpasses all experience that it becomes almost
inconceivable. It is futile to speak of it in terms of the heat
evolved by the combustion of hydrogen, which is the greatest that can
be produced by chemical means.
This effect is unaltered at low temperatures, as has been tested by
immersing a tube containing radium in liquid air. It should be stated
that these measurements were made after the radium had reached an
equilibrium with its products; that is, after waiting at least a month
after its preparation. The evolution of heat from radium and the
radio-active substances is, in a sense, a secondary effect, as it
measures the radiant energy transformed into heat energy by the
active matter itself and whatever surrounds it. Let us repeat,
therefore, that the total amount of energy pent up in a single atom of
radium almost passes our powers of conception.
Necessity for a Disintegration Theory
The facts gathered so far justify and necessitate a theory which shall
satisfactorily explain them, and since these phenomena are not caused
by nor subject to the influence of external agencies, they must refer
to changes taking place within the atom--in other words, a theory of
disintegration. In the main, these facts may be summed up as the
emission of certain radiations from known elemental matter: the
material alpha particles with positive charge, the beta particles or
negative electrons, and the gamma rays analogous to _X_ rays. The
emission of these rays results in the production of great heat. Then
there is the law of transformations by which whole series of new
elements are generated from the original element and maintain a
constant equilibrium of growth and decay in the series. Lastly, we
have the production of helium from the alpha particles.
Disintegration Theory
In explanation of these phenomena, Rutherford offered the hypothesis
that the atoms of certain elements were unstable and subject to
disintegration. The only elements definitely known to come under this
description are the two having atoms of the greatest known mass,
thorium (232) and uranium (238).
The atoms of uranium, for instance, are supposed to be not permanent
but unstable systems. According to the hypothesis, about 1 atom in
every 10^{18} becomes unstable each second and breaks up with a
violent explosion for so small a mass of matter. One, or possibly two
alpha particles are expelled with great velocity. This alpha particle
corresponds to an atom of helium with an atomic weight of 4, and its
loss reduces the original atomic weight to 234 with the formation of a
new element, having changed properties corresponding to the new atomic
weight. This new element is uranium X_{1}.
These new atoms are far more unstable than those of uranium, and the
decomposition proceeds at a new rate of 1 in 10^{7} per second. So at
a definite, measurable rate this stepwise disintegration proceeds. The
explosions are not in all cases equally violent in going from element
to element, nor are the results the same. Sometimes alpha particles
alone are expelled, sometimes beta, or two of them together, as alpha
and beta.
The new product may remain with the unchanged part of the original
matter. Thus there would be an accumulation of it until its own decay
balances its production, resulting eventually in a state of
equilibrium.
Constitution of the Atom
In order to explain the electrical and optical properties of matter,
the hypothesis was made that the atom consisted of positively and
negatively electrified particles. Later it was shown that negative
electrons exist in all kinds of matter. Various attempts were made to
work out a model of such an atom in which these particles were held in
equilibrium by electrical forces. The atom of Lord Kelvin consisted of
a uniform sphere of positive electrification throughout which a number
of negative electrons were distributed, and J. J. Thomson has
determined the properties of this type as to the number of particles,
their arrangement and stability.
Rutherford's Atom
According to Rutherford, the atom of uranium may be looked upon as
consisting of a central charge of positive electricity surrounded by a
number of concentric rings of negative electrons in rapid motion. The
positively charged centre is made up of a complicated system in
movement, consisting in part of charged helium and hydrogen atoms, and
practically the whole charge and mass of the atom is concentrated at
the centre. The central system of the atom is from some unknown cause
unstable, and one of the helium atoms escapes from the central mass as
an alpha particle.
There are, confessedly, difficulties connected with this conception of
the atom which need not, however, be discussed here. Much remains to
be learned as to the mechanics of the atom, and the hypothesis
outlined above will probably have to be materially altered as
knowledge grows. Perhaps it may have to be entirely abandoned in favor
of some more satisfactory solution. Until such time it at least
suffices as a mental picture around which the known facts group
themselves. In this picture energy and matter lose their old-time
distinctness of definition. Discrete subdivisions of energy are
recognized which may be called charged particles without losing their
significance. Some of these subdivisions charged in a certain way or
with neutralized charge exhibit the properties of so-called matter.
Scattering of Alpha Particles
This conception of the atom would doubtless fail of much support were
it not for certain experimental facts which lend great weight to it.
Certain suppositions can be based on this theory mathematically
reasoned out and tested by experiment. Predictions thus based on
mathematical reasoning and afterward confirmed by experiment give a
very convincing impression that truth lies at the bottom.
The first of these experimental proofs comes under the head of what is
known as the scattering of the alpha particles, a phenomenon which,
when first observed, proved hard to explain. If an alpha particle in
its escape from the parent atom should come within the influence of
the supposed outer electrical field of some other atom, it should be
deflected from its course and, the intensity of the two charges being
known, the angle of deflection could be calculated. For instance, if
it came to what might be called a head-on collision with the positive
central nucleus of another atom, it would recoil if it were itself of
lesser mass, or would propel the other forward if that were the
lighter.
The experiment is carried out by placing a thin metal foil over a
radio-active body, as radium _C_, which expels alpha particles with a
high velocity, and counting the number of alpha particles which are
scattered through an angle greater than 90° and so recoil toward their
source. This has been done by a number of investigators and it has
been found that the angle of scattering and the number of recoil
particles depend upon the atomic weight of the metal used as foil. For
example, if gold is used, the number of recoil atoms is one in
something less than 8,000.
Taking the atomic weight of gold into consideration, Rutherford
calculated mathematically that this was about the number which should
be driven backward. But he went further and calculated also the number
which should be returned by aluminum, which has an atomic weight of
only about one-seventh that of gold. Two investigators determined
experimentally the number for aluminum and their results agreed with
Rutherford's calculations.
The metals from aluminum to gold have been examined in this way. The
number of recoil particles increases with the atomic weight of the
metal. Comparing experiment with theory, the central charge in an atom
corresponds to about one-half the atomic weight multiplied by the
charge on an electron, or, as it is expressed, 1/2 Ae.
There is only one lighter atom than helium, namely, hydrogen, which
has a mass only one-fourth as great. When alpha particles are
discharged into hydrogen, a few of the latter atoms are found to be
propelled to a distance four times as great as that reached by the
alpha particles.
Stopping Power of Substances
Parallel with the experiments mentioned, there is what is called
the stopping power of substances. This means the depth or thickness
of a substance necessary to put a stop to the course of the alpha
particles. This gives the range of the alpha particles in such
substances and is connected in a simple way with the atomic weight,
that is, it is again fixed by the mass of the opposing atom. This
stopping power of an atom for an alpha particle is approximately
proportional to the square root of its atomic weight.
Considering gases, for instance, if the range in hydrogen be 1,
then the range in oxygen, the atomic weight of which is 16, is only
(1/16)^{1/2} or 1/4. Generally in the case of metals the weight of
matter per unit area required to stop the alpha particle is found to
vary according to the square root of the atomic weight of the metal
taken.
CHAPTER VI
RADIO-ACTIVITY AND CHEMICAL THEORY
Influence upon Chemical Theory
It can easily be seen that the revelations of radio-activity must have
a far-reaching effect upon chemical theory, throwing light upon, and
so bringing nearer, the solution of some of the problems which have
been long discussed without arriving at any satisfactory solution. The
so-called electro-chemical nature of the elements will certainly be
made much clearer. The changes in valence should become intelligible
and valence itself should be explained. A fuller understanding of the
ionization of electrolytes also becomes possible. As these matters are
debatable and the details are still unsettled, it is scarcely
appropriate to give here the hypotheses in detail or to enter into any
discussion of them. But the promise of solution in accord with the
facts is encouraging.
The Periodic System
Such progress has been made, however, in regard to a better
understanding of the Periodic System that the new facts and their
interpretation may well be given. No reliable clue to the meaning of
this system and the true relationship between the elements had been
found up to the time when new light was thrown upon it by the
discoveries of radio-activity. The underlying principle was unknown
and even the statement of what was sometimes erroneously called the
Periodic Law was manifestly incorrect and its terms were ignored.
Basis of the Periodic System
The ordinary statement of the fundamental principle of the Periodic
System has been that the properties of the elements were periodic
functions of the atomic weights, and that when the elements were
arranged in the order of their atomic weights they fell into a natural
series, taking their places in the proper related groups.
In accepting this, the interpretation of function was both
unmathematical and vague, and the order of the atomic weights was not
strictly adhered to but unhesitatingly abandoned to force the group
relationship. Wherever consideration of the atomic weight would have
placed an element out of the grouping with other elements to which it
was clearly related in physical and chemical properties, the guidance
of these properties was accepted and that of the atomic weights
disregarded. Such shiftings are noted in the cases of tellurium and
iodine; cobalt and nickel; argon and potassium. It was most helpful
that, following the order of atomic weights, the majority of the
elements fell naturally into their places. Otherwise the
generalization known as the Periodic System might have remained for a
long time undiscovered and the progress of chemistry would have been
greatly retarded.
Influence of Positive Nucleus
It is evident that the order of the elements is determined by
something else than their atomic weights. From the known facts of
radio-activity it would seem that this determining factor is the
positive nucleus. And this nucleus also determines the mass or weight
of the atom. Taking the elements in their order in the Periodic Series
and numbering the positions held by them in this series as 1, 2, 3,
etc., we get the position number or what is called the atomic number.
This designates the order or position of the element in the series.
We must learn that this number marks a position rather than a single
element, a statement which will be explained later.
Determination of the Atomic Number
Since the atomic weight is unreliable as a means of settling the
position of an element in the series and so fixing its atomic number,
how is this number to be determined? Of course, one answer to this
question is that we may rely upon a consideration of the general
properties, as has been done in the past. Fortunately, other methods
have been found by which this may be confirmed. For instance, the
stopping and scattering power of the element for alpha particles has
been suggested and successfully used.
Use of X-Ray Spectra
A most interesting method is due to Moseley's observations upon the
_X_-ray spectra of the various elements. It has been found that
crystals, such as those of quartz, have the power of reflecting and
defining the _X_ rays. The spectra given by these rays can be
photographed and the wave lengths measured. These _X_ rays are emitted
by various substances under bombardment by the cathode rays (negative
electrons) and have great intensity and very minute wave lengths.
Moseley made use of various metals as anti-cathodes for the production
of these rays. These metals ranged from calcium to zinc in the
Periodic System. In each case he observed that two characteristic
types of _X_ rays of definite intensity and different wave lengths
were emitted. From the frequency of these waves there is deduced a
simple relation connected with a fundamental quantity which increases
in units from one element to the next. This is due to the charge of
the positive central nucleus. The number found in this way is one less
than the atomic number. Thus the number for calcium is 19 instead of
20 and that for zinc is 29 instead of 30. So, by adding 1 to the
number found the atomic number is obtained.
The atomic weight can usually be followed in fixing the atomic number,
but where doubt exists the method just given can be resorted to. Thus
doubt arises in the case of iron and nickel and cobalt. This would be
the order according to the atomic weights. The _X_-ray method gives
the order as iron, cobalt, and nickel, and this is the accepted order
in the Periodic System.
Changes Caused by Ray Emission
On studying the properties of the elements in a transformation series
in connection with the ray emission which produced them, it was seen
that these properties were determined in each case by the nature of
the ray emitted from the preceding transformation product or parent
element.
Atomic Weight Losses
Each alpha particle emitted means a loss of 4 in the atomic weight.
This is the mass of a helium atom. Thus from uranium with an atomic
weight of 238 to radium there is a loss of three alpha particles.
Therefore, 12 must be subtracted from 238, leaving 226, which agrees
closely with the atomic weight of radium as actually determined by the
ordinary methods. Uranium X_{1}, then, would have an atomic weight
of 234 and that of ionium would be 230. The other intermediate
elements, whose formation is due to the loss of beta particles only,
show no decrease in atomic weight.
Lead the End Product
From uranium to lead there is a loss of 8 alpha particles, or 32 units
in atomic weight. This would give for the final product an atomic
weight of 206. The atomic weight of lead is 207.17. It is not at all
certain that the final product of this series is ordinary lead. The
facts are such that they would lead one to think that it is not. It is
known only that the end product would probably be some element closely
resembling lead chemically and hence difficult or impossible to
separate from it. Several accurate determinations of lead coming from
uranium minerals, which always carry this element and in an
approximately definite ratio to the amount of uranium present, show
atomic weights of 206.40; 206.36; and 206.54. Even the most rigid
methods of purification fail to change these results. The lead in
these minerals might therefore be considered as coming in the main
from the disintegration of the uranium atom and, though chemically
resembling lead, as being in reality a different element with
different atomic weight.
Furthermore, in the thorium series 6 alpha particles are lost before
reaching the end product, which again is perhaps the chemical analogue
of lead. The atomic weight here should be 232 less 24, or 208.
Determinations of the atomic weight of lead from thorite, a thorium
mineral nearly free from uranium, gave 208.4.
The end product of the actinium series is also an element resembling
lead, but both the beginning and ending of this series are still in
obscurity.
Changes of Position in the Periodic System
The loss of 4 units in the atomic weight of an element on the
expulsion of an alpha particle is accompanied by a change of chemical
properties which removes the new element two groups toward the
positive side in the Periodic System.
Thus ionium is so closely related to thorium and so resembles it
chemically that it is properly classed along with thorium as a
quadrivalent element in the fourth group. Ionium expels an alpha
particle and becomes radium, which is a bivalent element resembling
barium belonging to the second group. Radium then expels an alpha
particle and becomes the gas, radium emanation, which is an analogue
of argon and belongs to the zero group. Other instances might be cited
which go to show that in all cases the loss of an alpha particle makes
a change of two places toward the left or positive side of the System.
Changes from Loss of Beta Particles
The loss of a beta particle causes no change in the atomic weight but
does cause a shift for each beta particle of one group toward the
right or negative side of the System. Two such losses, then, will
counterbalance the loss of an alpha particle and bring the new element
back to the group originally occupied by its progenitor. Thus uranium
in the sixth group loses an alpha particle and the product UX_{1}
falls in the fourth group. One beta particle is then lost and UX_{2}
belonging to the fifth group is formed. With the loss of one more beta
particle the new element returns to the sixth group from which the
transformation began.
The table on page 48, as adapted from Soddy, affords a general view of
these changes.
Isotopes
An examination of the table will show a number of different elements
falling in the same position in a group of the Periodic System
irrespective of their atomic weights. These are chemically inseparable
so far as the present limitations of chemical analysis are concerned.
Even the spectra of these elements seem to be identical so far as
known. This identity extends to most of the physical properties, but
this demands much further investigation. For this new phenomenon Soddy
has suggested the word isotope for the element and isotopic for the
property, and these names have come into general use.
[Illustration: RADIO-ACTIVE ELEMENTS FROM URANIUM AND THORIUM
PLACED IN THE PERIODIC SYSTEMS Adapted from Soddy]
Manifestly, we have come across a phenomenon here which quite
eliminates the atomic weight as a determining factor as to position in
the Periodic or Natural System or of the elemental properties in
general. All of the properties of the bodies which we call elements,
and consequently of their compounds and hence of matter in general,
seem to depend upon the balance maintained between the charges of
negative and positive electricity which, according to Rutherford's
theory, go to make up the atom.
It is evident that any study of chemical phenomena and chemical theory
is quite incomplete without a study of radio-activity and the
transformations which it produces.
Radio-activity in Nature
In concluding this outline of the main facts of radio-activity, it is
of interest to discuss briefly the presence of radio-active material
on this planet and in the stars. Facts enough have been gathered to
show the probable universality of this phenomenon of radio-activity.
Whether this means solely the disintegration of the uranium and
thorium atoms, or whether other elements are also transformed under
the intensity of the agencies at work in the universe, is of course a
question as yet unsolved.
Radio-active Products in the Earth's Crust
The presence of uranium and thorium widely distributed throughout
the crust of the earth would lead to the conclusion that their
disintegration products would be found there also. Various rocks of
igneous origin have been examined revealing from 4.78 × 10^{-12}
to 0.31 × 10^{-12} grams of radium per gram of the rock. Aqueous
rocks have shown a lesser amount, ranging from 2.92 × 10^{-12} to
0.86 × 10^{-12} grams. As the soil is formed by the decomposition
of these rocks, radium is present in varying amounts in all kinds of
soil.
Presence in Air and Soil Waters
As radium is transformed into the gaseous emanation, this will escape
wherever the soil is not enclosed. For instance, a larger amount of
radio-activity is found in the soil of caves and cellars than in open
soils. If an iron pipe is sunk into a soil and the air of the soil
sucked up into a large electroscope, the latter instrument will show
the effect of the rays emitted and will measure the degree of
activity. Also the interior of the pipe will receive a deposit of the
radio-active material and will show appreciable radio-activity after
being removed from the soil.
This radium emanation is dissolved in the soil waters, wells, springs,
and rivers, rendering them more or less radio-active, and sometimes
the muddy deposit at the bottom of a spring shows decided
radio-activity.
The emanation also escapes into the air so that many observations made
in various places show that the radium emanation is everywhere present
in the atmosphere. Neither summer nor winter seems to affect this
emanation, and it extends certainly to a height of two or three miles.
Rain, falling through the air, dissolves some of the emanation, so
that it may be found in freshly-fallen rain water and also in
freshly-fallen snow. Radio-active deposits are found upon electrically
charged wires exposed near the earth's surface.
As helium is the resulting product of the alpha particles emitted by
the emanation and other radio-active bodies, it is found in the soil
air, soil waters, and atmosphere.
Average measurements of the radio-activity of the atmosphere have led
to the calculation that about one gram of radium per square kilometer
of the earth's surface is requisite to keep up the supply of the
emanation.
A number of estimates have been given as to the heat produced by the
radio-active transformations going on in the material of this planet.
Actual data are scarce and mere assumptions unsatisfactory, so little
that is worth while can be deduced. It is possible that this source of
heat may have an appreciable effect upon or serve to balance the
earth's rate of cooling.
Cosmical Radio-activity
Meteorites of iron coming from other celestial bodies have not shown
the presence of radium. Aerolites or stone meteorites have been found
to contain as much as similar terrestrial rock. Since the sun
contains helium and some stars show its presence as predominating,
this suggests the presence of radio-active matter in these bodies. In
addition, the spectral lines of uranium, radium, and the radium
emanation have been reported as being found in the sun's spectrum and
also in the new star, _Nova Geminorum 2_. These observations await
further investigation and confirmation. So far as the sun's
chromosphere is concerned, the possible amount of radium present would
seem to be very small. If this is true, radio-active processes could
have little to do with the sun's heat. The statement is made by
Rutherford that indirect evidence obtained from the study of the
aurora suggests that the sun emits rays similar in type to the alpha
and beta rays. Such rays would be absorbed, and the gamma rays
likewise, in passing through the earth's atmosphere and so escape
ordinary observation. All of this is but further evidence of the unity
of matter and of forces in the universe.
INDEX
Actinium, discovery of, 6
Activity, induced, 17
Alpha particles, effect of loss on Atomic Weight, 45
electrical charge of, 26
form helium, 27
nature of, 25
penetrating power of, 39
position of element changed by its loss, 46
recoil, 39
scattering of, 38
solid, 26
Atom, constitution of, 36
Kelvin's, 37
models of, 37
Rutherford's, 37
Atomic number, determination of, 43
Becquerel's experiments, 2
Beta particles, change in position of element by loss of, 47
Chalcolite, natural and artificial, 4
Constants, table of, 31
Curie unit, 22
Disintegration of the element, 25
Disintegration series, 24
Disintegration theory, 35
Electroscope, 12
Equilibrium series, 22
Helium, characteristics of, 30
discovery of, 29
Ionium, discovery of, 6
Ionization, application of electric field to, 10
experimental confirmation, 9
Ionization of gases, 7
theory of, 8
Ions, size and nature of, 10
Isotopes, 47
Lead, atomic weight varies with source, 45
radio-active, 6
the end product, 45
Life-periods of radio-active bodies, 21
Periodic system, 41
basis of, 42
Polonium, discovery of, 4
Positive nucleus, influence of, 43
Potassium, radio-activity of, 3
Radiations, action on phosphorescent bodies, 13
action on photographic plates, 11
discharge electrified bodies, 12
magnetic deflection of, 14
measurements of, 15
penetrating power of, 13, 15
Radio-active bodies, elemental nature of, 20
examination of, 20
life periods of, 21
Radio-activity, an atomic property, 3
cosmical, 51
influence on chemical theory, 41
products in atmosphere, 51
products in earth's crust, 50
products in soil waters, 50
Radium, action on organic matter, etc., 33
amount in pitchblende, 5
discovery of, 5
emanation, 22
energy evolved by, 34
properties of, 5, 32
Rays, alpha, 15, 16, 26
beta, 15, 16
gamma, 15, 16
identification of, 16, 25
magnetic deflection of, 14
photographing track of, 10
types of, 14
Rubidium, radio-activity of, 3
Spinthariscope, 13
Stopping power of substances, 39
Thorium X, discovery of, 18, 21
Uranium atom, disintegration of, 36
Uranium minerals, radio-activity of, 3
Uranium X, discovery of, 17, 21, 23
X-ray spectra, 44
Zinc sulphide screen, 13
TRANSCRIBER'S NOTES
1. Passages in italics are surrounded by _underscores_.
2. Images have been moved from the middle of a paragraph to the
closest paragraph break.
3. The original text includes certain Greek alphabets. For this text
version [alpha], [beta], [gamma] indicate first three letters of Greek
alphabet respectively.
4. In this version, the number following carat character ^ is to be
interpreted as follows. The expression 10^{-2} means multiplying by
0.01; 10^{10} means multiplying by 10,000,000,000.
5. In this version, the subscripted text has been replaced by an
underline character _ followed by the same with curly braces { and }.
For example, X_{1} indicates X with subscript 1.
6. The fractions are indicated with the help of forward character /.
For example, 1/4 indicates one-fourth.
7. Other than the changes listed above, the original text has been
reproduced as such.
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