Frederick Soddy.

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clear that a similar relationship must exist. One
of the important corollaries is that changes much
slower than the slowest known, namely, those of
uranium and thorium, would probably not be detect-
able, as, even were a- or /3-particles expelled, they
would be of too low velocity probably to ionise
gases or show fluorescent or photographic actions.
Indeed, for mesothorium-/ and actinium this appears
to be the case. No detectable radiation is expelled,
although the products conform to what would occur

1 The shortest wave-length so far resolved by the crystal
reflection method is 0-072 A. in the spectrum of the -y-rays of
radium- C. Ishino and Rutherford have recently concluded, how-
ever, that the main 7-radiation of radium-C must have a wave-
length lying between 0-02 and 0-007 A. (Phil. Mag., 1917, [vi.] 33,
129; 34. I53)-


in /3-ray changes. The period of both substances
is long, and it is probable that the /3-particle is
expelled, but is undetectable by ionisation methods.
For the slowest /3-ray change, that of radium-Z?,
with a period of twenty-four years, the /3-radiation
is of such low velocity as to be only capable of
detection by special care, and is far less penetrating
than average a-rays. These facts serve to show
that changes may be going on in the non-radio-
active elements which at present are beyond experi-
mental means of detection.


The law of radioactive change, which is the
same for all cases, is that of unimolecular reaction,
the rate of change, or quantity changing in unit of
time, being a fraction, designated by X and known
as the radioactive constant, of the amount present.
The value of X, although vastly different for different
radio-elements, is an absolute constant, so far as is
known, for any one element, independent of every
consideration whatever. The period of average life
is the reciprocal of this constant, but the actual life
of any one atom may assume any value. This is an
experimental fact very difficult to account for. For
example, it is quite easy to compare the value of X
for a collection of atoms ( i ) only just produced and
not in existence a short interval before, and (2) that
have remained undistinguished from an originally
very much greater number, and each of which has
been in existence many times the period of average
life. In both cases the value of X is the same. This
fact excludes from consideration as a conceivable
cause of disintegration any gradual progressive altera-
tion in the atom during its period of existence, as,
for example, was at one time suggested, a gradual


radiation of internal energy by the electrons in their
orbits within the atom. So far, we must admit, the
cause of atomic disintegration remains unknown,
although Lindemann (Phil. Mag 1 ., 1915, [vi.], 30,
560) has attempted, with some success, to frame a
theory to account for it.


The development of the various radioactive
sequences revealed that sometimes the series
branches, and that in the change of one radio-
element sometimes two products result, in general,
in different amounts. Thus the uranium series at
one point branches into the radium and actinium
series, in proportion 92 to 8 out of 100 atoms dis-
integrating. Again, in the case of radium- C and
thorium- C a similar branching occurs, and here in
one branch an a-ray change is followed by a /3-ray
change, and in the other branch the sequence is
reversed. These cases are sufficiently explained if
it be supposed that two simple radioactive changes
are in progress in the same substance simultaneously,
and that each obeys the law of simple change as
though the other did not occur. The distribution of
the original substance into the two products is then
proportional to the relative rates of the two changes.
If AX and A 2 are the radioactive constants of the two
changes, the proportion between the two products is
as A x to A 2 , and the constant of the double change
as a whole, \+^ 2 . For thorium- C, the ratio is as
65 to 35, but for radium-C 99-97 to 0-03. The first
is relatively easy, but the second extremely difficult
to follow experimentally. It is, for example, impos-
sible to follow further what occurs to the minor
branch owing to the minuteness of the quantity of
material, and although this has to be represented as


not further changing-, we have only negative evidence
to go on. This branching is very important as
showing how from one element two products or more
in very different quantity may result, and may be
the explanation of the excessive rarity of certain of
the elements in nature.


The second, and in many respects even more
revolutionary phase in the development of the study
of radioactive change arose out of the chemical
characterisation of the successive products, but some
historical comment on the various influences which
have gone to shape the current conception of the
chemical element may be of interest before dealing
with this development.

The analysis of matter into different chemical
elements was at first concerned with known materials
obtainable in abundance. The question, then, was
not as to the existence or otherwise of certain
elements, but whether certain thoroughly well-known
substances were elements or compounds. Boyle's
original celebrated definition was a purely practical
one. That was to be regarded as elementary which
could not by any means be separated into different
substances. Almost at once, however, there crept
into the interpretation of this conception two fallacies,
or two aspects of the same fallacy, implicit in all
the later characterisations of the elements, right up
to the present time, namely, first, that chemical
analysis was necessarily the most fundamental and
searching kind of material analysis, known or to be
discovered, and, secondly, that chemical compounds
were necessarily more difficult to resolve than simple
mixtures. Any means soon came to mean any
chemical means, and the element, in consequence,


the chemical element. So was taken the first step
which ultimately was to make the term chemical
element, as it is at present understood, denote a
definite but highly complex chemical conception,
incapable of being defined or even understood with-
out long years of training in the science, and totally
different in every single respect from what a plain
man or a beginner in the subject might reasonably
suppose the term element ought to connote. The
elementary and even the homogeneous character
has departed from the conception of the chemical
element, but the conception remains, and, whatever
we choose to call it, will remain. The criterion of
the chemical element soon came to be, in fact, the
possession of a unique chemical character, distin-
guishing it and sufficing for its separation from all
other elements. To this Dalton added a new
criterion, the magnitude of the weight of the atom
of the element, and each element unique in chemical
character (as it happened) proved also to possess a
unique atomic weight.

The discovery of the periodic law introduced the
idea of families of chemically analogous elements,
the members of which recurred after regular intervals
when the elements were arranged in order of atomic
weight. With the exception of hydrogen, every
element became one of a group all totally distinct,
but with obvious similarities. Boyle's practical
definition of the element as that which could not be
further resolved, more and more, as the century
advanced, fell into desuetude. It became replaced
by a theoretical conception, to which subsequently I
propose to apply the term "heterotope," meaning the
occupant of a separate place in the periodic table of
elements. With this place came to be associated
the unique chemical character, unique atomic weight,
and later unique spectrum. On the claims of a



substance to the title of element, as in settling dis-
putes as to what multiple of the equivalent was to
be adopted as the atomic weight, the periodic law
became the court of appeal. Did a claimant to the
title of element fit into a vacant place in the family
of related elements? If it did, not only was there
no doubt as to its atomic weight, but it certainly
could scarcely be an ordinary compound or mixture.
Whatever the elements were, it was clear that they
were all of a class, the limits of chemical analysis,
and, if complex, then all probably of the same kind
of complexity.

Incidentally, also, the periodic law showed that
although there was a connection between atomic
weight and chemical character, there were exceptions,
like tellurium and iodine, where the atomic weights
appeared to have been reversed. This made it
perfectly plain that it was merely a chance that no
two elements happened to possess the same atomic
weight. Dalton, as we shall come to describe,
discovered in the atomic weight not merely a new
atomic property, but a new class of atomic property
which, until the present century, remained the only
one of the kind known, and is concerned with a
different region of the atom from that to which
physical and chemical character, position in the
periodic table, spectrum, and other identifying
characteristics are to be referred.

The discovery of spectrum analysis led to the
recognition of many new elements, caesium and
rubidium, thallium, indium, helium, and gallium all
being so recognised before anything at all was known
as to their other properties. In each case unique
spectrum was later found to correspond with unique
chemical character except for the argon gases, all
characterised by absence of chemical character and
unique atomic weight.


Again, the first-fruits of the discovery of radio-
activity were the recognition of the new elements
polonium, radium, and actinium by their unique
radioactive character in the first place. Then, in
the case of radium, its claim to the title of element
was confirmed, first by its exhibiting a unique
spectrum, then by its possession of unique chemical
character and atomic weight and by its occupying a
vacant place in the periodic table. The emanations,
next, as occupying a place in the family of argon
gases, were easily characterised, and for the radium
emanation unique spectrum was proved. Its origin
from radium by loss of one a-particle gives the atomic
weight as 222, which agrees with determinations
of its density and rate of diffusion. The chemical
characters of polonium and of actinium are different
from those of the elements they most closely resemble.
Polonium, or radium-7% by its close chemical analogy
to both bismuth and tellurium, was characterised as
an element of the sulphur family occupying the vacant
place contiguous to bismuth. Actinium, by its
resemblance in chemical character to the rare earths,
and especially to lanthanum, although capable of
being concentrated fractionally from that element,
was reasonably supposed to occupy the vacant place
in Group III, between radium and thorium. As will
later appear evident, both these elements in due
course may be expected to show unique spectra.

Further progress in the elucidation of the chemical
character of successive products then underwent an
abrupt and, at first, very puzzling change of direction.
As member after member in the series was dis-
tinguished and characterised by its unique radio-
active character, by its disintegration in definite and
characteristic ways at definite and characteristic
rates, no further chemically new elements were
discovered. Unique radioactive character does not


always, as it did with radium, imply unique chemical
and spectroscopic character. The new members
resembled known elements in chemical character so
closely that they could not be separated from them
by chemical analysis, although sharply differentiated
from them by the radioactive properties. Radiolead,
or radium-/?, cannot be separated from the lead
which, being a product of uranium, accompanies it
always in uranium minerals. Ionium, the direct
parent of radium, cannot be separated from thorium ;
but the most instructive case, historically, which
shows well how the new method of radioactive
analysis serves to distinguish different elements,
where chemical analysis fails, was the case of

Ramsay and Hahn, in the course of working up a
large quantity of thorianite for radium, observed in
fractionating the radium from the barium in the
usual way that the activity of the material concen-
trated at both ends of the fractionation. The activity
accumulating in the more soluble fractions was due
to a new product, which they termed radiothorium.
It produces thorium- X, the thorium emanation,
etc., in successive changes. Naturally enough, they
thought they had separated radiothorium by chemical
processes from thorium, but they had not, for that,
as we know, is quite impossible. Then Hahn found
along with the other end fraction, containing the
radium, a further new product, mesothorium, which
is intermediate between thorium and radiothorium.
The radiothorium they had separated from thorianite
was not that present in the mineral when they started,
but that which had re-formed from the mesothorium
after it had been separated from the thorium in the
mineral. Could any more elegant extension, not


merely of knowledge, but of the means of obtaining
knowledge, be imagined? Two different elements,
thorium and radiothorium, which on account of their
chemical resemblance could not be individually
recognised, and in the original interpretation of the
thorium disintegration series were taken as one,
became individually knowable, because the latter is
the product of the former through the intermediary
of a third member, mesothorium, possessing chemical
properties totally unlike either. Radioactive change
thus became the means of a new analysis of matter,
for which there is no counterpart outside the radio-

In turn, mesothorium suffered analysis into two
successive products, mesothorium- 1 and -2, the first
distinguished by long period of life and a rayless
disintegration into the second, which has a short life
and gives powerful /3- and y-radiation in its change
into radiothorium.

I then found that mesothorium- 1 was chemically
non-separable from radium, a discovery also made by
Marckwald at the same time, and in 1911 I pointed
out that in an a-ray change, such as ionium into
radium, radium into emanation, thorium into meso-
thorium- i, and other cases, the expulsion of the
a-particle causes the radio-element to shift its place
in the periodic table by two places in the direction of
diminishing mass and diminishing valency, whereas
in successive changes in which a-particles are not
expelled, it frequently reverts to its former position,
as, for example, radiothorium from mesothorium and
lead from radiolead.

To those actually engaged in the task of trying
to separate the successive products of radioactive
change by chemical analysis, it soon became clear
that the chemical resemblances disclosed between
certain of the members was such as to amount to


chemical identity. The most obstinate cases of
similarity previously known, among the rare earths,
for example, cannot be compared with them. In all
cases, radioactive methods afford the most delicate
means for detecting the least alteration in the
concentration of the constituents, and the most
prolonged and careful attempts fail to produce a
detectable separation.

At my request, Fleck undertook in my laboratory
a systematic chemical examination of all the members
of the series still imperfectly characterised, from the
point of view of first finding which known element
they most resembled and then finding whether or not
they could be separated from that element. His
researches were the means of finally unmasking
the extreme simplicity and profound theoretical
significance of the process of radioactive change.
All the members of the series so far chemically un-
characterised he found to be chemically non-separable
from one or other of the known elements, meso-
thorium-2 from actinium, radium-^4 from polonium,
the three /^-members and radium-/? from lead, the
three C-members and radium-/? from bismuth,
actinium-/? and thorium-/? from thallium.


In February, 1913, K. Fajans in Germany, from
electrochemical evidence, and in this country A. S.
Russell and I, independently, from Fleck's work,
pointed out the complete generalisation which
connects chemical character and radioactive change.
In addition to the shift of two places in the periodic
table caused by the expulsion of the a-particle, it was
now clear that the expulsion of the /3-particle caused
a shift of one place in the opposite direction. Since
the a-particle carries two atomic charges of positive


electricity and the ^-particle one atomic charge of
negative electricity, the successive places in the
periodic table must thus correspond with unit
difference of charge in the atomic structure, a con-
clusion reached later for the whole periodic table,
as far as aluminium, as the result of Moseley's
investigations on the frequency of Barkla's character-
istic ^T-radiations of the elements.

The non-separable elements, with identical chemi-
cal character, on this scheme were found all to
occupy the same place in the periodic table, and
on this account I named them isotopes. Conversely,
the different elements recognised by chemical analysis
should be termed "heterotopes," that is, substances
occupying separate places in the periodic table, but
themselves mixtures, actually proved or potential, of
different isotopes, not necessarily homogeneous as
regards atomic weight and radioactive character,
but homogeneous as regards chemical and spectro-
scopic character, and also physical character, so far
as that is not directly dependent on atomic mass.


As regards the spectrum, the first indication
that chemically non-separable elements probably
possessed identical spectra arose out of the failure
of Russell and Rossi and of Exner and Haschek
in 1912 to detect any lines other than those of
thorium in the spectrum of ionium-thorium prepara-
tions that might reasonably be supposed to contain
an appreciable, if not considerable, percentage of
ionium. The work of Honigschmid on the atomic
weight of ionium-thorium preparations has fully
confirmed this view. The isotopes of lead of
different atomic weight separated from uranium
and thorium minerals have been found to possess


identical spectra. For this element, lead, Rutherford
and Andrade have shown that the secondary y-radia-
tion excited by the impact of /8-rays on a block of
ordinary lead gave by crystal reflection two lines
identical in wave-length with the two strongest
lines in the y-ray spectrum of radium-./?, an isotope
of lead, as Fleck showed, of atomic weight 214.
This is of importance as indicating that A"-rays
and y-rays, although no doubt originating in a
deeper region of the atom than the ordinary light
spectrum, do not originate in the deepest region
of all to which the weight of an atom and its
radioactive properties are to be referred.


The generalisation, brought up to date, is set
forth in detail in the Tables on p. 134 and is illus-
trated by the accompanying figure, which is to be
read at an angle of 45, making the lines of atomic
weight horizontal and the division between the
successive places in the periodic table vertical.
Starting from uranium and thorium, the series run
in an alternating course across the table and extend
over the last twelve places as far as the element
thallium. At this point, it is interesting to note
that the expulsion of an a- instead of a /3-particle
would have resulted in the production of an isotope
of gold, and so literally have realised the goal of
the alchemist. As it happens, a ^-particle is
expelled and lead results, so far as the changes have
yet been traced, in all cases as the final product.

It has been necessary, in order to separate the
series from one another, to displace the actinium
series to the right and the radium series to the
left of the centre of the places, but this displacement
within the single place is not intended to express


any physical significance ; but for the fact that many
members would be superimposed, they would all
be represented in the centre of the places. The
periods of average life, which are always 1-443
times the periods of half-change, are shown for
each member above or below its symbol, a ? indicat-
ing- that the period is estimated indirectly from
the Geiger-Nuttall relation.

The figures at the head of each place represent
the atomic numbers or number of the place in the
periodic table, starting with hydrogen as unity,
helium as 2, lithium as 3, and so on. Moseley found
that the square-root of the frequency of the charac-
teristic ^-radiation of an element was, for the
^-series of radiations, proportional to integers less
by one than the atomic numbers. Strictly speak-
ing, there is no means of determining the absolute
value of the atomic number, but the starting point
having been fixed for any one element, the others
can then be found in terms of it. Moseley assumed
the atomic number of aluminium as 1 3, as it is the
thirteenth known element in the list starting with
hydrogen as unity. It is unlikely that any new
elements will be discovered between hydrogen and
aluminium, although if they were it would be
necessary to alter the whole of the subsequent
atomic numbers to correspond. For ^-radiations of
the other series, the square-roots of the frequencies
are not proportional to integers even, although the
differences are nearly integral for successive elements
in the periodic table. The actual numbers in the
figure, 92 for uranium, for example, are derived
from the assumption that the atomic number of
aluminium is 13, but it is well to remember that,
although relatively to one another based on experi-
mental evidence, the absolute value is to some
extent arbitrary.





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[Face page 134.



The simple connection between the sequence of
radioactive changes and the chemical character of
the products has effected an enormous simplifica-
tion, not only in the theory, but also in the practice
of radio-chemistry. The series extends over twelve
places, two, namely those in the families of the
halogens and the alkali metals, being- entirely
skipped. In the ten occupied places are forty- three
distinct types of matter, but only ten chemical
elements. Seven of these ten, thallium, lead, bis-
muth, emanation, radium, thorium, and uranium,
can now in every respect be considered, both
chemically and spectroscopically, thoroughly well
known. These seven places accommodate all but
nine of the known radio-elements, and these nine,
the isotopes of polonium, actinium, and ekatan-
talum respectively, are the only members the
chemistry and physics of which cannot be referred
to well-known elements obtainable in sufficient
quantity for ordinary chemical and spectroscopic

Of these three, polonium, although the element
of which at present the chemistry is best known,
is likely to remain the most difficult to bring into

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Online LibraryFrederick SoddyScience and life: Aberdeen addresses → online text (page 10 of 18)