chemical properties, discussed later, shows that a particle must be liberated but at too low a speed to detect with certainty. Actinium and mesothorium are examples of such products.
A number of new products have been discovered, particularly in the uranium and actinium series. The results are included in the appended table of radioactive elements.
TABLE OF RADIOACTIVE ELEMENTS
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Element
Atomic Weight
Atom- ic Num- ber
T
Rays
Range cms. o rays (15 and 760
mm.)
Uranium-Radi-
um Series.
Uranium I.
238-18
92
4-5Xioyrs.
a
2-50
Uranium Xi
234
90
23-8 days
0, 7
Uranium Xj
234
9'
1-15 min.
f,y
Uranium II.
234
92
about 2X10"
a
2-90
yrs.
Uranium Y
(3 per cent)
230
90
24-6 hrs.
Ionium
230
90
about 9X10'
a
3-07
yrs.
Radium
226
88
1700 yrs.
a
3-52
Radium
Emanation
222
86
3-85 days
a
4-16
Radium A
218
84
3-05 min.
a
4-75
Radium B
214
82
26-8 min.
ft, y
Radium C
214
83
19-5 min.
a.0, 7
6-94
Radium D
2IO
82
1 6 yrs.
0,7
Radium E
210
83
4-85 days
0,7
Radium F
(Polonium)
2IO
84
136-5 days
a.
3-83
Radium C (end-
product ura-
nium-lead)
2O6
82
Thorium Series.
Thorium
232-I
90
2-2Xlo'yrs.
a
2-72
Mesothorium I
228
88
6-7 yrs.
0,7
Mesothorium 2
228
89
6-2 nrs.
0,7
Radiothoritim
228
90
1-90 yrs.
a
3-87
1 horium X
224
88
3-64 days
a
4-30
Thorium
Emanation
22O
86
54 sees.
a
5-00
Thorium A
216
84
0-14 sec.
a
5-/o
Thorium B
212
82
10-6 hrs.
0,7
Thorium C
212
83
60 min.
a
/4-8o \ 8-60
Thorium D
208
81
3-2 min.
0,7
Thorium E
(end-product
thorium-
lead)
208
82
Actinium Series.
Protoactinium
23<>
91
about 10*
a
3-31
yrs.
Actinium
226
89
20 yrs.
Radioactinium
226
90
19 days
a
4-6
Actinium X
222
88
1 1 -2 days
a
4-26
Actinium
Emanation
218
86
3-9 sees.
a
5-6
Actinium A
214
84
002 sec.
a
6-3
Actinium B
2IO
82
36 min.
0.7
Actinium C
210
83
2-16 min.
a
5-'5
Actinium D
2O6
8l
4-76 min.
0,7
Actinium E
(end-product
actinium-
lead)
2Ofi
82
In the table T is the time-period of a product or the time required for the product to be half transformed. It will be seen that the value of T, which is a measure of the relative stability of atoms, varies between 2-2 Xio 10 years (Thorium) and -002 second (Actinium A). The atomic weights and atomic numbers of uranium, radium, ura- nium-lead, thorium, thorium-lead have been directly determined. The atomic weights and atomic numbers of the others are deduced on the assumption that the expulsion of an o particle (helium atom) of charge 2 and mass 4 lowers the atomic number of the succeeding element by 2 units and the atomic weight by 4. The expulsion of a ft particle raises the atomic number by I unit, but it is not sup- posed to influence the atomic weight to a detectable degree.
Branch Products. In the great majority of cases each of the radioactive elements breaks up in a definite way, giving rise to one a or /3 particle and to one atom of the new product. Un-
doubted evidence, however, has been obtained that in a few cases the atoms break up in two or more distinct ways, giving rise to two or more products characterized by different radio- active properties. A branching of the uranium series was early demanded in order to account for the origin of actinium. While the latter is always found in uranium minerals in constant proportion with the uranium, Boltwood showed that the activity of the actinium with its whole series of a-ray products in a uranium mineral was much less than that given by a single a-ray product of the main radium series. The head of the uranium series is believed to be uranium Y, the branch product of the uranium series first observed by AntonofE. The branching is supposed to occur in the product uranium Xi, 4% going into the actinium branch and the other 96 % into the main uranium series. The atomic weights of actinium given in the table are calculated on this basis. The recently discovered product, protoactinium, is the hitherto missing link between uranium Y and actinium.
The most striking cases of branching occur in the " C " products of radium, thorium and actinium, each of which breaks up in two or more distinct ways. In the case of radium C, a new substance called radium Ct was obtained by recoil from a nickel plate coated with radium C. This product emitted only rays and had a period of 1-4 minutes. Fajans estimated that the amount of the product wasonly 1/3,000 of that of radium C. To account for these results, the following scheme of transformation has been proposed :
- End.
Radium 19 min.
^ "*
c
^
o
Radium f, fc
-^ 1-4 min. L RaHiMm C, Q J
T06 sec.
Radium D.
etc.
where in the main branch a particle is first expelled, giving rise to radium C, which emits an o particle. The reverse process is assumed to take place in the other branch. Radium Ci, which emits a swift o particle, is supposed to have an exceedingly short period of trans- formation. It is uncertain whether the radium C 2 branch ends after the expulsion of a particle. The resulting product is an isotope of lead like radium D in the main branch.
In the case of thorium C, two sets of o particles are observed, one- tliird of the total number of range 5-0 cms., and the remainder of range 8-6 centimetres. Recently another set of rays, about 1/10,000 of the total, has been found by Rutherford and Wood to have the great range in air of 11-3 cms. and has been shown to consist of a particles. These results suggest that the thorium C atom breaks up in three distinct ways, each marked by the expulsion of an a particle with characteristic but different velocity. Marsden showed that actinium C emits two sets of a particles, 99-84% of range 5-15 cms. and 16% of range 6-4 cms., indicating a dual transformation of actinium C. It is quite possible that a close examination of radioac- tive substances may reveal other examples of such complex methods of transformation, for, after the violent explosion that occurs during the breaking-up of an atom, more than one state of temporary equilibrium may be possible for the residual atom.
Relation between Range of a. Rays and Period of Transformation. We have seen that each a-ray product emits o particles of charac- teristic velocity which have a definite range in air. It was early ob- served that there appeared to be a connexion between the period of transformation of a product and the velocity of the a particles emitted. The shorter the period of transformation, the swifter is the velocity of expulsion of the a particle. This relation was brought out clearly by the measurements of Geiger and Nuttall, where it was shown that if the logarithm of the range was plotted against the logarithm of X, the constant of transformation, all the points lay nearly on a straight line. A similar result has been observed for the thorium and actinium products. This relation is at present purely empirical and no doubt only approximate, but it is of great in- terest as indicating a possible relation between the stability of the radioactive nucleus and the velocity of the expelled helium atom. This relation has proved very useful in forming estimates of the period of transformation of ionium and other substances before the results of more direct determinations were available. From this relation also the change which gives rise to the swift a particle of radium C is believed to be exceedingly rapid, and a similar con- clusion is drawn for the products emitting the very swift o particles from thorium C.
Chemistry of the Radioelements. Apart from uranium and thorium and a few special cases like radium and polonium, the radioactive products of short life exist in too small quantity to examine by the ordinary chemical methods, but, by the use of the radioactive method of analysis, it was possible to form some idea of their chemical behaviour. Certain very interesting points
