334 



NATURE 



[May 12, 1921 



Isotopes and Atomic Weights.^ 



By Dr. F. W. Aston. 



POSSIBLY the most important generalisation 

 in the whole history of chemistry is the 

 atomic theory put forward by John Dalton in 1803, 

 and it is a striking tribute to the shrewd intuition 

 of that observer that of his five postulates only 

 one seems to be in the least degree faulty, and 

 more than a century of active and unremitting 

 investigation has been necessary to detect the flaw 

 in that. 



The postulate in question states that " atoms of 

 the same element are similar to one another and 

 equal in weight." Of course, if we take this as a 

 definition of the word "element," it becomes a 

 truism ; but, on the other hand, what Dalton 

 meant by an element and what we understand by 

 the word to-day is a substance such as hydrogen, 

 oxygen, chlorine, or lead, which has unique chem- 

 ical properties, and cannot be resolved into more 

 elementary constituents by any known chemical 

 process. For many of the well-known elements 

 Dalton 's postulate still appears to be strictly true, 

 but for others, probably the majority, it needs 

 some modification. 



The general state of opinion at the end of 

 last century may be gathered from the following 

 quotations from Sir William Ramsay's address to 

 the British Association at Toronto in 1897 : 



There have been almost innumerable attempts to 

 reduce the differences between atomic weights to 

 regularity by contriving some formula which will ex- 

 press the numbers which represent the atomic weights 

 with all their irregularities. Needless to say, such 

 attempts have in no case been successful. Apparent 

 success is always attained at the expense of accuracy, 

 and the numbers reproduced are not those accepted 

 as the true atomic weights. Such attempts, in my 

 opinion, are futile. Still, the human mind does not 

 rest contented in merely chronicling such an irregu- 

 larity ; it strives to understand why such an irregu- 

 larity should exist. . . . The idea . . . has been ad- 

 vanced by Prof. Schiitzenberger, and later by Mr. 

 Crookes, that what we term the atomic weight of rn 

 element is a mean ; that when we say the atomic 

 weight of oxvgen is 16, we merely state that the 

 average atomic weight is 16 ; and it is not inconceiv- 

 able that a certain number of molecules have a weifrht 

 somewhat higher than 32, while a certain number 

 have a lower weight. 



That such conjectures were then regarded as 

 wildly speculative shows how strong was the faith 

 in Dalton 's postulate, which is all the more re- 

 markable when we consider that at that time not 

 one single direct experimental proof of it had 

 been offered. Such proof, obviously, can be ob- 

 tained only by some method which measures the 

 masses of atoms individually, and at that time 

 none had been developed. 



The first direct evidence that the atoms of an 

 element were, at least approximately, equal in mass 

 appears to be that obtained by Sir J. J. Thomson 

 in 1910 by his well-known method of analysis of 



1 Discourse delivered at the Royal Infttitution on Friday, February u. 



NO. 2689, VOL. 107] 



positive rays. The fact that sharply defined para- 

 bolic streaks were obtained at all proves that the 

 ratio of the masses of the separate particles caus- 

 ing them to the charges of electricity they carry is 

 constant. The latter was known to be a definite 

 unit, or a simple multiple of it, so that if the 

 masses of the individual atoms varied amongst 

 each other in an arbitrary manner, an indistinct 

 blur would result instead of a clear-cut parabola. 

 Before going on to consider the evidence of posi- 

 tive rays in greater detail, it will be as well to 

 re-state briefly the evidence upon which the theory 

 of isotopes was founded. The first indication 

 that it might be possible to obtain substances 

 having identical chemical properties, but different 

 atomic weights, was afforded by the brilliant re- 

 searches on the radio-active elements made by 

 Sir E. Rutherford and his colleagues. Investiga- 

 tions on the transformations of the diff'erent radio- 

 active families showed that certain products, such 

 as lead, could be formed in several ways. Each 

 of the leads so formed was found to have chemical 

 properties identical in every respect with those of 

 ordinary lead, but their method of production pre- 

 cluded any possibility of them all having the 

 same atomic weight. Such bodies, although 

 having different atomic weights, must occupy the 

 same position in the periodic table of the 

 elements, and on this account have been called 

 " isotopes " by Prof. Soddy. 



Moseley's epoch-making discovery has shown 

 us that chemical properties depend, not upon 

 atomic weight, but upon something much more 

 fundamental, namely, atomic number. The 

 atomic number of an element is the number of 

 units of positive electricity on the nucleus of its 

 atoms; the nuclear charge of hydrogen is i, of 

 helium 2, of lithium 3, and so on. We see, there- 

 fore, that isotopes are elements having the same 

 atomic number, but different atomic weights. 



The theory of isotopes was triumphantly vindi- 

 cated during the war bv the researches of Soddy, 

 Richards, Honigschmid, and others on the 

 atomic weights of lead found in various radio- 

 active minerals. Quantities were obtainable 

 which were ample for the most accurate deter- 

 minations by chemical methods, and the atomic 

 weights were found to differ from each other and 

 from ordinary lead by quantities altogether out- 

 side possible experimental error. Long before 

 this convincing proof was forthcoming, the theory 

 of isotopes was discussed with the greatest in- 

 terest in connection with atomic weights in 

 general. If isotopes occurred among the heavy 

 elements, why should thev not be possible among 

 the lighter non-radio-active ones, in which case 

 elements with fractional atomic weights might 

 clearly be mixtures, the constituents having 

 atomic weights equal to whole numbers? This 

 explanation was a very attractive one, for the 



