82 



NATURE 



[September i6, 1920 



pertics, for the X-rays this periodicity disappears, 

 and is replaced by a perfectly orderly projjression 

 from element to element. Kach spectrum can be 

 derived from the last by a reduction of the vvave- 

 lengths according- to a simple rule. Thus an 

 examination of a sequence of spectra reveals the 

 trrie order of the elements, and gaps are shown 

 by a missing term in the sequence. It will be 

 seen that Moseley's method only gives differences 

 of atomic number, and not the numbers them- 

 selves. To determine these we must find one of 

 them in some other way. This is not difificult, for 

 we know from observation on o-particles that 

 helium must have atomic number 2 ; and 

 though X-rays have not been observed for the 



sion of an a- or a )B-particle from the nucleus, and 

 this emission transmutes the atom into a new 

 element. From a study of the chemical behaviour 

 of these successive elements it was concluded that 

 the emission of an a-ray shifts the element two 

 places to the left in the periodic table, while the 

 emission of a ;8-ray shifts it one place to the right. 

 This fits in exactly with the conception of atomic 

 number, for when the nucleus loses an o-particle 

 (that is, a helium nucleus of atomic number 2) 

 its own number must be reduced by two, whereas 

 the loss of a /3-particle out of the nucleus must 

 raise the number by one, since the jS-particle is 

 an electron and has a negative charge. By a 

 studv of all the radio-elements we can work out 



92 

 91 

 90 

 89 

 88 

 87 

 86 

 85 



Z^\Po/on/um 

 \ 



83 



82 

 81 



Uranium U.j[ 



il.Xi Ionium 



Radium 



Protoach'nium 

 U.Y \ Radioact. 



Rci. Emanstion 



Ra.A 



Ac.Ei 



m. 



Pb. Ra.D Pb. 

 \ 



Ra.B 



Ra.C^ 



AcA 



Ac.B 



Thorium Radiotb. 



Mth. 



■s 



Mesoi-h.i Th.X 



Th.Em. 



Th.B 



Th.D 



92 



91 



90 



89 



%^ 



87 



86 



as 



84 

 83 

 hi 

 8] 



P"lG. I. — The Radio-active Transformations. In every case a step two downwards is accompanied by the emission of an a-partide, 

 and one upwards by a /S-farticle. Actinium is derived -rom uranium or an isotope, but the connection shown is rather 

 conjectural. Thorium C has a third branch about which little is known. The final products in all cases are believed to be lead, 

 but as they aie not radio-active it is uncertain how many of the isotopes are really different. In the Table of Atomic Numbers 

 they were counted as all different, and one was added on to allow for the existence of common lead. One was also added 

 there for 8i and 83 to represent the non-radio-active types of thallium and bismuth. 



first dozen elements, the periodic table is perfectly 

 regular in that region, and it is impossible to 

 believe that it could contain any gaps. More re- 

 markable still, if the formula for the wave- 

 length of the X-ray spectrum is extrapolated right 

 back to hydrogen, it reproduces the fundamental 

 line of the famous Balmer series. The numbers 

 which were given in the table are therefore cer- 

 tainly correct. 



In formulating the principle of atomic number 

 we said that it, and not the weight, completely 

 determines the chemical character of the element. 

 This statement is justified by a line of evidence 

 which we must now consider. The process of 

 radio-active transformation consists in the emis- 

 NO. 2655, VOL, 106] 



chains of transformations such as are shown in 

 Fig. 1. These chains start at uranium 92 and 

 thorium go, and, so far as we know, all end at lead 

 82. It will be seen that most of the numbers are 

 occupied several times over. For example, 90 has 

 ionium, the parent of radium, as well as thorium 

 and several others. It has been found that the 

 spectra of ionium and thorium are exactly the 

 sarne, and that all chemical methods are quite 

 powerless to .separate them from each other. They 

 are chemically identical because they have the 

 same atomic number, but thev differ in the nature 

 of their nuclei, as witnessed bv the difference in 

 their radio-active behaviour. Such substances are 

 called isotopes. 



