RADIOACTIVITY— ARTIFICIAL AND NATURAL 299 



nucleus is much more massive than the electrons, and this is one of the 

 reasons for comparing the atom with the solar system, in which the 

 sun is much more massive than the planets which perpetually swing 

 in orbits around it. A less hackneyed and newer simile is that of 

 Bragg, just now, by the way, appointed as Rutherford's successor in 

 the Cavendish chair at Cambridge, who likens the atom to a man's 

 head with a swarm of gnats buzzing around it. Normally — that is to 

 say, when the atom is complete and electrically neutral — ^the negative 

 charges of all the electrons put together just balance the positive 

 charge of the nucleus. If Z be used to stand for the number of 

 electrons in the normal neutral atom, and —e for the charge of the 

 negative electron, then +Ze is the amount of the charge on the 

 nucleus. Z is called the "atomic number" of the element in question. 



13 14 15 16 17 18 19 20 21 22 23 



6 C O ^ 



8 O 



10 Ne 



11 Na 



Fig. 3 — Isotopes of the elements numbered 6 to 11. 



This it is, of which all the isotopes of any one element have the same 

 value. This it is which distinguishes an element, and is common to 

 all of the different atoms of that element whatever their masses may 

 be. For hydrogen it is 1 ; for helium 2; for lithium 3; for uranium 92. 

 Each row in Fig. 2 and Fig. 3 is marked on the left not only with 

 the chemical symbol of the element to which the row belongs, but also 

 with the atomic number thereof. 



Radioactivity is a feature of the nucleus. This accounts for some of 

 its remarkable aspects, which greatly surprised the world of physicists 

 and chemists when they were first established. Being a quality of 

 the nucleus, it varies from one isotope * to another of any element, 

 much more drastically than does the mass. Being a quality of the 

 nucleus, it is immune to the physical state of the atom — -i.e. it is the 



* Isotopes were first distinguished from each other (by Soddy) by virtue of their 

 differences in radioactivity. 



