3G2 



DISCOVERY 



a conipKJuiul. Hy passing an electric current through 

 it its elements, liydrof^en and oxygen, arc obtained. 



An clement or an elementary substance is supposed 

 to be an aggregate of minute particles which are called 

 atoms. These atoms are supposed to be the saine in 

 cverj' respect ; they arc indivisible. What they look 

 like is of no imixirtance, for nobody will ever see them — 

 they are much too small — but most people imagine 

 them as little hard spheres like tiny balls of steel. All 

 atoms of any particular element have the same size 

 and weight, but the weights of atoms of different 

 elements are very different. The atom of uranium, 

 the element whose atom is the heaviest, is 236 times 

 that of hj'drogen, the lightest ; that of iron is three 

 and a half times heavier than that of o.xygen ; that of 

 oxygen is four times the weight of the atom of helium ; 

 that of helium nearly four times that of hydrogen. 



These weights cannot conceivably be measured 

 directly, i.e. by weighing the atoms singly in turn. 

 The atomic weight of an element has to be determined 

 indirectly, following certain laws and assumptions which 

 are at the very root of the science of chemistry. How 

 this is done need not concern us here. Suffice it to 

 say that when the atomic weights of different elements 

 are determined, these values are expressed not as vciy 

 small fractions of a very small weight such as an ounce, 

 a grain, or a gramme, but more simply as nimibers on 

 a scale. One must remember that these numbers, 

 although called " atomic weights," are for purposes 

 of comparison only. They arc not the actual weights 

 of the atoms. The atomic weight of o.xygcn is chosen 

 as 16. It is the standard of comparison. Uranium is 

 then found to have an atomic weight of 238, so that 

 the uranium atom is W- times heavier than that of 

 oxygen. Lead has an atomic weight of 207 ; tin of 

 119 ; copper, 63 ; iron, 56 ; cldorine, 35-5 ; nitrogen, 

 14 ; and hydrogen just a little greater than i. From 

 these numbers one can work out, if one wishes, the 

 relative weights of these elements with regard to each 

 other or to oxygen. With a table of the atomic weights 

 of the elements before one, a glance is sufficient to 

 show what are the relative weights of the elements, 

 and it is as easy to arrange the chemical elements in 

 increasing order of their atomic weights as it is to 

 arrange a series of numbers so that each is less than 

 the one which follows it. 



\Miy is one atom heavier than another ? One 

 simple answer to this question was that given by the 

 chemist Prout. He imagined that all the elements 

 were in some way built up of hydrogen atoms ; con- 

 sequently, the more hydrogen atoms it contained, the 

 heavier was the atom. But if an atom be composed 

 of a number of the lightest (hydrogen) atoms, it must 

 be composed in a \ery special way, for if it is a com- 

 position of the kind usually met with in chemistry, one 



would imagine that in time someone would manage 

 to split it up into simpler parts. This, however, has 

 not been done. An clement is an clement, and cannot, 

 except in a very special manner, as we shall sec later, 

 be split up. Without passing any judgment on this 

 idea of Prout's, let us answer the question by saying 

 that one atom differs in weight from another because 

 it has a different structure. 



Now, the structure of the atom is so new a subject 

 that it will be several years yet before one has a pretty 

 definite theory of just what that structure is. A 

 short account of the principal points of the theory 

 may, however, be given now. It represents in outUne 

 the present ideas. 



The atoms of all the elements arc supposed to have 

 the same general structure. A description of one 

 atom, therefore, answers for a description of all. An 

 atom is supposed to be composed of positive and 

 negative electricity only. The positive electricity is 

 supposed to be at the centre of the atom, forming a 

 core there ; or, as it is called, a nucleus. Surrounding 

 this nucleus are separate particles of negative elec- 

 tricity (called electrons), which are spaced out to 

 occupy the rest of the atom. The important things 

 about the nucleus are : (a) its size, which is ver>' 

 minute compared with the size of the atom ; (b) the 

 fact that " practically the whole " of the mass (or 

 weight) of the atom is concentrated on this tiny nucleus, 

 paradoxical as this may app>ear to be ; and (c) the fact 

 that the nucleus maj- have in it both negative and 

 positive electricitj' " slumf>ed together " ; but it 

 appears to be positive electricity only, because there is 

 always more of the latter than of the former. The 

 negative electricity, on the other hand, is not " slumped 

 together," but exists as separate little entities 

 (electrons), which may re\-olve round the nucleus 

 analogously to the movement of the planets round the 

 sun, or may simply be stationarj-. The size of each of 

 these entities of negative electricity is very small 

 compared with the diameter of the atom, but it is 

 somewhat larger than the uliolc of the positive nucleus. 

 As there is but one nucleus, and as the number of 

 electrons in an atom is never greater than 92 (e.vperi- 

 mcnt proves this),' and as all are very minute compared 

 with the size of an atom, it follows that most of 

 an atom — more than 99-99 per cent, of it — is empty 

 space. Consider an atom so magnified as to appear 

 superficially like Ireland. On that scale the nucleus, 

 situated at Athlone, would be a ball of S feet in radius 

 only. Round that, at different distances from .Athlone 

 to the sea, would be some forty or fifty (on the average) 

 spheres of about the same radius, and llusi: and the 

 tiiiclciis arc the uholc of the atom. 



An atom, then, in some respects resembles the solar 

 system. The solar system, if you conic to think of it, 



