578 



SCIENCE 



[N. S. Vol. L. No. 1304 



by the statement that the number of positive 

 charges on the nucleus is equal to the number 

 of negative electrons, since it is known that 

 the hydrogen ion carries a positive charge 

 equal to the negative charge on the negative 

 electron. 



The atom is similar to the solar system in 

 a second sense, for the planetary electrons are, 

 relative to their size, about as far from each 

 other and from the nucleus, as the planets 

 and the sun. Thus it need not be surprising 

 from this point of view, that Rutherford has 

 found that the very minute nucleus of a 

 heliimi atom, often called the alpha particle, 

 may be shot directly through many thousands 

 of other atoms, without hitting a single 

 nucleus or a single negative electron, just as 

 a planet like the earth might be shot through 

 thousands of solar systems like our own with- 

 out hitting a single sun or planet. 



The atom is like the solar system too, in 

 that the nucleus, like the sun, possesses nearly 

 the entire mass of the system, since in general 

 the nucleus is more than two thousand times 

 heavier than all of the electrons which sur- 

 round it. While the atom is so small that a 

 row of fifty million atoms would be only 

 about one inch long, if they were put as 

 closely together as they are in solids; and so 

 small too, that there are 180 thousand billion 

 billion atoms of carbon in one cubic centi- 

 meter of diamond; the atom is so large com- 

 pared with the electron, that, if we take the 

 dimensions usually accepted for the electron, 

 there would be space in a single atom suffi- 

 cient to contain ten million billion electrons, 

 while the atom which contains the greatest 

 number of planetary electrons, uranium, ac- 

 tually has only 92 of these. According to the 

 work of Rutherford, Geiger, Darwin and 

 Marsden, the nucleus of even the heaviest 

 atom, is not very much larger than a nega- 

 tive electron. Thus the atom may be said 

 to be very sparsely populated with electrons. 

 The atom is unlihe the solar system in that 

 the planetary electrons are arranged more 

 or less symmetrically in space around the 

 nucleus, at least that is the idea expressed 

 in papers by the American chemists, Parsons, 

 Harkins, Lewis and Langmuir, the last two 



having paid the most attention to the details 

 of the arrangement. Also, while the solar 

 system is held together by the gravitational 

 attraction between the large mass in the sun 

 and the smaller masses in the planets, the 

 atoms are held together by the positive elec- 

 trical charges in the nucleus and the negative 

 charges of the electrons, together with what- 

 ever magnetic effects are produced by the 

 rotation of the electrons. 



THE BUILDING OF ATOMS 



While chemists have not as yet synthesized 

 any atoms, it is also true that they have only 

 recently begun the study of their structure. 

 l^ow, when a chemist wishes to build up even 

 such a simple thing as an organic molecule, 

 he first studies its structure, and often many 

 years intervene between the working out of 

 the structure of the molecule and its first 

 synthesis. In the synthesis of an organic dye 

 there may be two steps which we may have to 

 consider. Suppose for example that the first 

 of these consists of a complex set of reactions 

 which are very difficult to carry out, while the 

 second stej) will occur by itself if the inter- 

 mediate product is only left standing in the 

 air. It is evident that the practical chemist 

 will need to put almost his whole attention on 

 the first step of the synthesis. The building 

 of atoms is similar in that the first step, the 

 building of the nucleus of an atom, has not 

 yet been accomplished, while, if the nucleus 

 were once built, it would of itself pick up the 

 whole system of planetary electrons which 

 would turn it into a complete atom. For ex- 

 ample in the disintegration of the nuclei of 

 certain radioactive atoms, alpha particles 

 which are the nuclei of helium atoms, are 

 shot out as rapidly as twenty thousand miles 

 per second, so rapidly that they pass through 

 as many as half a million other atoms before 

 coming to comparative rest. 'Now Rutherford 

 has proved that when these nuclei finally slow 

 down, they give the ordinary spectrum of 

 helium, which indicates that each alpha 

 particle has picked up the two negative 

 electrons which are essential to convert it 

 into a complete helium atom. 



