48 SCIENCE PROGRESS 



crystal structure, has made it possible to obtain a clearer idea 

 of the relative positions of the atoms in the chemical molecule. 

 The crystal structure is composed of atoms, or groups of atoms, 

 arranged in a definite pattern, the unit of which is repeated at 

 regular intervals in three dimensions. By means of X-ray 

 analysis, the dimensions of the pattern, and the relative posi- 

 tions of the individual atoms, can all be measured with a high 

 degree of accuracy. We can find the distance between any given 

 atom and the neighbouring atoms, and so get an estimate from 

 a new source of the size of the atomic structure. 



In order to realise the significance of the atomic arrange- 

 ments in crystals, it is necessary to consider them in the light 

 of recent theories of atomic and molecular structure. Ruther- 

 ford has shown that the positively charged part of the atom 

 consists of a nucleus, whose dimensions are small compared with 

 those of the atom, and with which practically the whole of the 

 mass of the atom is associated. The positive charge on the 

 nucleus is an integral multiple of the unit of electrical charge, 

 the negative counterpart of which is the charge borne by the 

 electron. The place of an element in the periodic series is 

 determined by the charge on the atomic nucleus, the hydrogen 

 nucleus having a single positive charge, the helium nucleus a 

 double positive charge, and so on through the periodic table to 

 the element uranium, whose atom has a nuclear charge of 92 

 positive units. The number of electrons surrounding the 

 nucleus of the uncharged atom is equal to the number of positive 

 units of charge on the nucleus, and so is identical with the 

 " Atomic Number " of the element, a number the importance 

 of which was revealed in so striking a manner by Moseley's 

 work on X-ray spectra. In passing from one element to the 

 next in the periodic table, one unit is added to the charge on 

 the nucleus and one electron to those in the atomic structure. 



The arrangement of electrons in the atom of an inert gas 

 may be supposed to be an exceedingly stable one, since these 

 atoms do not enter into chemical combination with other ele- 

 ments. The chemical behaviour of the three successive elements 

 such as chlorine (atomic number 17), argon (18), and potassium 

 (19), may be understood by assuming this stability of structure 

 in the case of an inert gas. The potassium atom has one more 

 electron than the stable argon atom, and in consequence the 

 additional electron is easily detached from the atom, which then 

 becomes a singly charged positive ion. The chlorine atom has 

 one electron less than the number in the argon atom, and there 

 is a tendency for an additional electron to be absorbed into the 

 structure, forming an anion with unit negative charge. The 

 calcium atom (20) tends to lose two electrons, the sulphur 

 atom (16) to gain two, and so for other elements near one of the 



