The Structure of the Atom. 7 



neighbouring atoms upon it, it must be in the state of a gas at low 

 pressure. 



In 1897, J. J. Thomson found that if high electric forces were 

 applied to a gas at low pressure contained in a glass vessel a new 

 form of matter, which he called the electron, made its appearance. 

 The electron had the following properties : — It was always negatively 

 charged with electricity and the amount ol charge was always the 

 same. Its weight was only one two- thousandth that of the lightest 

 particle previously known, namely, the hydrogen atom. These 

 two properties never varied, whatever the nature of the gas in the 

 tube, or the material of the tube itself. 



Here then was a realization of something underlying the atom. 

 Further research shewed that the electron appeared under more 

 simple conditions. A red-hot wire gives off clouds of electrons. 

 Light falling on a zinc plate releases electrons from its surface. 

 The hot gases from a Bunsen burner are full of the same particles. 

 In all cases the charge and weight of the electrons are the same. 



Numerous attempts to isolate a positively charged electron 

 have failed. Positive electricity seems always to be associated 

 with the atom itself Further, it appears that the number of 

 electrons in the atom is comparatively small, about half the atomic 

 weight, i.e., hydrogen has only one electron and oxygen eight, so 

 that the weight of the atom cannot be due to its electrons. 



We may now anticipate and describe the form of atom which 

 Thomson and Rutherford have suggested, which explains the 

 experimental facts very fully. On their view the atom is composed 

 of a system of electrons rotating about a central nucleus, much as 

 the planets rotate about an immensely heavier sun. This nucleus 

 is very small and carries a positive charge equal to one-half the 

 atomic weight. The mass of the atom is concentrated in the 

 nucleus, and strange though it may appear, when we have small 

 charged particles, the snialler the particle which carries a fixed 

 charge, the more massive will that particle be. The structure of 

 the nucleus is unknown but it is the essential part of the atom. 

 We can detach electrons from the atom by subjecting it to violent 

 collisions, but it will always re-form round its nucleus. Thus the 



