2l6 



PHYSICAL SCIENCE 



the central force is now supposed to be connected 

 with a minute positively electrified nucleus and 

 not with a sphere of positive electricity as large 

 as the atom. 



Thomson thus accepted the nuclear atom, and 

 revised his theory in its terms. He imagines 

 that within a given distance from the nucleus its 

 attraction for an electron changes to a repulsion. 

 At this critical distance a single electron will 

 rest in equilibrium. Two electrons will rest on 

 opposite sides of the nucleus, three at the corners 

 of an equilateral triangle, four at the corners of 

 a regular tetrahedron, and so on up to eight. 

 With nine electrons, eight will form an inner 

 shell round the nucleus, and one will stay outside, 

 further from the centre of the atom. This outer 

 layer increases regularly in number as we pass 

 to heavier atoms and more electrons are added, 

 till it too contains eight electrons. 



Now, while radio-activity is an affair of the 

 nucleus, the chemical properties of the atom 

 depend on the outer electrons. As we shall see 

 below, the atoms of hydrogen and lithium have each 

 one electron, and the number will rise as we pass 

 up Mendeleeff's Periodic Table, as shown in the 

 following list of a few of the lighter elements : — 



