COLLOIDS AND ADSORPTION 31 



same electron, and the two ionised atoms are held 

 together in sodium chloride by their opposite charges. 

 In the case of non-ionised compounds, such as the majority 

 of organic compounds, the bond is formed by the sharing 

 of a pair of electrons between the atoms, one electron 

 of each pair being supplied by each atom to give a co- 

 valent bond : — 



H 



•C- + 4 -H > H:C:H. 



By this sharing of electrons the carbon atom acquires, 

 in effect, the stable octet structure and the hydrogen 

 atoms have two electrons each, as in the inert gas, 

 helium. A double bond is formed by the sharing of two 

 pairs of electrons and a triple bond by the sharing of three 

 pairs. Since there is no actual transfer of electrons, the 

 molecules are neutral and uncharged ; but one of the 

 atoms may have a stronger pull on the electron pair than 

 its neighbour, and accordingly the electrons will be 

 displaced to some extent from the equilibrium position 

 so that the atom to which they are more strongly attracted 

 will have a relatively greater negative charge than the 

 atom from which they tend to be pulled away. As a 

 result, a group of atoms in w^hich this occurs wdll act as if 

 it were a minute magnet with two poles, e.g. 



+ 0: 



Such groups are known as polar groups. The most 

 commonly occurring of such polar groups are those which 

 involve oxygen, especially doubly linked oxygen, nitrogen 

 or halogens. The polar strength of the groups varies 

 considerably from group to group, but is approximately 

 constant for any one group. As examples of such groups, 

 in order of their strength, may be mentioned 



