valence of representative elements is usually (though not necessarily) equal to the number of electrons in the outermost orbitals and/or equal to eight minus the number of outermost electrons as shown below. Nowadays the term oxidation state is frequently used for valence. Consider the two oxygen containing compounds: OF and Na O. The order of electronegativity of the three elements involved in these compounds is F > O > Na.
Each of the atoms of fluorine, with outer electronic configuration s p , shares one electron with oxygen in the OF molecule. Being highest electronegative element, fluorine is given oxidation state – . Since there are two fluorine atoms in this molecule, oxygen with outer electronic configuration s p shares two electrons with fluorine atoms and thereby exhibits oxidation state + . In Na O, oxygen being more electronegative accepts two electrons, one from each of the two sodium atoms and, thus, shows oxidation state – .
On the other hand sodium with electronic configuration s loses one electron to oxygen and is given oxidation state + . Thus, the oxidation state of an element in a particular compound can be defined as the charge acquired by its atom on the basis of electronegative consideration from other atoms in the molecule. Problem . Using the Periodic Table, predict the formulas of compounds which might be formed by the following pairs of elements; (a) silicon and bromine (b) aluminium and sulphur.
(a) Silicon is group element with a valence of ; bromine belongs to the halogen family with a valence of . Hence the formula of the compound formed would be SiBr . (b) Aluminium belongs to group with a valence of ; sulphur belongs to group elements with a valence of . Hence, the formula of the compound formed would be Al S .
Some periodic trends observed in the valence of elements (hydrides and oxides) are shown in Table . . Other such periodic trends which occur in the chemical behaviour of the elements