Explain the term ‘valency’ of an element. How does it vary in a period and in a group in the periodic table?
The chemical properties of elements depend upon the number of electrons in the outermost shell of an atom. These electrons are called valence electrons and thus determines the valency of the atom (or element). In representative elements, the valency is generally equal to either n or (8 – n), where n is no. of. valence electrons in the atom. In a period valence electrons increase from 0 to 8 on moving from left to light. The valency of an element w.r.t. H and Cl increase from 1 to 4 and then decrease to zero. However w.r.t. oxygen, valency increases from 1 to 7 and then becomes zero in noble gases. In a group, the number of valence electrons remains the same, and therefore all elements in a group exhibit the same valency, e.g., all elements of group I have valency one and those of group 2 have valency two. However, the transition elements exhibit variable valency.
