Q.The electronic configuration of the elements A and B are 1s² 2s² 2p⁶ 3s² and 1s² 2s² 2p⁵ respectively. The formula of the ionic compound that can be formed between these elements is

An atom's valence is its combining capacity relative to hydrogen, usually equal to its number of valence-shell electrons, or to $8 - (\text{number of valence electrons})$ once the valence shell is more than half full; oxidation state is the more practically useful modern substitute for valence.

Down a group, valence stays constant, since the number of valence electrons stays constant -- Group 1 elements are always valence 1, and Group 15 elements always show valences 3 and 5 (5 valence electrons, either used directly or via the $8-5=3$ rule).

Across a period, valence generally increases, since the number of valence electrons keeps increasing -- period 2 valences run 1 (Li), 2 (Be), 3 (B), 4 (C), then 5 or 3 (N), 6 or 2 (O), 7 or 1 (F), 8 or 0 (Ne), as the $8-n$ rule starts applying alongside the direct electron count.

Many elements additionally show variable valence/oxidation state: most Group 15 elements show both 3 and 5, and transition and inner-transition metals characteristically show several different oxidation states, a consequence of their closely-spaced $ns$ and $(n-1)d$ (or $(n-2)f$) orbital energies.