Give a brief account of valence ?

Valence. The electrons present in the outermost shell (or valence shell) of the elements are called valence electrons and they normally represent the valency of the element.
In the case of representative elements (s-and p-block elements), valency is either equal to the number of valence electrons or eight minus the number of valence electrons. For example
In HCl, the valency of H =1
In CaCI₂. the valency of Ca = 2 (valence electrons = 2)
In NH₃, the valency of N = 3 {8-5} (valence electrons = 5)
Variation along a period: The number of electrons in the valence shell increases from one to eight as we move from left to right in the periodic table. The valency with respect to hydrogen initially increases from 1 to 4 and then decreases from 4 to 0.
LiH  BeH₂  BH₃  CH₄   NH₃   H₂O  HF   Ne
1      2       3      4      3      2      1     0

In a similar manner, the valency with respect to chlorine also initially increases from 1 to 4 and then decreases from 4 to 0.
Variation in a group: The valency of the element depends on upon its valence shell configuration. Since in a group, the valence shell configuration of the elements remains almost the same, they exhibit a common valency. For example, all the elements of group 1 have valency one and those of group 2 have valency two.