Question

1. how do we calculate the valency of an element from its electronic configuration?
2. how does the valency vary in a period?

Answer 1

1) Valency of an element is simply the number of electrons that need to be added, or removed from an atom so that it achieves its octet, which is a stable state. (Octet means eight electrons in the outermost shell)
Thus valency can be found out from the number of Valence electrons, which are the electrons present in the outermost shell.

Suppose an atom has 13 electrons. Thus that atom has 3 valence electrons. For it to complete the octet, 5 electrons have to be added; or 3 electrons need to be subtracted so that it can get perfectly eight electrons in the outermost shell.
Valency can also negative as well as positive. If the atom receives electrons then it is negative and vice versa.

2) Valency is a period first increases from 1 to 4, and then decreases from 4 to 0. (All inert gases have valency as 0)

For example in period number 2,
Li (1) Be (2) B (3) C (4) N (3) O (2) F (1) Ne (0)
Valencies of the elements are given in the brackets.

Answer 2

valency is the no of electrons lost or gained during an electron transfer
Generally in when complete electron transfer takes place the maximum number of electrons an atom can take is 3
So valency of all elements except Group 14 elements exhibit valency in the range of -3 to +3
Group 14 elements having 4 electrons in valence shell do not form compounds by electron transfer but by sharing of electrons

By gaining or losing electrons an atom tries to attain the stable electronic configuration of nearest noble gas
so
Group 1 exhibits +1
.....
Group 3 exhibits +3
......
Group 15 exhibits -3
Group 16 exhibits -2
Group 17 exhibits -1
Group 18 exhibits 0


In case of D block and F block elements concept of variable valency comes