How to calculate valency electrons ?

Valence electrons are the number of electrons present in the outer most shell of an atom.

Valency is the combining capacity of one atom with another atom to form chemical bonds.  

If you consider Oxygen, it has the atomic number 8, and thus has eight electrons. It has 6 electrons in the outer most shell and requires only 2 electrons to attain stable electronic configuration. It is easier for oxygen to gain 2 electrons rather than loosing 6 electrons. Hence its valency is 2.

The number of valence electrons of elements increases from left to right in the periodic table.

If the no of valence electrons less than or equal to 4; then valency is equal to no. of valence electrons.

If the no of valence electrons more than 4; then valency = 8 – no.of valence electrons.

Group in the periodic table: 1  2  13  14  15  16   17   18

             Valence electrons:  1   2   3    4    5    6    7     8

                           Valency:   1   2   3    4   3,5  2,6  1,7  0,8

This does not include transition metals as they not change the no. of valence electrons.