Question

How can We know the valency of any ion ?

Answer 1

it have some vancy of all elements and we have to learn them the we will understand

Answer 2

Valency can be known by the number of electron in last shell.
If the electrons are less than 4 it is itself the valency of that element.
But if the number of electrons exceeds 4 then we have a formula. 8- number of electrons.

Hope this helps.