Question

how can you find the valency of an element

Answer 1

by cross criss method
eg: valency of copper in co2
so valency of copper is 2 and oxygen is 1.

Answer 2

An element's valency is determined by the number of electrons in its outer shell. Hence, the number of valence electrons obtained from the electronic configuration of the element gives the valency i.e. the number of electrons lost, gained or shared by the element to attain the noble gas configuration....