Question

how to find valency of different elements

Answer 1

first find ectronic configuration of element than you will understand that the element is metal or non metal if it is metal than the number of electrons it lose for completing its last shell is its valency and if it is a non metal than the number of electron it need gain completing it last shell is its valency

Answer 2

Valency of an element was defined as the combing power of an element. 1) With respect to hydrogen is the number of hydrogen atoms with which one atom of that element chemically combines. 2)With respect to oxygen is twice the number of oxygen atoms with which one atom of that element chemically combines. For example one atom of Na chemically combines with one atom of H to give NaH. Therefore the valence of Na is 1.