Question

how can i calculate valency

Answer 1

Learn periodic table till 50 elements and then according to their position number divide them in the electron shells Like for K 2 electrons and for rest al 8 electron. and if the last shell is not filled with the required number then the electrons which are needed to full it will be your valence electron or valency
Ex- Oxygen=2,6
so valency will be 2 becoz it need 2 electron to fulfill it's shell