Question

How to find valency of compound

Answer 1

we can find valency like this-

This strategy is based on the octet rule, which tells us that elements and radicals combine so as to produce a stable outer shell of eight electron.
we use this we can find the valency

Answer 2

oxygen atomic no.8
then electronic configuration is 1s2 2s2 2p4
oxygen two valance electron no. is absorb then oxygen valance electron is full field
so oxygen charge 2 negative