Question

what is valency?? explain by taking example of oxygen and carbon.

Answer 1

Valency is the combining capacity of an atom. (or)
It is the number of electrons an atom can gain or lose to become stable or to reach to the state of octet.

for example:-
oxygen: it's electronic configuration is 2 6
so it's gains 2 electrons so it's valency is 2.

carbon: it's electronic configuration is 2 4
so it shares 4 electrons so it's valency is 4.

hope it helps...

Answer 2

Valency of an element is the number of electrons it can lose or gain to attain stability. It is also defined as the combining strength or capability of an element. 

Atomic number of oxygen = 8
Therefore, electronic configuration = 2,6
Hence, Oxygen will gain 2 electrons to attain stability and satisfy the octet rule.
Its valency would thus be -2

Atomic number of carbon = 6
Electronic configuration = 2,4
Hence, it will share 4 electrons to obtain stability by satisfying the octet rule.
Its valency would thus be 4