Question

What is the valency of N O and C

Answer 1

Valency Of N =3
C=4
O=2

Answer 2

O - oxygen - Atomic no. - 8 Electronic configuration - 2, 6
So, valence electrons = 6
and 
as it is easier to obtain two electrons compared to giving six electrons to complete the octet or duplet, thats why its valency will be 2 

N - nitrogen - Atomic no. - 7 Electronic
Configuration - 2, 5
The valency of Nitogen will be 3 due to the same reason that Oxygen will have 2

C - Carbon - Atomic no - 6 Electronic Configuration - 2, 4
Carbon has 4 valence atoms, so it can take the electrons as well as donate the electrons which make Carbon a very versatile atom and that is why its configuration is 4 .