Question

Chemical formula of carbon di oxide is co2. But valency of carbon is 4 and valency of oxide is 2. So can we write it as c2o4?

Answer 1

take 2 as common, you'll get CO2
in the molecular structure of CO2, it is O=C=O
ie. the oxygen atoms are bonded to carbon via double bonds. so even though the concept of C2O4 is not incorrect don't confuse it with the actual molecular structure.