Finding the oxidation number.....?

Question

Hey, can you set it up like an algabraic equation and set it equal to zero? For H (subscript 2)CO.
H2 +C? + O = 0
1+(2) + C? + (2-) = 0
(2+) + C? + (2-) = 0
0 + C? =0
C?=0
so the charge for Carbon would be 0?
Does this make sense or has Chemistry driven me crazy?
thanks

Answer 1

nope.
you need the products of the reaction to apply the law of conservation of charges.
hope that helps.

Answer 2

All non combined elements have oxidation number = 0
Also molecules formed by egual atoms such H2, Cl2, F2, ...