( ) NO + ( )O2 ------------------> ( 2)NO2
Coefficients appear to the left of the molecules
but before we have to balance the reaction.
On the right you have 1 atom of Nitrogen and 2 atoms of Oxygen = NO2
On the left side of the reation you have 1 of N and 3 of O, so we have to balance.
First look the N atoms
N -----------------------------> N
Both sides have 1 atom.
Now the problem is the Oxygen:
2 NO + 1 O2 -------------------------------------> 2 NO2
We put 2 on the right so now we have 2 N and 4 O and then on the left you have to put a 2 on the left of NO.
Now if you count the reaction is balanced.
Left: 2x1 N = 2 and 2 x 1 O = 2 (2 NO),
then 1 x 2 O = 2 (1 O2)
In left we have a total of 2 Nitrogen and 4 Oxygen
Right of the reaction:
2 x 1 N = 2
2 x 2 O = 4
Now the answer is 2,1,2