Why don't equal masses of carbon atoms and oxygen molecules contain the same number of particles?
This is because of their size difference.
Carbon has bigger atomic radius than oxygen so if i take 1 gram of both the substances, oxygen will have more particles than carbon in it.
You can understand it in this way:
Imagine you have to take 1 spoon of beads from two bowls.
a) have small radius and another b) little bigger radius . so while taking it you'll realize that from bowl 'a' you get more beads and from bowl 'b' you have less.
hope it helps.good luck