Question

Why don't equal masses of carbon atoms and oxygen molecules contain the same number of particles?

Answer 1

This is because of their size difference.

Explanation:

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