The number of oxygen atoms are 6 atoms
<u><em>Explanation</em></u>
According to the law of mass conservation the number of reactant atoms is equal to number of product atoms.
For given equation to obey the law of mass conservation it need to be balanced as below
4 Al + 3 O₂ → 2 Al₂O₃
Al₂O₃ is the product,therefore the number of oxygen atom
= coefficient in front of Al₂O₃ × subscript of O
=2 x 3 = 6 atoms
