If a piece of metal, such as calcium or magnesium, reacts with atmospheric oxygen, would you expect the product to have a greater or lesser mass that the reacting metal? Why?
Should you not tell us this?
A metal is oxidized; i.e. it combines with atmospheric oxygen:
And certainly mass is conserved in every chemical reaction.
If mass is conserved, what should be the mass of the oxide with respect to the starting metal?