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 than the reacting metal?

A piece of metal reacting with atmospheric oxygen will have a greater mass than the reacting metal. This is because this type of reaction will add the atmospheric oxygen to the metal, thus, making the result heavier. Basically, adding something (even something as light as oxygen) to a metal will make it heavier.