What is the difference between an atom and a compound?

An atom cannot be broken down any smaller whereas a compound is made up of atoms and can be broken down into smaller pieces (the individual atoms that make it up)
Hope this helps!

A molecule is formed when two or more atoms join together chemically. A compound is a molecule that contains at least two different elements. All compounds are molecules but not all molecules are compound. The smallest bit of each of these substances would be referred to as a molecule.

The mass has escaped into a gas which you can not see so the law stays true.