<span>An element is made of only one kind of atom, while a compound contains the atoms of two or more elements. For example, gold is made of only one kind of atom, so it is an element, but water is a mix of hydrogen and oxygen, so it is a compound.</span>
When iron rusts and forms iron oxide, the iron oxide has more mass than the iron because there are more iron atoms in iron oxide than in pure iron.
The process of rust occurs when pure iron is exposed to air and moisture. Rust is the oxidation of pure iron to iron II oxide (Fe2O3).
We can see that there are two iron atoms per mole of Fe2O3 whereas there is only one iron atom in each mole of pure iron.
Therefore, iron oxide has more mass than the iron because there are more iron atoms in iron oxide than in pure iron.
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