Should you want to know, the atomic number of an atom is the number of protons it contains. The atoms of different elements all have different numbers of protons.
Meanwhile, the mass number of an atom consists of the total number of protons and neutrons it contains. An atom’s mass number can never be smaller than its atomic number, and while it can be the same it is normally larger. Sounds simple enough, right?
We use the relative atomic mass of an element as opposed to its actual mass, because the actual mass is very small. If you’re interested in the nitty gritty of why this is, it’s because the symbol used for relative atomic mass is Ar. So, we can calculate the relative formula mass (Mr) of a compound by adding together the relative atomic masses of the elements which it is formed from.
Still with us? Great! For example, the relative atomic mass of oxygen is 16 and that of hydrogen is one. So the relative formula mass of water (H2O) would be 16 + (1 x 2) = 18.