The atomic number is the number of proton or number of electron in an atom while the atomic mass is the total mass of the atom. The higher the atomic number, the higher is the mass of atom. The reason is that with the increase in atomic number number of proton and number of electron increases. This enhances the mass of atom because the sum of the total mass of all of the sub-atomic particles increases. That is why the atomic masses increases in the periodic table with the ascending atomic numbers.
