Relative mass

“Relative” means that the masses are being compared on a convenient scale: a proton and a neutron each have a relative mass of approximately 1. An electron’s relative mass is so small that it is often treated as zero when accounting for the mass of an atom. Consequently, almost all the mass of an atom is concentrated in its nucleus, even though the nucleus occupies only a minute fraction of the atom’s volume.

To compare these particles conveniently, chemists use relative charge and relative mass. These are comparisons, so they do not need units.