Most of the mass of an atom is concentrated in the nucleus, even though the nucleus is tiny. This is because the nucleus contains the protons and neutrons, and each of these has a relative mass of 1.
Electrons have a very small mass, about one two-thousandth of a proton, so they add almost nothing to the mass of an atom. The total mass of the electrons in an atom is so small that it can usually be ignored.
For example, a carbon atom has 6 protons, 6 neutrons and 6 electrons. The protons and neutrons in its nucleus give a relative mass of 12, while the 6 electrons add almost nothing.