Question

Why does The mass of an atom is not dependent on the number of electrons in an atom.

Answer 1

This is really an extended comment to Geoffrey's answer, so please upvote Geoffrey's answer rather than this.

The mass of a hydrogen atom is 1.67353270×10−27 kg. If you add the masses of a proton and electron together then they come to 1.67353272×10−27 kg. The difference is about 13.6eV, which is the ionization energy of hydrogen (though note that the experimental error in the masses isn't much less than the difference so this is only approximate).

This shouldn't surprise you because you have to add energy (in the form of a 13.6eV photon) to dissociate a hydrogen atom into a free proton and electron, and this increases the mass in accordance with Einstein's famous equation

Answer 2

The mass of the atom, its atomic mass, depends upon the sum of the number of protons and the number of neutrons present in the nucleus. Since the properties of the electrons depend upon the number of protons in the nucleus of an atom, each different atomic number corresponds to a qualitatively different kind of atom.