When an atom loses or gains electrons to form an ion, it does indeed experience a change in its electrical charge, but this process does not significantly alter the mass of the atom.
The mass of an atom is primarily determined by the sum of the masses of its protons, neutrons, and electrons. Protons and neutrons reside in the atom's nucleus and account for the majority of its mass, while electrons are significantly lighter and orbit around the nucleus in electron shells.
When an atom loses or gains electrons to become an ion, the number of protons and neutrons in the nucleus remains the same, as electrons do not contribute significantly to the overall mass of an atom. The mass of an electron is about 1/1836th the mass of a proton or neutron, so the change in mass resulting from electron loss or gain is negligible.
For example, let's consider the formation of a cation, where an atom loses one or more electrons. The loss of electrons reduces the negative charge of the atom, resulting in a positively charged ion. However, the number of protons and neutrons in the nucleus remains unchanged, meaning the mass of the nucleus remains the same.
In summary, losing or gaining electrons does not significantly affect the mass of an atom because electrons are much lighter than protons and neutrons, which are the primary contributors to an atom's mass.