Electrons, protons, and neutrons are three subatomic particles of an atom. If the atom undergoes a change and becomes a positive ion, which subatomic particle is lost in this process?

electron

Explanation:

When a neutral atom lose an electron, it becomes a positively charged ion. That means the atom change when the electron is lost or gained. Hence, if the atom undergoes a change and becomes a positive ion, in this process the lost subatomic particle is electron.