A and Shave identical size and same mass. A becomes ${{A}^{2+}}$ and B becomes ${{B}^{2-}}$. Will ${{A}^{2+}}$ and ${{B}^{2-}}$ still have the same mass? Why?
No, they will not have same mass. ${{B}^{2-}}$ has more mass as, it has gained two electrons, whereas ${{A}^{2+}}$ has lost two electrons.