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Anions are negatively charged ions and cations are positively charged ions. The difference in the masses has to do with how the ions are made. Cations are formed by the loss of electrons. Since the atom is losing particles (electrons), the resulting ionic mass is lower. Anions are formed by gaining extra electrons. Since the atom is gaining particles (electrons), the resulting ionic mass is higher. This is why positive ions are smaller than their parent atoms and negative ions are larger than their parent atoms. It should be noted, however, that the majority of the mass of any atom is contained in the nucleus, so the mass differences in the ions is relatively very small even on the atomic scale.
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