In a negative ion, or anion, electron(s) have been added to the atom. This means the like charges within the electron cloud are now repelling each other more than they were before. The proton count, however, has not changed. So, the electron cloud, in response to the added electron(s) expands with that extra repulsion. the negative Ions will have gained an electron fill an electron shell while a positive Ion will have lost an electron in order to shed its outermost shell, thereby making the atom smaller.
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