And that means anions. When they gain electrons, this causes our atom or element, to grow in size. So cations, or the formation of cations, decrease the size of our atom, while the formation of anions increases the size of our atom. Now let's look at one last thing. Now isoelectronic atoms are those atoms that have the same number of electrons, but different number of protons. Suppose we have an atom with ten electrons and ten protons, and an atom with ten electrons and eleven protons.