When elements occur as ions, they are either a cation or an anion. A cation is a positively charged ion, while an anion is a negatively charged ion. When a cation and an anion combine to form a stable and neutral compound, the superscripts cross-multiple and turn into subscripts. Since, the superscripts are all - or -1, then the subscripts would also be 1. Therefore, there is only 1 anion in each formula for Na combined with any of the anions given.
