The easiest way to create chemistry formulas is to use the Advanced Editor.
There are two buttons, subscript and superscript, that can be used to appropriately format the atoms and ions.

i.e. PO43−
If you prefer to use the Quick Editor, you can use the following HTML tags: <sub> </sub> and <sup> </sup>.
i.e. PO<sub>4</sub><sup>3−</sup>