Chemical formulas use what to tell how many atoms of that element are in one unit of that compound?
This is done by inserting a subscript immediately after the symbol for that element, as in
If there is no subscript after the symbol for an element, you are to assume there is one atom of that type in the formula. If there is more than a single atom, a subscript will be placed after that element. The subscript applies only to the element that is immediately before it.
Brackets are used when a group of atoms (such as a polyatomic ion) appears more than once in the same molecule.
It is useful to use the brackets in this way as it helps to identify the nature of the compound (ionic in this case).