Question

When must parentheses be used in chemical formulas?

Answer 1

When you want to express a substance that has polyatomic ion or functional group (in organic chemistry).

Explanation:

There are two cases: polyatomic ions and polyatomic functional group.

[1] For example, Ammonium Chrolide `NH_4Cl` needs no parentheses because there is single `NH_4` ion in the formula.
Ammonium Sulfate should be expressed as `(NH_4)_2SO_4`, as two ammonium ions and one sulfate ion are needed to balance the electron.

It must not be `N_2H_8SO_4`. People would wonder what `N_2H_8^(2+)` ion is...

[2] Another example can be seen in organic chemistry.
Xylene molecule has two methyl group on the benzene ring. It is expressed as `C_6H_4(CH_3)_2`
Ethylbenzene molecule has one ethyl group on the benzene ring and its condensed formula is `C_6H_5C_2H_5`.
Both xylene and ethylbenzene have `C_8H_10` formula but their structures are different.