Question

multiple - choice question
when do we need parenthesis in the compound formula?
any time there is a polyatomic ion
when there is a monatomic element
when there is multiple of 1 polyatomic ion

Explanation:

Polytatomic ions are groups of atoms with a charge. Parentheses are used in compound formulas when there is more than one of the same polyatomic ion to indicate the quantity. For example, in $Al_2(CO_3)_3$, the carbonate ($CO_3^{2 - }$) is a polyatomic ion and there are three of them. Just having a polyatomic ion doesn't always mean parentheses are needed (e.g., $NaNO_3$ has the nitrate polyatomic ion but no parentheses). Monatomic elements don't relate to the use of parentheses for polyatomic - ion quantity indication.

Answer:

When there is multiple of 1 polyatomic ion