Question 13
When are brackets used in a chemical formula?
The right answer is the one marked above.
When more than one polyatomic ion is included in the formula of a compound.
Compare two formulae to see it. Sodium nitrate is NaNO3 — one nitrate ion, so no brackets are needed and nothing is ambiguous. Magnesium nitrate is Mg(NO3)2 — two nitrate ions, and the brackets are what make the 2 apply to the whole NO3 group rather than to the oxygen alone.