# When must parentheses be used in chemical formulas?

Oct 5, 2017

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 $N {H}_{4} C l$ needs no parentheses because there is single $N {H}_{4}$ ion in the formula.
Ammonium Sulfate should be expressed as ${\left(N {H}_{4}\right)}_{2} S {O}_{4}$, as two ammonium ions and one sulfate ion are needed to balance the electron.

It must not be ${N}_{2} {H}_{8} S {O}_{4}$. People would wonder what ${N}_{2} {H}_{8}^{2 +}$ 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 ${C}_{6} {H}_{4} {\left(C {H}_{3}\right)}_{2}$
Ethylbenzene molecule has one ethyl group on the benzene ring and its condensed formula is ${C}_{6} {H}_{5} {C}_{2} {H}_{5}$.
Both xylene and ethylbenzene have ${C}_{8} {H}_{10}$ formula but their structures are different.