When must parentheses be used in chemical formulas?

1 Answer
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 #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.