When writing the chemical formula of a compound
that contains a polyatomic ion, in order to indicate more than one polyatomic
ion in the formula, put parentheses around the polyatomic ion before writing
the subscript.

This does not apply if the subscript is
'1'. When the subscript is '1', there is no need for any parentheses
as it is usually not written.

For example,

Chemical Name

Correct Chemical Formula

Incorrect Chemical Formula

ammonium sulfate

Ions: NH_{4}^{+}, SO_{4}^{2-}

(NH_{4})_{2}SO_{4
}

Not sure how to figure out this formula?
Click to review section 5.8.2.

(NH_{4})_{2}(SO_{4}) There is an extra bracket around the 'SO_{4}'
unit.

(NH_{4})_{2}(SO_{4})_{1}
Subscript of '1' is usually not written.

aluminum sulfate

Ions: Al_{3}^{+}, SO_{4}^{2-}

Al_{2}(SO_{4})_{3}

Not sure how to figure out this formula?
Click to review section 5.8.2.

(Al)_{2}(SO_{4})_{3} There is an extra bracket around 'Al'.

calcium chloride

Ions: Ca^{2+}, Cl^{-}

CaCl_{2}

Not sure how to figure out this formula?
Click to review section 5.8.1.