Writing chemical formulae

Follow these simple steps to write chemical formulas for ionic compounds.

1. Involving simple ions
2. Involving polyatomic ions
3. When to use parentheses

Involving simple ions

Write the chemical formula involving calcium and nitrogen.

Step 1

• Write the symbols (including charge) of the cation and anion, with the cation listed first.
  • Calcium, Ca, belongs to Group IIA. It likes to form the Ca⁺² ion.
  • Nitrogen, N, belongs to Group VA. It likes to form the N^-3 ion.

From step 1: Ca⁺² N^-3

Step 2

• Determine the smallest number of each kind of ion needed to give an overall charge of zero.
  • Drop the signs on the charges and determine the lowest common multiple (LCM) between "2" and "3". For Ca⁺² and N^-3, the LCM is 2 x 3 = 6.
  • To determine the number of ions needed, LCM / charge of ion = number of ions

From step 2:
Number of Ca⁺² ions: 6/2 = 3 Ca⁺² ions
Number of N^-3 ions: 6/3 = 2 N^-3 ions

Step 3 Write the chemical formula, using the appropriate subscripts to make the compound neutral in charge.

• Use subscripts to show the number of Ca⁺² ions and N-^-3ions in the formula.

From step 3: Ca₃N₂

Involving polyatomic ions

Write the chemical formula involving strontium and nitrate.

Step 1

• Write the symbols (including charge) of the cation and anion, with the cation listed first.
  • Strontium, Sr, belongs to Group IIA. It likes to form the Sr⁺² ion.
  • Nitrate ion is a polyatomic ion. It has the formula, NO₃^-.

From step 1: Sr⁺² NO₃^-

Step 2

• Determine the smallest number of each kind of ion needed to give an overall charge of zero.
  • Drop the signs on the charges and determine the lowest common multiple (LCM) between "2" and "1". For Sr⁺² and NO₃^-, the LCM is 2 x 1 = 2.
  • To determine the number of ions needed, LCM / charge of ion = number of ions

From step 2:
Number of Sr⁺² ions: 2/2 = 1 Sr⁺² ions
Number of NO₃^- ions: 2/1 = 2 NO₃^- ions

Step 3 Write the chemical formula, using the appropriate subscripts to make the compound neutral in charge.

• Use subscripts to show the number of Sr⁺² ions and NO₃^- ions in the formula. Showing 1 Sr+2ion is easy. But showing 2 NO₃^- ions will involve putting a bracket around NO₃^- and a subscript 2 outside the bracket, (NO₃)₂, to represent that we need to units of NO₃^- in the chemical formula.

From step 3: Sr(NO₃)₂

When to use parentheses

In general, 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 one. When the subscript is '1', there is no need for any parentheses as it is usually not written.

Chemical name	Correct chemical formula	Incorrect chemical formula
ammonium sulfate Ions: NH4+, SO42-	(NH4)2SO4	(NH4)2(SO4) There is an extra bracket around the 'SO4' unit. (NH4)2(SO4)1 Subscript of '1' is usually not written.
aluminum sulfate Ions: Al3+, SO42-	Al2(SO4)3	(Al)2(SO4)3 There is an extra bracket around Al.
calcium chloride Ions: Ca2+, Cl-	CaCl2	Ca(Cl)2 There is an extra bracket around Cl.

Content suitability

BCIT courses: CHEM 0011