Using brackets in chemical formulae

Elements and simple compounds

First off, elements and simple compounds never need brackets.

C is carbon. O₂ is oxygen. NO₂ is nitrogen dioxide. CH₄ is methane. Al₂O₃ is aluminium oxide.

No brackets. That’s it. Simple!

Compounds formed from complex ions

Brackets are only needed for compounds formed from complex ions. But what is a complex ion? Well, it’s an ion that is made up of more than one element.

The most common examples of complex ions are:

• Hydroxide  OH^–
• Sulfate  SO₄^2-
• Nitrate  NO₃^–
• Carbonate  CO₃^2-
• Ammonium  NH₄⁺

You’re expected to know the formulae of these complex ions (and their charges) for the exam.

If there’s only one complex ion, then brackets are not needed

Examples:  NaOH     MgSO₄     NH₄     NO₃     CaCO₃

If there’s more than one complex ion in the compound, then brackets are needed

The reason is that you need to put a bracket around the complex ion to show how many of those whole complex ions there are in the compound.

Examples:  Mg(OH)₂     Al₂(SO₄)₃     (NH₄)₂SO₄

That’s it for brackets! Except that if you’re not sure how to decide whether there’s only one ion in a compound, or two or three, then have a look here: Top tips for writing chemical formulae where you’ll learn about balancing the charges of positive and negative ions.

Best of luck!

Ligaya Batten