When to use brackets
Brackets around polyatomic ions.
Is it AlOH₃ or Al(OH)₃?
Aluminium ions are Al³⁺ and hydroxide ions are OH⁻, so three hydroxide ions go with each aluminium ion.
Which way do you think the formula should be written?
What this lesson covers
The idea
When a formula has two or more polyatomic ions of the same type, the ion is put in brackets with the subscript outside; no brackets are needed for only one polyatomic ion.
Is it AlOH₃ or Al(OH)₃?
Aluminium ions are Al³⁺ and hydroxide ions are OH⁻, so three hydroxide ions go with each aluminium ion.
Which way do you think the formula should be written?
- Al(OH)₃
- AlOH₃
- Al₃OH
Brackets or not?
Sort each compound by whether its formula needs brackets.
A bracket groups an ion
The bracket keeps the atoms of one polyatomic ion together and the subscript outside the bracket tells how many such ions there are. With only one such ion there is nothing to count, so no brackets are needed.
When a formula has two or more polyatomic ions of the same type, the ion is put in brackets with the subscript outside. No brackets are needed for only one polyatomic ion.
Notes
Two or more of the same polyatomic ion: put it in brackets with the subscript outside, as in Al(OH)₃. For only one, no brackets: CaCO₃.
Check yourself
What does the formula Al(OH)₃ tell us?
Mg has charge 2+ and OH has charge 1−. What is the formula of magnesium hydroxide?
How many sulfate ions are there in Al₂(SO₄)₃?
Answer: 3
The subscript 3 outside the bracket shows three sulfate ions.
Why does CaCO₃ have no brackets?
- One hydroxide ion is bound to three aluminium ions. The subscript 3 outside the bracket counts the hydroxide ions.
- Three aluminium ions are bound to one hydroxide ion. The subscript 3 outside the bracket counts the hydroxide ions, and there is one aluminium ion.
- Three hydroxide ions are bound to one aluminium ion — correct. Yes!
- Mg(OH)₂ — correct. Yes!
- MgOH₂. Without brackets, the 2 would count only the hydrogen atoms. The brackets show two hydroxide ions.
- Mg₂OH. The charge of OH, 1, becomes the subscript of Mg and is not written. The charge of Mg, 2, becomes the subscript of OH.
- Carbonate is not a polyatomic ion. CO₃²⁻ is a polyatomic ion, made of carbon and oxygen. It needs no brackets here because only one is present.
- It has only one carbonate ion — correct. Yes!
- Calcium cannot form brackets. The rule depends on the number of polyatomic ions of the same type, not on the metal.