A regular, repeating three-dimensional arrangement of ions.

Why is the formula of an ionic compound not a molecular formula?

There are no molecules; it shows the simplest ratio of ions in the giant lattice.

When do you need brackets in an ionic formula?

When more than one of a compound ion is needed, e.g. Ba(OH)₂.

How do you check an ionic formula?

The total positive charge equals the total negative charge.

Ionic bondingAQA A-Level Chemistry: Flashcards

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Define ionic bonding.

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Define ionic bonding.: The electrostatic attraction between oppositely charged ions in a lattice.
What is a lattice?: A regular, repeating three-dimensional arrangement of ions.
Why is the formula of an ionic compound not a molecular formula?: There are no molecules; it shows the simplest ratio of ions in the giant lattice.
Charge on ions of Groups 1, 2 and 3?: 1+, 2+ and 3+.
Charge on ions of Groups 5, 6 and 7?: 3−, 2− and 1−.
Formula and charge of the ammonium ion?: NH₄⁺
Formula and charge of the sulfate ion?: SO₄²⁻
Formula and charge of the carbonate ion?: CO₃²⁻
Formula and charge of the nitrate and hydroxide ions?: NO₃⁻ and OH⁻
Formula of aluminium sulfate?: Al₂(SO₄)₃
Formula of calcium nitrate?: Ca(NO₃)₂
When do you need brackets in an ionic formula?: When more than one of a compound ion is needed, e.g. Ba(OH)₂.
How do you check an ionic formula?: The total positive charge equals the total negative charge.

Exam questions on Ionic bonding

Magnesium oxide is used to line industrial furnaces because it has a very high melting point. Aluminium sulfate is used to treat drinking water.
Describe the bonding in magnesium oxide.2 marks
A garden fertiliser is a mixture of ammonium nitrate and ammonium sulfate. The manufacturer lists the ingredients using their formulas.
Explain why the formula of ammonium sulfate contains two ammonium ions for every sulfate ion.2 marks
A geologist records three minerals in a rock sample: bauxite, which contains aluminium oxide; limestone, which contains calcium carbonate; and a salt deposit containing calcium nitrate.
Use the position of the elements in the Periodic Table to predict the charge on the ions in aluminium oxide, and construct its formula.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).