How are cations and anions formed?

Cations form when atoms lose electrons (metals); anions form when atoms gain electrons (non-metals).

What ions form from Group 2 and Group 6 atoms?

Group 2 form 2+ ions (e.g. Mg²⁺); Group 6 form 2− ions (e.g. O²⁻).

What does a dot-and-cross diagram of an ionic compound show?

Only outer-shell electrons, with each ion in square brackets and its charge at the top right.

What outer electrons does [Cl]⁻ show in a dot-and-cross diagram?

Eight: seven from the chlorine atom and one transferred from the metal.

Why does MgO have a higher melting temperature than NaCl?

Mg²⁺ and O²⁻ have greater charges and are smaller, so the electrostatic attraction is stronger and more energy is needed.

How does ionic radius change down a group?

It increases, because each ion has an extra electron shell.

What are isoelectronic ions?

Ions with the same number of electrons and the same electronic configuration, e.g. N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺.

Why does ionic radius decrease from N³⁻ to Al³⁺?

Same number of electrons, but more protons, so the nuclear attraction on the electrons is stronger.

Why are ionic compounds brittle?

A force moves a layer of ions so like charges line up; they repel and the crystal splits.

Why do ionic solids not conduct electricity but molten ones do?

In the solid the ions are fixed in the lattice; when molten the ions are free to move and carry charge.

What is the evidence for ions from filter paper experiments?

Coloured ions migrate to the electrode of opposite charge, e.g. blue Cu²⁺ moves to the negative electrode and purple MnO₄⁻ to the positive.

Is a cation larger or smaller than its atom?

Smaller, because it has lost its outer shell and the same protons attract fewer electrons.

Ionic bonding and ionic compoundsEdexcel A-Level Chemistry: Flashcards

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Define ionic bonding.

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Define ionic bonding.: The strong electrostatic attraction between oppositely charged ions.
How are cations and anions formed?: Cations form when atoms lose electrons (metals); anions form when atoms gain electrons (non-metals).
What ions form from Group 2 and Group 6 atoms?: Group 2 form 2+ ions (e.g. Mg²⁺); Group 6 form 2− ions (e.g. O²⁻).
What does a dot-and-cross diagram of an ionic compound show?: Only outer-shell electrons, with each ion in square brackets and its charge at the top right.
What outer electrons does [Cl]⁻ show in a dot-and-cross diagram?: Eight: seven from the chlorine atom and one transferred from the metal.
Why does MgO have a higher melting temperature than NaCl?: Mg²⁺ and O²⁻ have greater charges and are smaller, so the electrostatic attraction is stronger and more energy is needed.
How does ionic radius change down a group?: It increases, because each ion has an extra electron shell.
What are isoelectronic ions?: Ions with the same number of electrons and the same electronic configuration, e.g. N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺.
Why does ionic radius decrease from N³⁻ to Al³⁺?: Same number of electrons, but more protons, so the nuclear attraction on the electrons is stronger.
Why are ionic compounds brittle?: A force moves a layer of ions so like charges line up; they repel and the crystal splits.
Why do ionic solids not conduct electricity but molten ones do?: In the solid the ions are fixed in the lattice; when molten the ions are free to move and carry charge.
What is the evidence for ions from filter paper experiments?: Coloured ions migrate to the electrode of opposite charge, e.g. blue Cu²⁺ moves to the negative electrode and purple MnO₄⁻ to the positive.
Is a cation larger or smaller than its atom?: Smaller, because it has lost its outer shell and the same protons attract fewer electrons.

Exam questions on Ionic bonding and ionic compounds

Magnesium oxide is used to line industrial furnaces because it stays solid at temperatures above 2800 °C. Sodium chloride, by comparison, melts at about 800 °C. Both compounds are solids made of giant ionic lattices.
Explain why magnesium oxide has a much higher melting temperature than sodium chloride.2 marks
A chemist is studying the ions N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺ and Al³⁺. All of these ions have the electronic configuration 1s²2s²2p⁶.
Explain why the ionic radius decreases from N³⁻ to Al³⁺.2 marks
A student places a drop of aqueous copper(II) chromate(VI) at the centre of a strip of damp filter paper connected to a 20 V direct current supply. After several minutes a blue colour has moved towards the negative electrode and a yellow colour has moved towards the positive electrode.
Explain what these observations show about the particles in copper(II) chromate(VI).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).