Ionic bonding and ionic radii

Forming ions

Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions, usually reaching a noble gas configuration. Dot-and-cross diagrams show only the outer electrons, with square brackets and charges around each ion, for example [Na]+ [Cl]- with 8 electrons on the chloride.

The lattice

Ionic compounds form giant ionic lattices: each ion is surrounded by ions of opposite charge, and the attraction acts in all directions. Bonding is stronger (higher melting point) when ions are smaller and more highly charged: MgO melts far higher than NaCl.

Evidence for ions

• Physical properties: high melting points, brittle, conduct when molten or dissolved but not when solid.
• Electron density maps show nearly spherical ions with almost no electron density between them.
• Migration of ions: in electrolysis of a coloured salt on wet filter paper, coloured ions move towards the oppositely charged electrode (for example purple MnO4- moves to the positive electrode).

Ionic radii

Down a group, ionic radius increases (more shells). In an isoelectronic series (same number of electrons), radius decreases as nuclear charge increases: N3- > O2- > F- > Na+ > Mg2+ > Al3+. Positive ions are smaller than their atoms; negative ions are larger.

Worked example

Explain why magnesium oxide has a much higher melting point than sodium chloride.

1. Mg2+ and O2- have charges of 2+ and 2-, compared with 1+ and 1- in NaCl.
2. Mg2+ is also smaller than Na+.
3. So the electrostatic attraction between ions is much stronger.

Answer: More energy is needed to overcome the stronger ionic bonding in MgO.

Check yourself

1. Which ion has the largest radius?
   • O2-
   • Na+
   • Al3+
   • N3-
   Show answer

   N3-. All have 10 electrons; N3- has the fewest protons (7), so the weakest pull.

2. Which compound would you expect to have the highest melting point?
   • NaCl
   • MgO
   • KBr
   • NaF
   Show answer

   MgO. MgO has doubly charged ions, giving the strongest attraction.

3. Which observation is direct evidence that ions exist?
   • Ionic compounds are white
   • Ionic compounds dissolve in hexane
   • Ionic compounds have low boiling points
   • Coloured ions move towards an electrode during electrolysis
   Show answer

   Coloured ions move towards an electrode during electrolysis. Migration shows charged particles moving in an electric field.

4. Why does solid sodium chloride not conduct electricity?
   • The ions cannot move
   • There are no ions present
   • The electrons are delocalised
   • The bonds are covalent
   Show answer

   The ions cannot move. In the solid, ions are fixed in the lattice; when molten or dissolved they can move.

5. How does the radius of Group 1 ions change down the group?
   • It decreases
   • It stays the same
   • It increases
   • It first increases then decreases
   Show answer

   It increases. Each ion has an extra shell of electrons.

6. What is an ionic bond?
   • Strong electrostatic attraction between oppositely charged ions
   • A shared pair of electrons
   • Attraction between metal ions and delocalised electrons
   • Weak attraction between molecules
   Show answer

   Strong electrostatic attraction between oppositely charged ions. That is the definition the specification uses.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page
• Pearson IAL Chemistry (2018) transition guide

Last updated 7 November 2026. Spotted a mistake? Tell us.