Ions and ionic bonds
Formation of ions by electron transfer, ionic bonding, and the properties of ionic compounds.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 2.4.1Describe the formation of positive and negative ions by loss or gain of electrons.
- 2.4.2Describe ionic bonding as the electrostatic attraction between oppositely charged ions in a giant lattice.
- 2.4.3Explain the properties of ionic compounds in terms of their structure and bonding.Supplement
7 minute read
Ionic bonding
Ionic bonding happens between a metal and a non-metal. Electrons are transferred, not shared.
The sodium chloride story
Sodium (2,8,1) has one electron in its outer shell. Losing it leaves a full outer shell and a Na⁺ ion.
Chlorine (2,8,7) needs one more electron for a full shell. Gaining one makes a Cl⁻ ion.
Sodium hands its outer electron to chlorine. Both now have full outer shells, and the oppositely charged ions attract.
The bond itself
An ionic bond is the strong electrostatic attraction between oppositely charged ions. That exact phrase is the definition examiners want.
The ions do not exist as pairs. They build a giant ionic lattice — a regular three-dimensional structure in which every Na⁺ is surrounded by Cl⁻ ions and vice versa.
Properties, explained by the structure
- High melting and boiling points: there are many strong electrostatic attractions throughout the lattice, so a lot of energy is needed to separate the ions.
- Conduct electricity when molten or dissolved, but not when solid: charged ions must be free to move. In a solid lattice they are held in fixed positions.
- Often soluble in water, and generally brittle: knocking the lattice so that like charges align makes it split.
Notice how every property traces back to two facts — the ions are charged, and they are locked in a lattice.
Working out the formula
The compound must be electrically neutral. Mg²⁺ with Cl⁻ needs two chloride ions: MgCl₂. Al³⁺ with O²⁻ needs the charges to balance at 6 each: Al₂O₃.
Think of it like this
An ionic lattice is like a stack of alternating positive and negative magnets — enormously strong as a block, but if you slide one layer so like poles line up, the whole thing flies apart. That is why ionic solids are brittle.
Worked examples
Method, step by step
Describe, in terms of electron transfer, how magnesium oxide is formed from magnesium and oxygen atoms.
- 1Magnesium has the configuration 2,8,2 and loses its two outer electrons to form Mg²⁺.
- 2Oxygen has the configuration 2,6 and gains two electrons to form O²⁻.
- 3Both ions now have full outer shells.
- 4The oppositely charged ions attract electrostatically in a giant lattice.
Mg transfers 2 electrons to O, forming Mg²⁺ and O²⁻ ions held by strong electrostatic attraction: MgO.
Common misconceptions
- Saying electrons are shared in ionic bonding. They are transferred — sharing is covalent.
- Writing "NaCl molecules". Ionic compounds form giant lattices, not molecules.
- Claiming ionic solids conduct electricity. They only conduct when molten or in solution, because only then are the ions free to move.
In the exam
- Define an ionic bond as "the strong electrostatic attraction between oppositely charged ions" — vaguer wording loses the mark.
- In dot-and-cross diagrams, show only the outer shell unless told otherwise, and put square brackets with the charge outside for each ion.
- For conductivity questions, always say *why*: "the ions are free to move and carry charge".