Ionic Bond – IGCSE Chemistry Definition
IGCSE Chemistry definition of ionic bond: electrostatic attraction between oppositely charged ions formed by electron transfer. Covers properties of ionic compounds.
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Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Ionic bonding is the transfer of electrons from a metal to a non-metal, creating oppositely charged ions that attract each other. The 0620 syllabus requires you to describe how ionic bonds form, draw dot-and-cross diagrams showing electron transfer, and explain the properties of ionic compounds.
The 0620 definition
An ionic bond is the strong electrostatic attraction between oppositely charged ions. It forms when electrons are transferred from a metal atom to a non-metal atom.
How ionic bonds form
- A metal atom loses one or more outer shell electrons to form a positive ion (cation)
- A non-metal atom gains those electrons to form a negative ion (anion)
- Both ions now have full outer shells (stable electron arrangements)
- The opposite charges attract — this electrostatic attraction is the ionic bond
Example: sodium chloride (NaCl)
- Na (2, 8, 1) loses 1 electron to become Na+ (2, 8)
- Cl (2, 8, 7) gains 1 electron to become Cl- (2, 8, 8)
- Na+ and Cl- attract each other
Example: magnesium oxide (MgO)
- Mg (2, 8, 2) loses 2 electrons to become Mg²+ (2, 8)
- O (2, 6) gains 2 electrons to become O²- (2, 8)
Properties of ionic compounds
| Property | Explanation |
|---|---|
| High melting and boiling points | Strong electrostatic attractions between ions require much energy to overcome |
| Conduct electricity when molten or dissolved | Ions are free to move and carry charge |
| Do not conduct when solid | Ions are held in fixed positions in the lattice |
| Often soluble in water | Water molecules can separate the ions |
| Hard but brittle | Strong lattice, but displacing layers causes like charges to align and repel |
| Form crystals | Regular arrangement of ions |
Giant ionic lattice
Ionic compounds do not exist as individual molecules. They form a giant ionic lattice — a regular three-dimensional arrangement of alternating positive and negative ions. Each ion is surrounded by ions of the opposite charge. NaCl has a cubic structure where each Na+ is surrounded by 6 Cl- ions and vice versa.
Worked exam question
Describe the formation of an ionic bond in calcium chloride, CaCl₂. Include electron arrangements. (4)
Mark scheme
Calcium atom (2, 8, 8, 2) loses 2 electrons [1]; to form Ca²+ (2, 8, 8) [1]; each chlorine atom (2, 8, 7) gains 1 electron to form Cl- (2, 8, 8) [1]; two chlorine atoms are needed to accept the 2 electrons / strong electrostatic attraction between Ca²+ and Cl- ions [1]
Common exam mistakes
- Defining an ionic bond as “the transfer of electrons”. Transfer is the process, not the bond. The bond itself is the electrostatic attraction between the resulting ions.
- Forgetting to balance electron transfer. If a metal loses 2 electrons, enough non-metal atoms must be present to accept them all (e.g. CaCl₂ needs two Cl atoms).
- Saying ionic compounds conduct electricity as solids. They only conduct when molten or dissolved — ions must be free to move.
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