Diatomic – IGCSE Chemistry Definition
IGCSE Chemistry definition of diatomic: a molecule made of two atoms. Covers the seven diatomic elements, formulae, and exam tips for Cambridge 0620.
Published by IGCSEChemistry.com.my
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.
Getting the diatomic elements right is a basic skill that affects equation balancing throughout IGCSE Chemistry. If you write Cl instead of Cl₂ or O instead of O₂, you will lose the balancing mark every time.
The 0620 definition
A diatomic molecule is a molecule consisting of exactly two atoms. A diatomic element is an element that naturally exists as molecules containing two atoms of the same element.
The seven diatomic elements
| Element | Formula | Type of bonding |
|---|---|---|
| Hydrogen | H₂ | Covalent (single bond) |
| Nitrogen | N₂ | Covalent (triple bond) |
| Oxygen | O₂ | Covalent (double bond) |
| Fluorine | F₂ | Covalent (single bond) |
| Chlorine | Cl₂ | Covalent (single bond) |
| Bromine | Br₂ | Covalent (single bond) |
| Iodine | I₂ | Covalent (single bond) |
All four halogens on the syllabus are diatomic. Hydrogen, nitrogen, and oxygen are the other three you must remember.
Why this matters for equations
When writing equations, you must use the correct formula for elements. An element that is diatomic must be written as the diatomic molecule:
Correct: 2Na + Cl₂ → 2NaCl
Incorrect: 2Na + 2Cl → 2NaCl (Cl does not exist on its own)
Similarly:
Correct: 2H₂ + O₂ → 2H₂O
Incorrect: 4H + 2O → 2H₂O
Diatomic vs monatomic
Noble gases are monatomic — they exist as individual atoms (He, Ne, Ar), not molecules. Their full outer shell means they have no need to share electrons with another atom.
Metals in the solid state exist as giant metallic structures, not molecules, so the concept of “diatomic” does not apply to them.
Worked exam question
Write the balanced equation for the reaction of aluminium with bromine. [2]
Mark scheme
Correct formulae: 2Al + 3Br₂ → 2AlBr₃ [1 for correct formulae, 1 for balancing]
Note: writing Br instead of Br₂ loses the formulae mark.
Common exam mistakes
- Writing O, N, H, Cl, Br, I, or F as single atoms in equations instead of O₂, N₂, H₂, Cl₂, Br₂, I₂, F₂.
- Treating noble gases as diatomic — they are monatomic (single atoms).
- Forgetting nitrogen is diatomic, leading to wrong formulae in equations involving ammonia production or nitrogen reactions.
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