Diatomic
Summary: A diatomic molecule consists of two atoms chemically bonded together. The halogens (F₂, Cl₂, Br₂, I₂) are diatomic non-metals. Other diatomic elements include H₂, N₂, and O₂. Tags: igcse chemistry definition periodic-table Created: 2026-07-14 Last Updated: 2026-07-23
A diatomic molecule is a molecule composed of two atoms bonded together. The atoms may be of the same element (homonuclear, e.g. Cl₂) or different elements (heteronuclear, e.g. HCl).
Diatomic Elements
The elements that naturally exist as diatomic molecules at room temperature:
| Element | Formula | State at r.t.p. | Type |
|---|---|---|---|
| Hydrogen | H₂ | Colourless gas | Non-metal |
| Nitrogen | N₂ | Colourless gas | Non-metal |
| Oxygen | O₂ | Colourless gas | Non-metal |
| Fluorine | F₂ | Pale yellow gas | Halogen |
| Chlorine | Cl₂ | Yellow-green gas | Halogen |
| Bromine | Br₂ | Red-brown liquid | Halogen |
| Iodine | I₂ | Grey-black solid | Halogen |
Mnemonic: “Have No Fear Of Ice Cold Beer” — H₂, N₂, F₂, O₂, I₂, Cl₂, Br₂
Diatomic Molecules in the IGCSE Syllabus
Halogens as Diatomic Non-Metals
The Group 7 halogens are described as diatomic non-metals in the syllabus:
- They exist as X₂ molecules (F₂, Cl₂, Br₂, I₂)
- Each halogen atom shares one electron with another → single covalent bond
- They are NOT monatomic like the noble gases
Why Are Halogens Diatomic?
Halogen atoms have 7 electrons in their outer shell. By sharing one electron with another halogen atom (forming a single covalent bond), each atom achieves a stable noble gas configuration (8 electrons in the outer shell).
Diatomic vs Monatomic
| Property | Diatomic (e.g. Cl₂) | Monatomic (e.g. Ar) |
|---|---|---|
| Number of atoms | 2 | 1 |
| Bond | Covalent bond between atoms | No bonds (single atom) |
| Example group | Group 7 (Halogens) | Group 0/8 (Noble gases) |
| Reactivity | Reactive | Unreactive (inert) |
Key Facts
- Diatomic = molecule with two atoms
- All halogens (Group 7) are diatomic: F₂, Cl₂, Br₂, I₂
- Other diatomic elements: H₂, N₂, O₂
- Noble gases (Group 0) are monatomic, NOT diatomic
- Halogens are described as “diatomic non-metals” in the IGCSE syllabus
- Diatomic molecules are held together by covalent bonds
See Also
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.