Molecule – IGCSE Chemistry Definition

A molecule is a group of atoms joined by covalent bonds. The 0620 syllabus expects you to distinguish molecules from individual atoms and from ionic structures. You must also know the common diatomic elements and understand the difference between molecular formulae and empirical formulae.

The 0620 definition

A molecule is a group of two or more atoms chemically bonded together by covalent bonds.

A molecule can consist of:

• Atoms of the same element (e.g. O₂, N₂, Cl₂) — this is still an element, not a compound
• Atoms of different elements (e.g. H₂O, CO₂, CH₄) — this is a compound

Diatomic elements

Seven common elements exist as diatomic molecules (two atoms bonded together) at room temperature:

Element	Formula
Hydrogen	H₂
Nitrogen	N₂
Oxygen	O₂
Fluorine	F₂
Chlorine	Cl₂
Bromine	Br₂
Iodine	I₂

When writing equations, always use the diatomic formula for these elements, not the single atom symbol.

Molecules vs ionic structures

Feature	Molecular substance	Ionic substance
Bonding	Covalent bonds within molecules	Ionic bonds (electrostatic attraction between ions)
Particles	Discrete molecules	Ions in a giant lattice
Example	Water, H₂O	Sodium chloride, NaCl
Melting point	Usually low	Usually high
Conductivity	Does not conduct	Conducts when molten or dissolved

Important: the formula NaCl represents the simplest ratio of ions, not a molecule. You should never call NaCl “a molecule of sodium chloride.”

Simple molecular vs giant structures

Simple molecular substances have small, discrete molecules with weak intermolecular forces between them — hence low melting and boiling points. Giant structures (giant covalent or giant ionic) have continuous networks of strong bonds and very high melting points.

Worked exam question

State whether each of the following is an atom, a molecule, or an ion: (a) Na (b) H₂O (c) Cl- (d) N₂ (4)

Mark scheme

(a) Atom [1]

(b) Molecule [1]

(c) Ion [1]

(d) Molecule [1]

Common exam mistakes

• Calling NaCl or MgO “a molecule”. These are ionic compounds — they form lattices of ions, not molecules.
• Writing O instead of O₂ in equations. Oxygen exists as diatomic molecules. Using O implies a single atom, which loses the balancing mark.
• Confusing “molecule” with “compound”. O₂ is a molecule but not a compound (only one element). H₂O is both a molecule and a compound.