Molecule

Summary: A group of two or more atoms held together by covalent bonds. The smallest particle of a covalent compound that can exist independently. Tags: igcse chemistry definition atomic-structure bonding Created: 2026-07-14 Last Updated: 2026-07-14

A molecule is a discrete group of two or more non-metal atoms held together by covalent bonds — shared pairs of electrons — and represents the smallest particle of a covalent compound capable of independent existence. Simple molecules such as water (H₂O), carbon dioxide (CO₂), methane (CH₄), and chlorine (Cl₂) have fixed numbers of atoms and relatively low melting and boiling points because only weak intermolecular forces, not covalent bonds, need to be overcome during state changes. It is essential at IGCSE level to understand that not all substances are molecular: ionic compounds (e.g., NaCl) form giant ionic lattices with no discrete molecules, and giant covalent substances (e.g., diamond, SiO₂) consist of infinite networks of covalent bonds rather than separate molecular units. The term “molecule” should therefore be reserved for substances that exist as individual covalently bonded groups, and applying it to ionic compounds is a common error penalised in exams. Diatomic elements such as H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂ are also molecules, and IGCSE students are expected to recall these seven elements that naturally exist as pairs of atoms.

Key term — IGCSE Chemistry (0620/0971) glossary.