Molecule

A molecule is a particle made of two or more atoms chemically joined together. Malay: molekul · Chinese: 分子.

The atoms in a molecule are held together by chemical (covalent) bonds. The joined atoms may be the same kind, as in a molecule of an element, or different kinds, as in a molecule of a compound. A molecule is one of the three kinds of particle you meet in this chapter, alongside the atom and the ion.

Example. A water molecule, H₂O, is made of two hydrogen atoms and one oxygen atom bonded together. An oxygen molecule, O₂, is made of two oxygen atoms joined; because both atoms are oxygen, O₂ is a molecule of an element, while H₂O is a molecule of a compound. Other familiar examples are carbon dioxide (CO₂), hydrogen (H₂) and chlorine (Cl₂).

Confusion to avoid. Do not confuse a molecule with an atom or an ion. An atom is a single particle; a molecule is two or more atoms bonded together. An ion carries an overall electrical charge, whereas a molecule is electrically neutral. Students also mix up “molecule” and “compound”: a molecule is a particle, while a compound is a substance, and although every molecule of a compound contains different elements, a molecule of an element (like O₂) is not a compound at all.

In the exam, particle diagrams often ask you to label whether a substance is made of atoms, molecules of an element, molecules of a compound, or ions, so practise reading these diagrams carefully. The diatomic molecules H₂, O₂, N₂ and Cl₂ are among the examples tested most often.

In the Matter and the Atomic Structure chapter, thinking of matter as tiny particles is the key idea, and molecules are one of the particle types that make up gases, liquids and solids. Being able to tell an atom, a molecule and an ion apart is a skill our teachers build early because it underpins bonding and formulae later.