Definition · Plain-language
Molecule
A molecule is a group of two or more atoms held together by chemical bonds, forming the smallest unit of many substances.
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Atoms bonded into a unit
A molecule is what you get when atoms join through chemical bonds to form a discrete, electrically neutral particle. The bonds are usually covalent, meaning the atoms share electrons. A molecule is the smallest portion of a substance that retains its chemical identity: split a water molecule and you no longer have water, but hydrogen and oxygen. Molecules range from the tiny, like the two-atom hydrogen molecule, to the enormous, like proteins and DNA that contain thousands or millions of atoms.
Elements, compounds and diatomic molecules
A molecule can be made of a single element or of several. When two or more different elements are bonded, the molecule is also a compound — water (H₂O) and carbon dioxide (CO₂) are molecular compounds. When the atoms are all the same element, the molecule is not a compound: oxygen gas (O₂) is a molecule but a pure element. Several elements naturally exist as diatomic molecules — two bonded atoms — including hydrogen, nitrogen, oxygen and the halogens, which is why we write them as H₂, N₂ and O₂.
When substances are not molecular
Not everything is built from molecules. Ionic compounds such as sodium chloride form giant lattices of alternating ions rather than discrete molecules, so referring to a "molecule of salt" is technically inaccurate — its formula gives a ratio, not a molecule. Metals consist of a lattice of atoms in a sea of shared electrons, and covalent network solids like diamond are effectively one giant bonded structure. The molecular model applies best to covalently bonded substances that exist as separate, countable units.
Key facts
At a glance
- Definition: two or more atoms held together by chemical bonds
- Bond type: usually covalent (shared electrons)
- Smallest unit of: many substances, keeping their chemical properties
- Same or different elements: O₂ (element) or H₂O (compound)
- Diatomic examples: H₂, N₂, O₂, the halogens
- Not molecular: ionic compounds, metals, covalent networks
Common misconceptions
What people often get wrong
Often heard: Every molecule is a compound.
Actually: Only molecules made of two or more different elements are compounds. A molecule of a single element, such as O₂ or N₂, is a molecule but not a compound.
Often heard: All substances are made of molecules.
Actually: Ionic compounds form lattices, metals form atomic lattices in a sea of electrons, and network solids are giant bonded structures. The molecule model fits covalent substances that exist as separate units.
Often heard: A molecule is always larger than an atom and always tiny overall.
Actually: A molecule is built from atoms so contains at least two, but molecules span an enormous range — from two-atom hydrogen to proteins and DNA with millions of atoms.
Going deeper
