An atom is the smallest unit of a chemical element, while a molecule is two or more atoms bonded together — which can be the same element or different ones.
An atom consists of a nucleus (protons and neutrons) surrounded by electrons, and it's the smallest particle that still retains the chemical identity of an element, like oxygen or carbon.
Molecules can be made of a single element (like O₂ or N₂) or multiple different elements (like H₂O or CO₂, called compounds). Every compound is a molecule, but not every molecule is a compound.
Last reviewed: September 2026