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What is a molecule?

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Quick answer

A molecule is two or more atoms bonded together, acting as a single unit. Molecules are how atoms combine to form everything from the water you drink (H₂O) to the oxygen you breathe (O₂) and the sugars that fuel your body.

Key things to understand

  1. A molecule forms when atoms share electrons in chemical bonds.
  2. It can be two of the same atom (O₂, oxygen gas) or different atoms (H₂O, water).
  3. Which atoms it has and how they're arranged decide its properties — water and hydrogen peroxide share atoms but behave very differently.
  4. A compound is a molecule made of at least two different elements.
  5. Molecules range from tiny (two atoms) to enormous (a DNA molecule has billions).

People also ask

What's the difference between a molecule and a compound?
Every compound is a molecule, but not every molecule is a compound. A compound contains two or more different elements; O₂ is a molecule but not a compound because it's all one element.
What holds a molecule together?
Chemical bonds — most often atoms sharing electrons (covalent bonds) — hold a molecule's atoms together.
What is the smallest molecule?
Hydrogen gas (H₂), made of just two hydrogen atoms, is the smallest and lightest molecule.