Molecule
An electrically neutral group of two or more atoms held together by covalent bonds, acting as the smallest unit of a covalent substance.
Unit
Dimensionless
In depth
Molecules may be homonuclear (one element, such as O₂) or heteronuclear (several elements, such as H₂O). Their geometry, polarity and bond strengths determine boiling point, solubility and reactivity. Ionic compounds such as NaCl form extended lattices rather than discrete molecules, so their smallest repeating unit is called a formula unit.
Examples in the real world
Water (H₂O), carbon dioxide (CO₂), ammonia (NH₃), and the diatomic nitrogen (N₂) that makes up 78% of air.