An atom is the smallest unit of a single element, while a molecule is two or more atoms bonded together, which may be the same element or different ones. Think of atoms as the building blocks and molecules as what you build with them: a water molecule is made of two hydrogen atoms and one oxygen atom.
The core difference
An atom is one particle of an element; a molecule is a group of atoms bonded together.
- atom — the smallest unit of a chemical element, with a nucleus and surrounding electrons: every atom of oxygen has eight protons.
- molecule — two or more atoms bonded together, keeping the chemical properties of a substance: a water molecule contains two hydrogen atoms and one oxygen atom.
How to tell them apart
A simple way to keep them straight: an atom is singular and can stand alone, while a molecule is a combination. If you can point to a single sphere in a diagram, that is an atom; if you are pointing at two or more spheres joined by a line, that is a molecule.
The mix-up usually comes up in chemistry class when people use the words interchangeably to mean "a tiny bit of stuff." Atom also has a separate, informal sense meaning a minimal or trace amount of something, as in "there isn't an atom of truth in that rumour," which has nothing to do with chemistry at all.
