What's the Difference Between an Atom and a Molecule?
An atom is the smallest unit of a chemical element, while a molecule is a group of two or more atoms bonded together. In other words, atoms are the building blocks, and molecules are the structures you can build from them.
What is an atom?
An atom is the smallest particle of an element that still has the properties of that element. Every atom is made of three types of subatomic particle:
- Protons, positively charged, found in the nucleus.
- Neutrons, no charge, also in the nucleus.
- Electrons, negatively charged, moving around the nucleus.
The number of protons decides which element the atom is. Hydrogen always has one proton; carbon always has six. Atoms of the same element that differ in their number of neutrons are called isotopes.
What is a molecule?
A molecule forms when two or more atoms join together through chemical bonds. The atoms can be the same or different:
- Same element: an oxygen molecule (O₂) is two oxygen atoms bonded together.
- Different elements: a water molecule (H₂O) is two hydrogen atoms bonded to one oxygen atom.
Molecules can be tiny, like hydrogen (H₂), or enormous, like the DNA in your cells, which contains millions of atoms.
Atom vs molecule: a quick comparison
| Atom | Molecule | |
|---|---|---|
| What it is | Smallest unit of an element | Two or more atoms bonded together |
| Can it exist alone? | Yes, but often reactive | Yes |
| Example | A single oxygen atom (O) | An oxygen molecule (O₂) |
The key point
Every molecule is made of atoms, but not every atom is part of a molecule. Some atoms, such as those of the noble gas helium, are stable on their own and do not normally bond into molecules at all.
For precise, easy-to-read definitions of terms like atom, molecule, and isotope, explore our Chemistry Dictionary.