Atom
The smallest particle of an element that retains the chemical identity of that element.
Unit
Dimensionless
In depth
An atom consists of a dense nucleus of protons and neutrons surrounded by electrons occupying quantised orbitals. The proton count (atomic number) fixes the element; the electron arrangement fixes the chemistry. Atoms are typically 0.1–0.5 nm across, with the nucleus roughly 100,000 times smaller than the atom itself.
Examples in the real world
A single carbon atom in a diamond lattice, or an argon atom drifting freely in the atmosphere as a monatomic gas.