Particle in atomic physics
An atom is made up of protons, neutrons, and electrons.
When an atom is charged, it becomes an ion.
There's not an atom of dirt in her house because she never stops cleaning.
A molecule may be defined as the smallest part of a substance which can exist alone; an atom as the smallest part of a substance which can exist in combination.
Dalton believed that the molecules of the elementary gases consisted each of one atom; his diagram for hydrogen gas makes the point clear.