An atom is the smallest constituent unit of ordinary matter
Atom (A very small particle)
Requires understanding of particle concept.
Development of this idea - ancient Greece. Scientist/Democritus - thought experiment lead to idea of matter being ultimately made of atoms after the atomos meaning undivisible.
Atom (The smallest unit of a chemical substance)
Development of this idea - Scientist/John Dalton - proposed that all matter was composed of atoms, indivisible and indestructible building blocks. While all atoms of an element were identical, different elements had atoms of differing size and mass.