- Molecule
- Atom
- Ion
- Compound
The atom is the fundamental building block of matter, representing the smallest unit of an element that retains all its chemical properties. The term originates from the Greek “atomos,” meaning indivisible, though we now know atoms are composed of subatomic particles: protons, neutrons, and electrons. Despite being divisible, an atom’s chemical identity and behavior remain intact; splitting it into subatomic particles destroys elemental characteristics. Atoms combine to form molecules and compounds through chemical bonds, but isolated atoms still exhibit elemental properties—for example, a single gold atom is still gold, though bulk properties like conductivity require atomic assemblies. Atoms of the same element are chemically identical, containing the same number of protons, though isotopes vary in neutron count. The concept, dating to ancient Greek philosophers Leucippus and Democritus, was scientifically validated through John Dalton’s atomic theory (1803-1808). Modern atomic theory explains chemical reactions as atomic rearrangements, with atoms conserved in ordinary chemical processes. The size scale is angstroms (10⁻¹⁰ meters), with millions fitting across a pinhead.
