Atomic theory

Atomic theory describes matter as composed of atoms with defined structures and interactions. It explains how atomic composition and arrangement account for the properties and transformations of substances.

Atom: The smallest unit of a chemical element that retains that element’s identity. Atomic theory treats atoms as matter’s basic units and explains their internal structure.
Atomic nucleus: The dense central region of an atom, containing protons and neutrons. The nucleus accounts for nearly all atomic mass and determines nuclear charge.
Democritus: An ancient Greek philosopher associated with the proposal that matter consists of indivisible particles called atoms. His atomism supplied an early philosophical precursor, not an experimental scientific theory.
Continuity theory of matter: The view that matter is continuous and can be divided without reaching fundamental discrete units. Atomic theory instead explains matter through distinct particles and their arrangements.
Periodic table: A tabular arrangement of chemical elements ordered by atomic number and recurring properties. Its organization reflects atomic structure and recurring electron configurations.
Chemical element: A pure substance whose atoms all have the same number of protons. An element’s identity is fixed by atomic number, a central feature of modern atomic theory.
Electron: A negatively charged subatomic particle found in atoms and many other systems. Electron arrangements govern much of an atom’s bonding and chemical behavior.
John Dalton: An English chemist who formulated an influential modern atomic theory in the early nineteenth century. Dalton used chemical regularities to argue that elements consist of characteristic atoms.
Plum pudding model: J. J. Thomson’s model of the atom as electrons embedded in diffuse positive charge. Rutherford’s scattering results displaced this model with a compact nucleus.
Mass spectrometry: An analytical technique that separates ions by mass-to-charge ratio. It measures isotopic masses and helps determine the atomic composition of samples.

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