Atom

An atom is the smallest unit of a chemical element that retains its identity. It consists of a nucleus surrounded by electrons.

Atomic nucleus: The dense central region of an atom, containing protons and neutrons. It holds nearly all an atom’s mass and determines its element through its proton count.
Quantum mechanics: The physical theory describing matter and energy at atomic and subatomic scales. It explains why electrons occupy quantized states rather than classical orbits.
Chemical bond: An attractive interaction that holds atoms or ions together in molecules and solids. Atoms form bonds through interactions involving their electrons.
Democritus: An ancient Greek philosopher associated with the idea that matter consists of indivisible particles called atoms. His atomism supplied an early philosophical account of matter’s discrete constituents.
Molecule: A group of two or more atoms held together by chemical bonds. Molecules are bonded assemblies, whereas an atom is a single elemental unit.
Electron: A negatively charged elementary particle found in atoms and other forms of matter. Electrons occupy the space around the nucleus and govern most chemical interactions.
Electromagnetic interaction: The fundamental interaction between electrically charged particles. Its attraction between electrons and nuclei binds atoms together.
Periodic table: A tabular arrangement of chemical elements ordered by atomic number and recurring properties. Its patterns arise largely from the electron structures of atoms.
John Dalton: An English chemist who developed an influential modern atomic theory in the early nineteenth century. Dalton used atoms to explain fixed proportions in chemical compounds.
Ion: An atom or molecule with a net electric charge from gaining or losing electrons. An ion differs from a neutral atom in electron count and charge.

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