Atom

An atom contains a compact, positively charged nucleus surrounded by electrons. Its nucleus contains protons and, except in ordinary hydrogen, neutrons. Most atomic mass is concentrated in this small central region; familiar diagrams greatly exaggerate its size relative to the whole atom.

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Proton number identifies an element. Changing neutron number produces a different isotope of that element, whereas gaining or losing electrons makes an ion. These distinctions separate nuclear identity from electrical charge and prevent ordinary ionization from being confused with the creation of another element.

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Bound electrons occupy quantum states rather than miniature planetary paths. Transitions between energy levels can absorb or emit light. The pattern of permitted transitions produces characteristic spectra, linking atomic models to measurements of laboratory substances and distant stars.

References: [1] [2]

Dalton’s chemical atoms were followed by models incorporating electrons and a central nucleus. Bohr introduced quantized conditions for hydrogen; later quantum mechanics described electronic states with wave functions. Atomic theory retained its chemical usefulness while abandoning the idea that atoms had no internal structure.

References: [2] [1]