History of Atomic Models

Thomson model

After J. J. Thomson identified the electron, he proposed that negative electrons were embedded within a diffuse region of positive charge. This model made the atom electrically neutral but predicted only modest deflections of fast charged projectiles.

Rutherford scattering evidence

Geiger and Marsden, working under Rutherford, directed alpha particles at thin metal foil. Most passed through with small deflection, showing that atoms are mostly empty space. A small fraction scattered through large angles, which required positive charge and most atomic mass to be concentrated in a very small nucleus.

Classical instability

Rutherford’s nuclear model did not explain atomic stability or discrete line spectra. In classical electromagnetism, an orbiting electron is accelerating and should radiate continuously, lose energy, and spiral inward. A classical orbit could also emit a continuous range of frequencies rather than the observed characteristic spectral lines.

Those failures motivated quantised atomic models and, eventually, quantum mechanics. This page is historical support rather than part of the maintained light-quantisation path; continue at the Quantum Theory of Light Hub.