Atoms

The breakthrough came with Rutherford’s Alpha-Particle Scattering Experiment. By bombarding a thin gold foil with alpha particles, Ernest Rutherford observed that while most particles passed straight through, a tiny fraction bounced backward. This led to the conclusion that the entirety of an atom's positive charge and most of its mass are concentrated in a tiny, dense core called the nucleus, with electrons orbiting around it.

To fix the stability flaws in Rutherford’s planetary model, Niels Bohr introduced the Bohr Model of the Hydrogen Atom. Bohr postulated that electrons revolve only in specific, non-radiating orbits called stationary states. An electron can only transition between these orbits by absorbing or emitting a photon of energy exactly equal to the difference between the energy levels. This quantization beautifully explained the discrete line spectra observed in hydrogen gas, paving the way for modern quantum theory.