Structure of Atom — Class 11 Chemistry

This unit traces the experimental discovery of the atom's internal structure and the models built from it. Cathode-ray experiments revealed the electron, whose charge-to-mass ratio J. J. Thomson measured () and whose charge Millikan's oil-drop experiment fixed (). Canal rays revealed the proton, and Chadwick discovered the neutron in 1932. Thomson's plum-pudding model gave way to Rutherford's nuclear model after the gold-foil -scattering experiment showed a tiny, dense, positively charged nucleus. The atomic number equals the proton count, the mass number equals protons plus neutrons, and isotopes and isobars are defined. Rutherford's model could not explain atomic stability or line spectra. Planck's quantum theory () and Einstein's explanation of the photoelectric effect established the dual wave-particle nature of radiation. Bohr quantised electron orbits and energy, successfully explaining the hydrogen line spectrum (Lyman, Balmer, Paschen, Brackett and Pfund series), but failed for multi-electron atoms. de Broglie's matter-wave relation and Heisenberg's uncertainty principle led to the quantum-mechanical model based on Schrödinger's equation, in which orbitals are described by four quantum numbers and electrons are filled following the Aufbau principle, Pauli exclusion principle and Hund's rule.