Timeline: Development of Atomic Models

Year	Scientist	Contribution
1803	John Dalton	Atomic theory: matter = indivisible atoms
1897	J.J. Thomson	Discovery of electron; "plum pudding" model
1900	Max Planck	Energy quantization: E = hν; birth of quantum theory
1905	Albert Einstein	Photoelectric effect explained by photons; quantum of light
1909	Millikan	Oil-drop experiment: measured charge of electron
1911	Ernest Rutherford	Nuclear model: dense positive nucleus; electrons in large empty space
1913	Niels Bohr	Quantized orbits for hydrogen; E_n = −13.6/ $n^{2}$ ; spectral series explained
1924	Louis de Broglie	Wave nature of matter: λ = h/mv
1925	Wolfgang Pauli	Exclusion principle: no two electrons with same 4 quantum numbers
1925	Werner Heisenberg	Matrix mechanics; uncertainty principle: $\Delta x$ · $\Delta p$ ≥ h/4π
1926	Erwin Schrödinger	Wave equation: ψ describes electron probability; orbitals replace orbits
1927	Friedrich Hund	Hund's rule: maximum spin multiplicity in degenerate orbitals

Evolution of the Nuclear Model: Thomson (uniform sphere) → Rutherford (nucleus + orbiting electrons) → Bohr (quantized orbits) → Quantum mechanical (orbitals/probability)