Development of Scientific Theories – Quick-Review Notes

Science & Scientific Method

  • Science (latin: scientia = knowledge)
    • Body of knowledge + process (scientific method).
  • Core loop:
    • Observation ➜ Hypothesis ➜ Prediction ➜ Test ➜ Revision
    • Hypotheses must be falsifiable.
  • Scientific theories: give reasons for phenomena.
  • Scientific laws: describe / predict without explanation (e.g. Law of Conservation of Energy).

Theory vs. Law (quick recall)

  • Theory = explanatory framework, repeatedly tested, integrates evidence.
  • Law = concise statement of regularity, often mathematical.

Development of Atomic Theory (key timeline)

  • \text{400 BCE} – Leucippus & Democritus: atomism (indivisible atoms in the Void).
  • 1661 – Robert Boyle: corpuscularism; empirical basis for molecular chemistry.
  • 1808 – John Dalton: atoms indivisible, identical within an element, combine in simple ratios.
  • 1827\,(Brown)\;\to\;1905\,(Einstein)\;\to\;1908\,(Perrin): Brownian motion → quantitative proof of atoms.
  • 1897 – J. J. Thomson discovers electron; Plum-Pudding model (negative electrons in positive ‘soup’).
  • 1911\text{–}1909 – Rutherford scattering (gold-foil): nuclear atom.
  • 1917 – Rutherford identifies proton.
  • 1913 – Niels Bohr: quantised electron orbits; good for H atom.
  • 1924 – Louis de Broglie: matter-wave hypothesis.
  • 1926 – Erwin Schrödinger: wave equation \hat H |\psi\rangle = E|\psi\rangle → wave-mechanical model, orbitals.
  • 1932 – James Chadwick: neutron completes basic sub-atomic set.

Key Models & Concepts

  • Plum-Pudding: electrons embedded in diffuse positive charge.
  • Rutherford Planetary: tiny dense nucleus, electrons orbit; could not explain spectra.
  • Bohr: quantised circular orbits; E_n \propto -\frac{1}{n^2}.
  • Wave-Mechanical (de Broglie/Schrödinger): electrons as standing waves → orbitals (s, p, d…).

Evidence Highlights

  • Brownian motion: random pollen movement quantified → atomic reality.
  • Alpha particle scattering: large deflections → concentrated nucleus.
  • Spectral lines: discrete energies → quantised orbits/waves.

Take-Home Messages

  • Theories evolve incrementally, each refining predecessors.
  • Require consistent, reproducible data & must remain falsifiable.
  • Multiple disciplines and researchers contribute: science = team effort.
  • Modern atomic theory integrates classical, quantum & experimental insights.