Structure of Atom
Atoms are divisible and made of three fundamental sub-atomic particles: electrons, protons and neutrons.
Discovery of particles
- Electron: discovered through cathode rays. J.J. Thomson (1897) measured the charge to mass ratio e/me = 1.758820 x 10^11 C per kg. Millikan's oil drop experiment found the charge = -1.602 x 10^-19 C. Mass = 9.109 x 10^-31 kg.
- Proton: positively charged particle from canal rays (lightest positive ion from hydrogen), characterised in 1919.
- Neutron: discovered by Chadwick (1932) by bombarding beryllium with alpha particles; neutral, slightly heavier than the proton.
Atomic models
- Thomson (plum pudding) model: sphere of uniform positive charge with electrons embedded. Could not explain later experiments.
- Rutherford (nuclear) model: from the alpha particle scattering experiment - a tiny dense positively charged nucleus with electrons revolving around it. Drawback: could not explain atomic stability (an orbiting electron should radiate energy and spiral inward).
- Bohr model: electrons in fixed circular orbits (stationary states) with quantised energy and angular momentum (mvr = n times h over 2 pi). Explained the hydrogen spectrum but failed for multi-electron atoms and ignored wave nature and the uncertainty principle.
Quantum mechanical model
Based on de Broglie's dual nature of matter (wavelength = h over momentum) and Heisenberg's uncertainty principle (position and momentum cannot both be known exactly). Solving the Schrodinger equation gives orbitals (one-electron wave functions) defined by quantum numbers n, l, ml, with ms as the fourth. Orbitals are filled following the Aufbau principle, Pauli exclusion principle and Hund's rule to give the electronic configuration.Key constants
- Planck's constant h = 6.626 x 10^-34 J s
- Speed of light c = 3.0 x 10^8 m per s
- Rydberg constant (energy form) RH = 2.18 x 10^-18 J