Evolution of Atomic Models: From Democritus to Quantum Mechanics
First Atomic Models
Democritus (450 BC)
Proposed the idea of atoms as indivisible particles.
Dalton Atomic Model
Introduced atomic-molecular theory:
- Matter consists of indivisible atoms.
- Atoms of the same element have identical mass and properties.
- Compounds form from combinations of different elements.
- Mass is conserved in chemical reactions.
- Law of multiple proportions.
Lavoisier: Law of Conservation of Mass
Proust: Law of Definite Proportions
Atomic Model – Thomson
Discovered electrons through cathode ray experiments.
Proposed the “plum pudding” model: a sphere of positive charge with embedded electrons.
Atomic Model – Rutherford
Gold foil experiment demonstrated a dense, positively charged nucleus.
Nuclear model: a small, positive nucleus surrounded by orbiting electrons.
Subatomic Particles
- Proton: Located in the nucleus, positive charge.
- Neutron: Located in the nucleus, no charge.
Isotopes: Atoms with the same number of protons but different neutrons.
Background to Bohr Atomic Model
Atomic Spectroscopy
Study of light interaction with atoms.
Wave Model of Light: Light as an electromagnetic wave.
Particle Model of Light: Light as photons (energy packets).
Photoelectric Effect: Emission of electrons when light shines on a metal.
Atomic Spectra
- Emission Spectra: Discrete lines of light emitted by excited atoms.
- Absorption Spectra: Dark lines where light is absorbed by atoms.
Spectral Series: Lyman, Balmer, Paschen, etc.
Bohr Atomic Model
Electrons orbit the nucleus in specific energy levels without radiating energy.
Energy is emitted or absorbed when electrons jump between levels.
Limitations of Bohr Model
- Only explains hydrogen-like atoms.
- Doesn’t account for finer spectral lines.
- Doesn’t explain stability of orbits.
Bohr-Sommerfeld Model
Introduced elliptical orbits in addition to circular orbits.
Quantum Mechanical Models
Current model: electrons exist in orbitals, regions of probability.
Wave-particle duality: Electrons exhibit wave and particle properties.
Heisenberg Uncertainty Principle: Limitation on simultaneously knowing position and momentum.
Schrödinger Wave Equation: Describes electron behavior.
Electronic Configuration
Arrangement of electrons in atomic orbitals.
- Pauli Exclusion Principle: No two electrons can have the same four quantum numbers.
- Aufbau Principle: Electrons fill orbitals in order of increasing energy.
- Hund’s Rule: Electrons fill degenerate orbitals individually before pairing up.
Noble gases: Stable elements with filled electron shells.