Introduction: Chemistry from Quantum Mechanics
The periodic table, discovered empirically in 1869, is now understood as a consequence of quantum mechanics. Shell structure, Pauli exclusion, and electron-electron repulsion explain all its features.
The Aufbau Principle
Electrons fill orbitals in order of increasing energy (approximately):
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → ...
Each orbital holds 2 electrons (spin up and down).
Shell Capacities
- \(n = 1\): 1s holds 2 → He at 2
- \(n = 2\): 2s + 2p hold 8 → Ne at 10
- \(n = 3\): 3s + 3p hold 8 → Ar at 18
- \(n = 4\): 4s + 3d + 4p hold 18 → Kr at 36
Periodic Properties
- Noble gases: Filled shells → chemically inert
- Alkali metals: One valence electron → highly reactive
- Halogens: One electron short of filled shell → eager to accept
- Transition metals: Filling d-orbitals → complex chemistry
The Quantum Connection
Every chemical property—reactivity, bonding patterns, color, magnetism—ultimately derives from quantum mechanics. The periodic table is the most successful application of quantum mechanics to the real world. Dirac said, "The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known."