3.3 Orbitals and Electron Configurations

Notes

The modern model places electrons in orbitals: regions where an electron is likely to be found. Each orbital holds at most 2 electrons.

• Sublevels: s (1 orbital, 2 e), p (3 orbitals, 6 e), d (5 orbitals, 10 e), f (7 orbitals, 14 e).
• Aufbau principle: fill from lowest energy up: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p.
• Pauli exclusion principle: two electrons in one orbital have opposite spins. Hund's rule: in equal-energy orbitals, place one electron in each before pairing.
• Example: sulfur (16 electrons) is $1s^2 2s^2 2p^6 3s^2 3p^4$.
• Noble gas notation shortens it: sulfur is $[\text{Ne}]3s^2 3p^4$.