Chapter 9 · Electrons in Atoms and the Periodic Table
9.6Quantum-Mechanical Orbitals and Electron Configurations
13 min · two checks
Predict
How do three electrons occupy the 2p subshell?
The idea
- Apply the Aufbau principle, the Pauli principle, and Hund’s rule.
- Write an orbital diagram and a configuration for a small atom.
Electrons fill the lowest available energy first (Aufbau). The order of filling begins 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. No orbital holds more than two electrons, and those two have opposite spins (Pauli). When several orbitals share an energy, electrons occupy them singly with parallel spins before pairing (Hund). An orbital diagram draws this with boxes and arrows. A configuration compresses it: oxygen is 1s² 2s² 2p⁴.
The 4s orbital fills before 3d even though its principal number is higher, because its energy is lower in the atoms where the filling happens. Write what the atom actually does, not a strict numerical order. When you practice, count the superscripts: they must add to the number of electrons. For a neutral atom that sum equals Z.
Keep these
- Fill order starts 1s 2s 2p 3s 3p 4s 3d 4p.
- Two electrons per orbital, opposite spins. Spread out before pairing.
- Superscripts sum to the electron count.
Worked path
Write the ground-state configuration of sodium, Z = 11.
Observe
11 electrons, lowest energy first.