1.5 Atomic Structure and Electron Configuration

Electron configuration basics

Electron configuration describes how electrons are arranged in orbitals around the nucleus, filling from lowest to highest energy. Orbitals are grouped into subshells (s, p, d, f), each holding a specific number of electrons: s=2, p=6, d=10, f=14. The Aufbau principle states electrons fill lowest-energy orbitals first.

Rules for filling orbitals

The Pauli exclusion principle says each orbital holds at most 2 electrons with opposite spins. Hund's rule states electrons fill degenerate (same-energy) orbitals singly before pairing up, minimizing repulsion. Together with the Aufbau principle, these rules predict ground-state configurations like carbon: 1s²2s²2p².

Valence electrons and periodic trends

Valence electrons are outermost electrons involved in bonding; their number relates directly to an element's group number and chemical behavior. Electron configurations explain periodic trends — atomic radius, ionization energy, and reactivity — since elements in the same group share similar valence configurations, giving similar properties.

Key terms

Aufbau principle

Electrons fill orbitals starting from the lowest energy level before moving to higher ones.

Pauli exclusion principle

Each orbital can hold a maximum of two electrons, and they must have opposite spins.

Hund's rule

Electrons fill degenerate orbitals singly before pairing up, to minimize repulsion.

Valence electrons

Outermost electrons that determine an element's chemical bonding and reactivity.

Subshell electron capacities

s=2, p=6, d=10, f=14 electrons maximum.

Try an AP-style question

What is the ground-state electron configuration of sulfur (Z = 16)?

1. 1s² 2s² 2p⁶ 3s⁴ 3p²
2. 1s² 2s² 2p⁶ 3s² 3p⁴
3. 1s² 2s² 2p⁸ 3s² 3p²
4. 1s² 2s² 2p⁶ 3s² 3p⁶