Atomic structure and electron configuration
Electron configuration is the arrangement of an atom's electrons among its energy levels, subshells, and orbitals, written in notation like 1s² 2s² 2p⁶. It follows from atomic structure and predicts an element's chemical behavior.
An atom consists of a dense nucleus of protons and neutrons surrounded by electrons occupying quantized energy levels. Electron configuration describes exactly how those electrons are distributed among shells (n = 1, 2, 3...), subshells (s, p, d, f), and orbitals. Sodium, for example, is 1s² 2s² 2p⁶ 3s¹ — eleven electrons filling the lowest available energy states.
Three rules govern the ground-state filling. The Aufbau principle says electrons fill the lowest-energy subshells first, following the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on. The Pauli exclusion principle limits each orbital to two electrons with opposite spins. Hund's rule says electrons occupy degenerate orbitals singly before pairing up. Subshell capacities follow from orbital counts: s holds 2 electrons, p holds 6, d holds 10, and f holds 14.
Configuration explains chemistry. The outermost (valence) electrons determine bonding and reactivity, and the periodic table's shape mirrors subshell filling — the s-block, p-block, d-block, and f-block. Elements in the same group share valence configurations and therefore similar behavior, while trends like atomic radius, ionization energy, and electron affinity track how tightly the nucleus holds the outer electrons. Ions form by gaining or losing electrons toward stable, often noble-gas, configurations.
The MCAT tests atomic structure and electron configuration in its chemical and physical foundations section: expect questions on writing configurations, identifying valence electrons, exceptions like chromium and copper, and connecting configuration to periodic trends.
Key takeaways
- Electron configuration lists how electrons fill shells, subshells, and orbitals, e.g. 1s² 2s² 2p⁶ 3s¹ for sodium.
- The Aufbau principle, Pauli exclusion principle, and Hund's rule govern ground-state filling.
- Subshells hold 2 (s), 6 (p), 10 (d), and 14 (f) electrons.
- Valence configuration determines bonding, group behavior, and periodic trends.
- The MCAT tests configurations, valence electrons, and their link to the periodic table's structure.