Topic overview

Start with the big picture

Configurations are built using the Aufbau principle, the Pauli exclusion principle, and Hund’s rule. Together, these explain orbital filling order, the limit of two opposite-spin electrons per orbital, and how electrons occupy equal-energy orbitals before pairing. Subshell and shell capacities help organize electrons, while quantum numbers specify an electron’s state. The lesson also connects subshell filling to periodic-table blocks and shows how noble-gas shorthand represents inner electrons. Some atoms have stability-related exceptions, and cations lose electrons from the highest principal shell first. Effective nuclear charge can change relative subshell energies; electron transitions between quantized levels are associated with atomic spectral lines.