Purdue University
Purdue University: Orbital Shells
This site from the Purdue University provides an overview of atomic orbitals and how quantum numbers specify these orbitals. Includes a model that can be rotated and examined in 3D. Includes learning exercises and answers.
Purdue University
Purdue University: The Aufbau Principle
At this site from Purdue University, Electron Configurations, the Aufbau Principle, Degenerate Orbitals, and Hund's Rule are detailed. Includes learning exercises and answers.