Which principle asserts that orbitals of equal energy fill singly before pairing occurs?

The correct answer is rooted in Hund's rule, which is instrumental in understanding how electrons are arranged in orbitals of equal energy, known as degenerate orbitals. According to this principle, when filling these orbitals, electrons will first occupy each orbital singly and with parallel spins before any orbital receives a second electron.

This approach minimizes electron-electron repulsions within an atom, leading to a more stable configuration. For example, when filling the p, d, or f orbitals where there are multiple orbitals of the same energy level, following Hund's rule results in a scenario where there is maximized electron spin and minimized repulsion, thereby enhancing stability.

In contrast, other principles such as the Pauli exclusion principle dictate that no two electrons can have the same four quantum numbers, and while the Aufbau principle describes the order of filling orbitals based on increasing energy, they do not specifically address the filling of degenerate orbitals. The octet rule primarily concerns the stability of atoms based on having a complete set of valence electrons, typically eight, and does not directly relate to the specific order in which orbitals fill.