Which electron configuration is correct for filling orbitals?

The correct principle for filling orbitals is that one must fill each orbital singly before pairing electrons within those orbitals. This concept is derived from Hund's Rule, which states that electrons will occupy degenerate (equal energy) orbitals singly and with parallel spins before any orbital is doubly occupied. This approach minimizes electron-electron repulsion, resulting in a more stable configuration. As a result, this filling order not only reflects how electrons are organized in an atom but also lays the groundwork for understanding chemical reactivity and bonding.

In stark contrast, pairing all electrons before filling new orbitals would violate this fundamental principle and lead to increased repulsion, thereby rendering the atom less stable. Skipping lower energy levels would also go against the Aufbau principle, which dictates that electrons should fill the lowest available energy levels first. Lastly, the notion of filling only as needed for stability does not accurately convey the systematic approach of filling orbitals that is guided by electron configuration rules. Thus, the principle of filling singly before pairing correctly represents the orderly method by which electrons occupy atomic orbitals.