What is the principle that electrons fill orbitals starting from lowest energy?

The principle that electrons fill orbitals starting from lowest energy is the Aufbau principle.

“Aufbau” is German for “building up,” reflecting the idea that an atom’s electron configuration is constructed by adding electrons to the lowest available energy subshells first. The usual sequence begins 1s, 2s, 2p, 3s, 3p, 4s, and 3d. This arrangement generally produces the most stable ground-state configuration.

The rule is closely associated with the Madelung rule, which predicts filling order using the n + ℓ pattern. It is not the same as Hund’s rule: Hund’s rule describes how electrons occupy orbitals of equal energy, while the Pauli exclusion principle limits each orbital to two opposite-spin electrons. The Aufbau principle was formulated in the early 1920s and is linked to Niels Bohr’s work on electron configurations.

It is an excellent general guide, but not an absolute law. Some transition-metal atoms have exceptions because subshell energies depend on the atom’s full electron arrangement. Ionization can also change the relative energies, so electrons are not always removed in the reverse order in which introductory diagrams show them being added.