Which orbital fills with electrons last during the Aufbau process?

During the Aufbau process, the filling of atomic orbitals follows a specific order based on their energy levels, which is influenced by both the principal quantum number (n) and the azimuthal quantum number (l). As a general rule, lower-energy orbitals are filled before higher-energy ones.

The F orbital, which corresponds to an azimuthal quantum number of 3 (l=3), is the highest-energy orbital compared to the S (l=0), P (l=1), and D (l=2) orbitals. As such, F orbitals are filled last in the sequence of electron configuration. This inclusivity of energy levels means that transitions into F orbitals occur only after all lower-energy orbitals have been filled.

The order of filling can generally be summarized by the Aufbau principle, and it demonstrates that the F orbitals have a higher energy level than the S, P, and D orbitals, which leads to their being filled last. Understanding this sequence is crucial for predicting an atom's electron configuration and behavior in chemical bonding.