Which orbital is filled last in the electron configuration?

The last orbital to be filled in the electron configuration is the 6s orbital. This is due to the Nature of the Aufbau principle, which states that electrons occupy the lowest energy orbitals first before they fill higher energy levels. The order of filling orbitals follows the sequence dictated by their increasing energy levels.

In the case of the 6s orbital, it is filled before the 5d and 7p orbitals because, despite being in higher principal energy levels, the s orbitals generally have lower energy than the d and p orbitals in the same principal energy level. The 4p, 5d, and 7p orbitals are higher in energy compared to the 6s, which is why the 6s orbital is the last to be filled before moving to the next levels.

As a result, when constructing electron configurations, you would complete the filling of the 6s orbital before moving to the 5d or 7p orbitals, confirming that it is indeed the last one to receive electrons as the atomic number increases.