lanthanum has one e- in 5d orbital even though it is before elements with e- in 4f orbital. why is it so as it violates aufbau principle? Answer: Electronic Configuration of Lanthanides: as the 4f and 5d electrons are so close in energy it is not possible to decide whether the electron has entered the 5d or 4f orbital.

There are two main exceptions to electron configuration: chromium and copper. In these cases, a completely full or half full d sub-level is more stable than a partially filled d sub-level, so an electron from the 4s orbital is excited and rises to a 3d orbital.

Hence Actinium violates the Aufbau’s Principle. According to the Aufbau principle, the orbital with the lower energy level must be filled first completely, before moving on to the next orbital. 4s orbital is a lower energy orbital as compared to 3d.

Orbitals fill in order of energy. So 5D fills before 4F in some cases simply because the 5D energy levels are lower than the 4F levels for some. The nuclear charge is insufficient to contract the 4F orbitals and lower their energy well below the 5D. 4F then fills first until Gadolinium is reached.

Re: Why 5d orbital filled first then 4f orbital? Answer: For the elements after Xe the valence orbitals become closer in energy, and therefore small changes in their values results in different orbitals being occupied.

Here, (n+l) of 6s orbital is 6 and that of a 4f orbital is 7 and hence 4f orbital is filled before 6s orbital.

Characteristics of lanthanides

5s, 5p, and 6s orbitals are all lower than 4f orbitals. In other words, before you can get an electron into a 4f orbital, you must first fill up the 5s orbitals, the 5p orbitals and the 6s orbitals.

The aufbau principle explains how electrons fill low energy orbitals (closer to the nucleus) before they fill higher energy ones. Similar confusion occurs at higher levels, with so much overlap between the energy levels that the 4f orbitals do not fill until after the 6s, for example.