The electronic configurations of the first four members of the actinide series cannot be written accurately in a regular manner because of

Small difference in energy between 5f and 6d orbitals is the reason the electronic configurations of the first four members of the actinide series cannot be written accurately in a regular manner. The Aufbau rule predicts that 5f orbital is more stable than 6d orbital and electrons should enter 5f first then 6d. But the reality is different. The energies of 5f and 6d orbitals are so similar that the incoming electrons have the option to go anywhere between these two orbitals. This is the reason why the first four actinoids don’t have an exact electronic configuration. 6. The electronic