Generally lanthanides and actinides have electron configurations that follow the Aufbau rule. There are some exception, in a few of the lanthanide and actinide elements.

The electronic configuration of the lanthanoids is4f1-145d0-16s2

Lanthanum, the d-block element preceding this series, has the electronic configuration [Xe]5d16s2.

The reason why gadolinium has a 5d1arrangement is that this leaves a half-filled 4f subshell, Lutetium has a 5d1arrangement because the f subshell is already full.General electronic configuration of actinoids is5f1-146d0-17s2

The difference in energy between the 5f and 6d orbitals is small for the first four elements, thorium, protactinium, uranium and neptunium, the electrons in these elements and their ions may occupy the 5f or the 6d levels, or sometimes both.

The reason why curiumhas a 6d1arrangement is that this leaves a half-filled 5f subshell, Lawrencium has a 6d1arrangement because the f subshell is already full.

[Rn]5f^(1-14)6d^(0-1)7s^2