Question

Actinoid contraction is greater than lanthanoid contraction because the shielding effect of 5f orbitals is poorer than that of 4f orbitals, then why is the ionisation enthalpies of actinoids less than that of the lanthanoids?

Answer 1

In the 5d-series of transitions elements, after lanthanum (La), the added electrons go to the next inner 4f orbitals. The 4f electrons have poor shielding effect. As a result, the outermost electrons experience greater nuclear attraction. This leads to higher ionisation energies for the 5d- series of transition elements.

Even though the actinoid contraction is greater,the greater nuclear attraction from the nucleus due to poor shielding of 4f orbit account for that contraction.

Also you can consider this too..

Ionisation enthalpy is the ability of elements to lose electrons. Down the group, the ionisation enthalpy decreases. This is due to the increase in the size of the atom due to the addition of one new orbit in which the Valence electrons are found. The nuclear pull in the Valence electrons in the seventh orbit is lesser than the nuclear pull on the electrons in the sixth orbit. Therefore actinoids have lesser ionisation enthalpy, than lanthanides.