Question

The reason for greater range of oxidation states in actinoids is attributed to:
A . 4f and 5d levels being close in energies.
B . The radioactive nature of actinoids.
C . Actinoid contraction.
D . 5f, 6d and 7s levels having comparable energies.

Answer 1

Hint:
Actinides are F-block elements(atomic number 89 to103) with the general electronic configuration of outermost shell is \[\left[ {{\text{Rn}}} \right]{\text{5}}{{\text{f}}^{{\text{1 - 14}}}}{\text{6}}{{\text{d}}^{{\text{0 - 1}}}}{\text{7}}{{\text{s}}^{\text{2}}}\]. Where the last electron enters to the inner 5f-orbital of the actinides. Actinides are also known as rare earth metals.

Complete step by step answer:
All actinides have atomic numbers greater than 83 i.e. why actinides are generally radioactive and unstable. Due to radioactivity these metals are also toxic.
Due to the low screening power of f-orbitals, addition of electrons to the 5f-orbital results in an increase of effective nuclear charge which causes the reduction of size of actinides and similarity of their chemical and physical properties. This effect is called actinoid contraction.
Now according to the Aufbau principle(L+S value) the energy order of the orbitals should be \[5f < 6d < 7s.\]but due to the more diffuse orbitals their energy becomes more or less the same. As a result, electrons can be excited easily . Due to this reason actinides shows greater range of oxidation states. But if we consider lanthanides due to the comparatively small size of 4f orbital they have a limited number of oxidation states.

So, the correct answer is D.

Note:
The orbitals of actinides are more diffuse compared to lanthanides. The energy difference of orbitals is very low for actinides.