Read the following passage and answer the next five questions based on it. (37-41)
Transition Series Elements:

Sc Ti V Cr MnFeCoNiCuZn

YZrNbMoTcRuRhPdAgCd
La Hf Ta WReOsIrPtAuHg

In any transition series, as we move from left to right the d-orbitals are progressively filled and their properties vary accordingly.
f-block Elements:

Ce Pr Nd PmSmEuGdTbDyHoErTmYbLu
Th Pa UNpPuAmCmBkCfEsFmMdNoLr

The above are the two series of f-block elements in which the chemical properties won’t change much. The 5f-series elements are radioactive in nature and mostly are artificially synthesized in laboratories and thus much is not known about their chemical properties.

Why do the actinoids exhibit a higher number of oxidation states than lanthanoids?

The correct option is: Energy difference between 5f and 6d is less with respect to the energy difference between 4f and 5d.

To understand why this is the case, we need to compare the electronic configurations and energy levels of lanthanoids and actinoids:
1. Lanthanoids involve the filling of the 4f orbitals. The energy gap between the 4f and 5d subshells is relatively large. Because of this large energy difference, electrons in the 4f orbitals are deeply buried and do not easily participate in chemical bonding alongside 5d and 6s electrons. Consequently, lanthanoids exhibit a limited number of oxidation states, with +3 being the most common and stable.
2. Actinoids, on the other hand, involve the filling of the 5f orbitals. The energy difference between the 5f, 6d, and 7s subshells is much smaller compared to the energy difference between the 4f, 5d, and 6s subshells of lanthanoids.

Due to this extremely small energy gap, electrons from the 5f orbitals, in addition to the 6d and 7s orbitals, can easily be shared or lost to participate in bonding. This allows actinoids to display a much wider range of oxidation states (varying from +3 up to +7 in some elements like Uranium, Neptunium, and Plutonium) compared to the lanthanoids.