Question

The electronic configuration of actinides cannot be assigned with degree of certainty because of:
(A) Overlapping of new orbitals.
(B) Free movement of electrons over all the orbitals.
(C) Small energy differences between $5f$ and $6d$ levels.
(D) None of the above

Answer 1

In the given question we are asked about the electronic configuration of Actinides which are an elemental series in the periodic table, they are separated from the main $7$ period and $18$ groups of the periodic table due to their characteristic property and their easy study.

Complete step by step solution:
Actinides are the elements from atomic number 90 to 103 starting from element Actinium. They consist of naturally occurring elements of thorium, uranium, and protactinium and eleven transuranic elements (artificially produced by nuclear reactions).
All of the elements in the series are radioactive. The term ‘actinide series’ is named due to the first element of the series, Actinium.
All actinide elements release a large amount of energy on radioactive decay. Thorium and Uranium are the most abundant naturally occurring actinides on earth, whereas plutonium is obtained artificially.
These elements are used in nuclear weapons and nuclear reactors. Uranium and thorium have wide current uses, whereas americium is used in modern smoke detectors.
The general electronic configuration of actinides is $[Rn]$ $5{f^{1 - 14}}$ $6{d^{0 - 1}}$ $7{s^2}$ . Here $[Rn]$ is the electronic configuration of the nearest noble gas which is Radium.
The maximum oxidation state first increases up to the middle of the series and then decreases i.e. it increases from $+ 4$ for $Th$ to $+ 5$ , $+ 6$ and $+ 7$ for $Pa$ , $V$ and $Np$ but decreases in the succeeding elements.
As we have known that the there is a very narrow gap between the energies of $5f$ and $6d$ orbitals and electrons can jump from $5f$ to $6d$ easily and vice-versa that’s why we cannot assign an electronic configuration to actinides with full certainty

Hence, the correct option is - (C) Small energy differences between $5f$ and $6d$ levels.

Note:
Actinides show variable oxidation states because of the narrow energy gap between $5f$ , $6d$ and $7s$ orbitals. Though ${3^ + }$ is the most stable oxidation state, other oxidation states are also possible due to the shielding of $f$ -electrons.