NEET Practice Questions (MCQs) with Answers & Solutions

The lanthanide contraction refers to the steady decrease in the ionic radii of the lanthanide elements with increasing atomic number. This contraction causes elements following the lanthanides in the periodic table to have similar sizes. Consequently, Zr and Hf, which are not lanthanides but are affected by the lanthanide contraction, have similar covalent and ionic radii.

Actinides exhibit a greater number of oxidation states compared to lanthanides primarily because the energy difference between the 5f and 6d orbitals in actinides is less than the energy difference between the 4f and 5d orbitals in lanthanides. This smaller energy difference allows electrons to be more easily excited to higher oxidation states in actinides. As a result, actinides can exhibit a wider range of oxidation states.

In the highest oxidation states, transition metals typically show acidic character due to the formation of high oxidation state oxides which are acidic in nature. For instance, Mn in +7 oxidation state forms $MnO_4^-$ which is acidic. Therefore, the statement 'In the highest oxidation states, the transition metals show basic.' is incorrect.

$Cr^{3+}$ is most stable in aqueous solution because it has a half-filled t2g configuration (3d^3) which provides extra stability due to exchange energy. This stability is greater compared to $V^{3+}$, $Ti^{3+}$, and $Mn^{3+}$ ions.

The maximum number of oxidation states is shown by elements with a partially filled d-subshell. In this case, $3d^5 4s^2$ (which corresponds to Manganese) has the most number of unpaired electrons and can exhibit multiple oxidation states ranging from +2 to +7. The half-filled $3d$ subshell provides stability across these states.