Periodic Properties

The third ionization enthalpy of transition elements increases as we move across the period due to increasing nuclear charge. Manganese has a half-filled d-subshell configuration, making it relatively stable and requiring more energy to remove the third electron. Hence, Manganese (Z = 25) has the highest third ionization enthalpy.

Lanthanoid contraction refers to the gradual decrease in the size of the lanthanoid ions (from La to Lu ) across the lanthanide series. Despite the increasing nuclear charge, the addition of electrons occurs mainly in the 4f orbitals, which do not effectively shield the increased positive charge. As a result, the effective nuclear charge experienced by the electrons increases, leading to a gradual contraction of the ionic radius. This contraction is particularly significant between elements such as La and Lu and affects the properties of the elements and their compounds, including the stability of their coordination complexes and their atomic size.
% Solution

Zr and Hf are very similar in size due to lanthanoid contraction, which makes it difficult to separate them effectively using conventional methods.