Question

Why is ionisation enthalpy of Zinc, Cadmium and Hg in the increasing order, when ideally, all periodic groups have decreasing order of enthalpy downward?

An argument could be that F and D orbitals show less screening effect, but this is true for all the transition metal groups.
Another argument about fully filled shells being more stable also arises,... but even 3rd, 4th and 5th groups have fully filled S shells. When it comes to D orbitals, how does it being fully filled affect the ionisation enthalpy? It is not the valence shell afterall.

Answer 1

The ionization enthalpy of zinc, cadmium, and mercury does not follow the regular trend due to pseudo noble gas configurations in the 3d subshell, which make the atoms more stable and increase the energy required to remove an electron.

Step-by-step explanation:

The trend of the ionization enthalpies in the periodic table is generally that they increase across a period and decrease down a group. However, there are some exceptions to this trend, and one of them is observed in the case of zinc, cadmium, and mercury.

The ionization enthalpy of zinc, cadmium, and mercury does not follow the typical trend because of the concept known as pseudo noble gas configurations. The 3d subshell in these elements is filled or almost filled, which makes the atom more stable. As a result, it requires more energy to remove an electron from these atoms, leading to a higher ionization enthalpy compared to what would be expected based on the group trend.

It is important to note that this exceptional behavior is specific to the transition metals and not observed in other groups of the periodic table.