Final answer:
The irregular variation of ionization enthalpies in the first series of transition elements is due to factors like shielding effect, electron-electron repulsion, and nuclear charge. The shielding effect results in decreased ionization energy due to the reduced effective nuclear charge, while electron repulsion within the same subshell and increased nuclear charge across a period affect the energy required to remove an electron. Option number b is correct.
Step-by-step explanation:
To account for the irregular variation of first and second ionization enthalpies in the first series of transition elements, we consider factors such as the shielding effect, electron-electron repulsion, exchange energy, and nuclear charge. The shielding effect refers to the phenomenon where inner electrons shield the outer electrons from the full attractive force of the nucleus, reducing the effective nuclear charge experienced by the valence electrons.
This effect contributes to irregular trends in ionization energies.
Electron-electron repulsion within the same valence shell also plays a role in the varied ionization energies of transition metals as electrons within the same shell do not shield each other effectively from the nucleus. When we consider nuclear charge, it becomes apparent that as the atomic number (Z) increases across a period, the net positive charge experienced by the electrons also increases, leading to a general rise in ionization energies. However, this trend is not uniform due to the various shielding effects and subshell configurations.
Group 13 elements exhibit lower ionization energies compared to Group 2 elements largely because of the difference in electron configurations; s electrons are lower in energy and harder to remove than p electrons within the same shell. Group 13 elements have their outermost electron in a p subshell, which experiences more shielding and is, therefore, higher in energy and easier to remove compared to the s electrons in Group 2 elements.