Definition of ionization enthalpy.
Ionization enthalpy is defined as the amount of energy required to remove the outermost electron completely from a gaseous atom in its ground state.
Explanation of ionization enthalpies of the d-block elements.
i. The ionization enthalpies of transition elements are higher than the ionization enthalpies of s-block elements and lower than the ionization enthalpies of p-block elements. This is due to the trends in atomic radii and nuclear charge.
ii. The atomic radii of transition elements are lower than the atomic radii of s-block elements and higher than the atomic radii of p-block elements.
iii. The nuclear charges of transition elements are higher than the nuclear charges of s-block elements and lower than the nuclear charges of p-block elements.
iv. As the atomic number increases across a transition series, the first ionization energy increases with some irregularities. The irregularities are due to shielding of the valence electrons from the nucleus by the added (n–1) d-electrons. Thus, the effect of increased nuclear charge is opposed by the screening effect.
v. The increasing order of the first ionization enthalpies of elements is: First transition series < second transition series < third transition series The third transition series comprises of elements having atoms which possess filled 4f-orbitals. On account of the peculiar diffused shape of 4f-orbitals, they exhibit poor shielding. Thus, the valence electrons experience greater nuclear attraction.
vi. As a result, greater amount of energy is required to ionise the elements of the third transition series.
Thus, the ionization enthalpies of the third transition series elements are much higher than those of the first and second series.