The ionization energies of the elements of first transition series are given below:

* in kJ mol^–1

The following generalizations can be obtained from the ionisation energy values given above.

(i) The ionisation energies of these elements are high and in the most cases lie between those of s- and p-block elements. This indicates that the transition elements are less electropositive than s-block elements.

Explanation : Transition metals have smaller atomic radii and higher nuclear charge as compared to the alkali metals. Both these factors tend to increase the ionisation energy, as observed.

(ii) The ionisation energy in any transition series increases in the nuclear with atomic number; the increase however is not smooth and as sharp as seen in the case of s and p-block elements.

Explanation : The ionisation energy increases due to the increase in the nuclear charge with atomic number at the beginning of the series. Gradually, the shielding effect of the added electrons also increases. This shielding effect tends to decrease the attraction due to the nuclear charge. These two opposing factors lead to a rather gradual increase in the ionisation energies in any transition series.

(iii) The first ionisation energies of 5d-series of elements are much higher than those of the 3d and 4d series elements.

Explanation : In the 5d-series of transitions elements, after lanthanum (La), the added electrons go to the next inner 4f orbitals. The 4f electrons have poor shielding effect. As a result, the outermost electrons experience greater nuclear attraction. This leads to higher ionisation energies for the 5d- series of transition elements.

Solved example 1: Ionisation potential values of d-block elements as compared to ionization potential value of f-block elements are

(a) Higher (b) Equal

(c) lower (d) All of these

Ans: a