Question

Explain the trend in atomic size of first row elements of transition series.

Answer 1

i. The atomic radii for transition metals are smaller than their corresponding s-block elements. 

ii. The atomic radii of the elements of a given series decrease with the increase in atomic number but this decrease becomes small after middle of the series.

Element	Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn
Atomic radii (pm)	144	132	122	118	117	117	116	115	117	125

In the first transition series, the atomic radius gradually decreases from scandium to chromium but from chromium to copper, it is nearly the same. Similar behaviour has been observed in the second and third transition series.

iii. The decrease in atomic radii in each series, in the beginning, is due to an increase in nuclear charge across the period, which tends to pull the ns electrons inward, i.e., it tends to reduce the size.

iv. The addition of extra electrons to (n1) d-orbitals provides the screening effect. As the number of d-electrons increases, the screening effect increases. Thus, there are two operating effects namely screening effect and nuclear charge effect which oppose each other. In the midway onwards of the series, both these effects become nearly equal and thus, there is no change in atomic radii inspite of the fact that atomic number increases gradually.

v. The values of atomic radii at the end of each series are slightly higher which is due to electronelectron repulsions among (n1) d-electrons. These repulsions become predominant at the end of each series and thus resulting in the increase in size.

vi. In a vertical row, the atomic radii is expected to increase from top to bottom. Therefore, the atomic radii of transition metals of second series have larger values than those of the first transition series. However, the transition metals of third series except the first member, lanthanum, have nearly the same radii as metals of second transition series above them which is due to the lanthanoid contraction.