Question

Which of the following options give the correct arrangement of the atomic radii of the 3d, 4d, and 5d transition series of elements? 

(a) atomic radii of 3d < atomic radii of 4d < atomic radii of 5d 

(b) atomic radii of 3d < atomic radii of 4d ≈ atomic radii of 5d 

(c) atomic radii of 3d ≈ atomic radii of 4d > atomic radii of 5d 

(d) atomic radii of 3d > atomic radii of 4d > atomic radii of 5d

Answer 1

(b) atomic radii of 3d < atomic radii of 4d ≈ atomic radii of 5d 

• d-block elements are the elements in which the differentiating electron enters into a d-orbital of the penultimate shell (n-1).

• These are thus the elements of group 3 to group 12 having outer configurations ns^1-2(n-1)d^1-10.

• The d-block elements consist of three series, ten elements are present in each series.

3d series (first series) from scandium (Sc) - zinc (Zn).

4d series (second series) yttrium (Y) - cadmium (Cd).

5d series (third series) lanthanum (La) - mercury (Hg)

Atomic radius trend:

• The radius of an atom increases with the increase in the number of shells, so the atomic radius of the 4d series is higher than that of the 3d series.

• But the atomic radius of the 5d transition series is virtually the same as those of the corresponding members of the 4d series due to the filling of 4f-orbitals which have poor shielding or screening effect (lanthanide contraction).

Therefore, atomic radius increases from the 3d series to the 4d series and atomic radius of the 4d series is virtually the same as the 5d series.

Hence, the required answer is option (b) atomic radii of 3d < atomic radii of 4d ≈ atomic radii of 5d.