The radii (metallic) of `Fe,CO and Ni` are nearly same.
This is due to:

Options

A. Lanthanide contraction
B. The fact that successive addition of d-electrons screens (4s) from the inward pull of the nucleus.
C. increase in radii due to increase in `n` is compensated by decrease in radii due to increase in effctive nulcear charge(Z).
D. Atomic radii do not remain constant but increase in a normal gradation.

Answer

B

Explanation

After mid way i.e. after `Cr` as the electrons enters the last but one shell, the added d-electron shield the outer most electrons. Hence with the increase in the d-electron screening effect increase . This counter balance the increased nuclear charge. As a result , the atomic radii remain practically same after chromium.