Question

Which of the following statements is true with respect to 3d series?

• A. Atomic radii gradually decreases from Sc to Zn
• B. Most of these elements have very similar atomic radii. There is hardly any appreciable variation at all
• C. Atomic radii gradually decreases from Sc to Cr
• D. Atomic radii gradually increases from Sc to Cr

Answer 1

The correct option is C Atomic radii gradually decreases from Sc to Cr

Atomic radii gradually decreases from Sc to Cr. This is due to increase in nuclear charge which tends to pull ns electron inward, i.e., it tends to reduce the atomic radius. Initially, with the progressive addition of an extra electron to (n-1) d-orbital, the screening effect is ineffective. Hence, the attractive force of the nucleus is felt more on the valence electrons. This is applicable from Sc to Cr.
Immediately after Cr, there is Mn, which shows an abrupt increase in atomic radius of 137 pm (from Chromium’s atomic radius of 129 pm). After Manganese, the three elements – Iron, Cobalt and Nickel all have very similar atomic radii. After Manganese, the electrons in the (n-1) d subshells begin to get paired and start to have appreciable screening effect on the valence ns electrons. Copper shows a marginal increase in its atomic radii at 128 pm from Nickel’s 125 pm. Zinc has a completely filled 3d orbital and has a significantly higher atomic radius of 137 pm.
Effective nuclear charge: The effective nuclear charge is always less than the total number of protons present in a nucleus due to shielding of valence electrons by the electrons occupying the inner orbitals. Since, the 3d orbital is progressively filled, can we say that the increasing effective nuclear charge is responsible for the decrease in atomic radii from Sc to Cr? Yes, definitely we can.