Question

Poor shielding of nuclear charge by $d$ or $f$ orbital electrons is responsible for which of the following facts?

• A. Atomic radius of Nb (4d-series) is comparable to that of Ta (5d-series)
• B. The 1st ionisation enthalpy of copper is less than that of zinc
• C. The value of electron gain enthalpy is more negative for sulphur than for oxygen
• D. The 1st ionisation energy for gold is greater than that the silver

Answer 1

Answer: (A), (D)

The correct options are
A Atomic radius of Nb (4d-series) is comparable to that of Ta (5d-series)
D The 1st ionisation energy for gold is greater than that the silver

Due to the diffuse size of the $d$ and $f$ orbitals, they do not shield the nuclear charge very effectively. Therefore, there is significant reduction in the size of the ions, just after $d$ or $f$orbitals have been filled completely.
Atomic radii of $Nb(N{b}^{3+}=0.72\stackrel{o}{A})$ and $Ta(T{a}^{3+}=0.72\stackrel{o}{A})$ are almost identical due to lanthanide contraction.
This is also the reason for the higher ionization energy of gold than silver.