Question

Which of the following has least first ionization enthalpy?

• A. C
• B. Pb
• C. Sn
• D. Ge

Answer 1

The correct option is C

Sn

Try to think about the factors affecting the value of ionization energy here.
It is mainly:
(a) Atomic size(b) Shielding effect
Down the group $(\downarrow )$, the first ionization enthalpy decreases. There is a sharp decreases from C to Si, whereas from Pb to Sn, there is a slight increase.
$C>Si>Ge>Pb>Sn$
Exception
A decrease in the first ionization enthalpy from C to Sn is due to increase in atomic size and shielding effect of the intervening electrons (i.e. the electrons present in-between) the nucleus and valence shell electrons) which overcomes the effect of increased nuclear charge.
An increase in the first ionization enthalpy from Sn to Pb is because the effect of increased nuclear charge overcomes the shielding effect caused by the presence of fully filled lesser shielding $4f^{14}and5d^{10}$ orbitals in Pb.
Therefore the correct option is not Pb.
It is Sn.