Question

Which of the following has the highest proton affinity?
A.Stilbene
B.Arsine
C.Phosphine
D.Ammonia

Answer 1

Hint: According to the Lewis acid-base theory, a base is defined as a lone pair donor while an acid is defined as the lone pair acceptor. The molecule in which the lone pair can be accommodated will have low proton affinity. Smaller the size of molecules, higher is its proton affinity.

Complete step by step answer:
Stilbene- the hydride of tin is called stilbene and has the formula, ${\text{Sn}}{{\text{H}}_3}$. Tin is the fourth member of the arsenic family and belongs to the 5th period. Hence its last shell is the fifth shell where the lone pair is there in the 5p orbital and due to the large size of the orbital, the lone pair is well-distributed.
Arsine- the hydride of arsenic with the chemical formula, ${\text{As}}{{\text{H}}_3}$. The lone pair on the arsenic atom are present in the 4p orbital and the size of 4p being large, the lone pair is accommodated well.
Phosphine- the hydride of phosphorus with the molecular formula \[{\text{P}}{{\text{H}}_3}\]. The lone pair on the phosphorus atom is in the 3p level, which is smaller than the 4p orbital but has enough space for the accommodation of the lone pair of electrons.
Ammonia- the hydride of nitrogen with molecular formula \[{\text{N}}{{\text{H}}_3}\]. The lone pair of electrons being in the small 2p subshell as most available to the positively charged protons.

Hence, the highest proton affinity will be shown by ammonia, option D.

Note:
All the elements of the group 15 have a lone pair of electrons in their valence shell that can be donated to a proton. The availability of the lone pair of electrons depends on the size of the orbital. Larger orbitals have more space for the accommodation of the lone pair so they are less easily donated.