Question

Which one of the following bonds would you expect to be the most polar?
A) B-H
B) N-H
C) P-H
D) Al-H
E) C-H

Answer 1

The polarity of the bond is the separation of electric charge along a bond. The terms non-polar and polar are used for covalent bonds primarily. The polarity of the bond can be found by the difference of the electronegativities of the atoms across the bond.

Complete answer:
The strength of the polarity depends on the electronegativities of the respective atoms that constitute the molecule. The molecules that have the highest difference in their electronegativities have the most polar bond. We will find out the difference in the electronegativities one by one. Difference in electronegativity can be found out by: $\Delta {X_{A - B}} = {X_A} - {X_B}$
Note that the electronegativity of Hydrogen is = 2.20
A) Electronegativity of B = 2.04
Difference in electronegativity will be : $\Delta {X_{B - H}} = 2.20 - 2.04 = 0.20$
B) Electronegativity of N = 3.04
Difference in electronegativity will be : $\Delta {X_{N - H}} = 3.04 - 2.20 = 0.84$
C) Electronegativity of P = 2.19
Difference in electronegativity will be : $\Delta {X_{P - H}} = 2.20 - 2.19 = 0.01$
D) Electronegativity of Al = 1.61
Difference in electronegativity will be : $\Delta {X_{Al - H}} = 2.20 - 1.61 = 0.59$
E) Electronegativity of C = 2.55
Difference in electronegativity will be : $\Delta {X_{C - H}} = 2.55 - 2.20 = 0.35$
The order of electronegativity difference can be given as: $N - H > Al - H > C - H > B - H > P - H$
Hence the N-H bond has the highest electronegativity difference and will be the most polar.
The correct answer is Option (B).

Note:
If highly electronegative atoms such as Fluorine, Oxygen are present, then their bond will be more polar than N-H bond. O-H bond, C-F bond are found to be highly polar. If there are no charge differences present across a bond, no polar bond will be present. Such molecules are said to be non-polar. Polar bonds can be symmetrical in nature too.