Question

What is the reason why Boron Trifluoride (\[B{F_3}\]) is a nonpolar molecule, whereas ammonia (\[N{H_3}\]​) is a polar molecule?
A. \[B{F_3}\]​ has no hydrogen bonding and \[N{H_3}\]​ does
B. \[B{F_3}\]​ is a triangular planar and \[N{H_3}\]​ is pyramidal
C. \[B{F_3}\]​ is a Lewis base and \[N{H_3}\]​ is a Lewis acid
D. the B-F bond is less polar than the N-H bond
E. boron is more electronegative than nitrogen

Answer 1

Hint: Firstly, Electron-negativity checks the difference between the electron-negativity of both the molecules.

Complete step by step solution:
\[B{F_3}\]is a molecule which consists of B-atom and F-atom belongs to the group 13 and group 17 respectively. The bond between the molecules is a single bond as they share a pair of electrons. Now, the F-atom requires only one electron to fulfill the octet which makes it the most electron-negative atom as it also has a small size which increases the attraction. The B-atom is an electro-positive atom. Thus, the bond B-F is polar bond but in a molecule the hybridisation of \[B{F_3}\] is \[s{p^2}\] and has geometry trigonal planar. In trigonal planar geometry, the atoms are at \[12{0^o}\]at equal length which cancel out each other having zero dipole moment. Therefore the\[B{F_3}\] is a nonpolar molecule.

\[N{H_3}\]​, molecules consist of N-atoms and H-atoms which belong to group 15 and group 1 respectively. The bond between the molecules is a single bond between the N-atom and H-atom sharing pair of electrons. Now, the N-atom has 5 electrons in the valence shell which take part in the bonding where the H-atom has one electron and require only one electron to complete its duplet. The N-atom is an electron-negative atom as it requires 3 electrons to attain noble gas configuration whereas the H-atom has only one electron and the most electron-positive atom. Thus, the N-H bond is a polar bond and the molecule is also a polar molecule because of the presence of the lone pair in the molecule. The hybridisation is sp3 and the geometry is tetrahedral having 1090 bond angles. But due to the presence of lone pairs at one side of tetrahedral geometry the bond angle decreases by bond pair-lone pair repulsion. In tetrahedral geometry or trigonal planar shape the bond angle is not same hence cannot cancel each other, in result, the dipole moment of the molecule is not zero. Therefore, the \[N{H_3}\] is polar molecule
The correct option is B.

Note: Geometry may be defined as the structure of the molecule in which all the bond pair and lone pair or non-bonding orbital is involved whereas Shape may be defined as the structure in which only bond pairs in the molecule are involved.