Knowing that H has an electronegativity value between those of B and N, what conclusion can be drawn from the fact that BH3 is nonpolar but NH3 is polar?
a) The boron atom in BH3 is sp3 hybridized, while the nitrogen atom in NH3 is sp2 hybridized.
b) The nitrogen atom in NH3 has a lone pair of electrons, whereas the boron atom in BH3 does not.
c) BH3 forms a ring structure.
d) The nitrogen atom in NH3 lacks enough electrons to complete its octet.
The correct answer and explanation is :
The correct answer is b) The nitrogen atom in NH3 has a lone pair of electrons, whereas the boron atom in BH3 does not.
Explanation:
To understand why BH3 (borane) is nonpolar and NH3 (ammonia) is polar, we need to examine their molecular structures and the nature of the bonds in these molecules.
- Electronegativity: Both boron and nitrogen are elements with electronegativity values between those of B (2.04) and N (3.04). However, electronegativity alone does not explain why the molecules have different polarities.
- Molecular Shape and Symmetry:
- BH3 (Borane): Boron in BH3 forms three single bonds with hydrogen atoms. The electron pairs involved in bonding are arranged in a trigonal planar geometry, where all the bond angles are 120°. Boron, however, does not have a lone pair of electrons. Due to its trigonal planar structure, the distribution of charge around the central atom is symmetrical. Because the bonds are evenly spaced and there is no lone pair of electrons to create an asymmetry, the molecule as a whole is nonpolar.
- NH3 (Ammonia): In NH3, nitrogen forms three single bonds with hydrogen atoms. However, nitrogen also has a lone pair of electrons, which significantly affects the shape of the molecule. The presence of the lone pair causes the molecular geometry to be trigonal pyramidal, rather than trigonal planar. The lone pair of electrons creates an electron-rich region that makes the molecule asymmetrical, and thus, the bond dipoles do not cancel out. This results in a polar molecule with a net dipole moment pointing towards the nitrogen atom.
- Why the Lone Pair Matters: The key difference between NH3 and BH3 is that nitrogen in NH3 has a lone pair, whereas boron in BH3 does not. This lone pair causes the asymmetry in NH3, making it polar, while the absence of such a lone pair in BH3 allows for a symmetrical, nonpolar structure.
Thus, answer b is correct, as it directly addresses the reason for the difference in polarity between the two molecules.