Although BH3 and CH2O have similar shapes, one is polar and the other is non-polar. Explain why.
The Correct Answer and Explanation is :
The polarity of molecules is primarily determined by the distribution of electron density and the presence of polar bonds, which are influenced by the molecular geometry and electronegativity differences between atoms. In this case, we will compare borane (BH₃) and formaldehyde (CH₂O).
Borane (BH₃) is a trigonal planar molecule. The central boron atom is bonded to three hydrogen atoms, and due to the small electronegativity difference between boron (2.04) and hydrogen (2.20), the B–H bonds are only slightly polar. Moreover, the symmetrical geometry of BH₃ leads to a cancellation of any dipole moments. The three B–H bonds are evenly distributed around the boron atom, resulting in a nonpolar molecule. The overall dipole moment is zero, as the vector sum of the individual bond dipoles cancels out due to their equal magnitude and symmetrical arrangement.
Formaldehyde (CH₂O), on the other hand, is a polar molecule. It has a trigonal planar structure but features a carbon atom double-bonded to an oxygen atom and single-bonded to two hydrogen atoms. The electronegativity of oxygen (3.44) is significantly higher than that of carbon (2.55), leading to a highly polar C=O bond. While the two C–H bonds are relatively nonpolar, the presence of the polar C=O bond creates a net dipole moment. The molecular geometry of formaldehyde does not allow for the cancellation of dipole moments, as the electronegative oxygen atom pulls electron density towards itself, creating a region of partial negative charge.
In summary, the key difference in polarity between BH₃ and CH₂O arises from their molecular structures and the differences in electronegativity between the constituent atoms. Borane’s symmetrical structure leads to a nonpolar nature, while formaldehyde’s asymmetrical distribution of charge results in a polar molecule.