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 difference in polarity between BH₃ (borane) and CH₂O (formaldehyde) can be attributed to their molecular geometries and the electronegativities of the atoms involved.
BH₃ (Borane) has a trigonal planar geometry, where the boron atom is at the center, and the three hydrogen atoms are arranged symmetrically around it. Boron has an electronegativity of 2.04, while hydrogen has an electronegativity of 2.20. The difference in electronegativity is minimal, leading to very weak dipoles in the B-H bonds. Moreover, since the three B-H bonds are arranged symmetrically and cancel each other out, BH₃ does not possess a net dipole moment. Therefore, BH₃ is classified as a non-polar molecule.
CH₂O (Formaldehyde), on the other hand, has a trigonal planar structure as well, but its geometry includes a double bond between carbon and oxygen, where carbon is bonded to two hydrogen atoms and one oxygen atom. The electronegativity of oxygen (3.44) is significantly higher than that of carbon (2.55) and hydrogen (2.20), resulting in a strong dipole moment across the C=O bond. This creates a partial negative charge on the oxygen atom and a partial positive charge on the carbon atom. The asymmetry in the molecular structure, due to the presence of the highly electronegative oxygen atom, results in a net dipole moment pointing towards the oxygen, making CH₂O a polar molecule.
In summary, the key factors that differentiate the polarity of BH₃ and CH₂O are the electronegativity differences between the constituent atoms and the symmetry of their molecular geometries. BH₃’s symmetrical arrangement of similar electronegative bonds leads to non-polarity, whereas CH₂O’s asymmetry and the presence of a highly electronegative oxygen atom result in a polar molecule.