Polarity and Molecular Dipole Moments in Water (H₂O)

Q38. Which drawing best accounts for the polarity of H₂O and its bond polarities?

Background

Topic: Molecular Polarity and Dipole Moments

This question tests your understanding of how molecular shape and bond polarities combine to produce a net dipole moment in a molecule, specifically water (H₂O).

Key Terms and Concepts:

• Polarity: A molecule is polar if it has a net dipole moment due to uneven distribution of electron density.

• Dipole Moment: A vector quantity representing the separation of charge in a molecule.

• Bond Polarity: Bonds are polar if there is a significant difference in electronegativity between the atoms.

• VSEPR Theory: Predicts the shape of molecules based on repulsion between electron pairs.

Step-by-Step Guidance

1. Recall that water (H₂O) has a bent molecular geometry due to two lone pairs on oxygen, resulting in a bond angle of about 104.5°.

2. Each O–H bond is polar because oxygen is more electronegative than hydrogen, so the dipole arrows point toward oxygen.

3. Consider how the individual bond dipoles combine: In a bent molecule, the dipoles do not cancel out, so there is a net dipole moment.

4. Examine the diagrams: Look for the one that shows both the correct bond polarities (arrows toward O) and a net dipole moment (resultant arrow).

Try solving on your own before revealing the answer!

Final Answer: Drawing (3)

Drawing (3) correctly shows the bent shape of H₂O, the bond dipoles pointing toward oxygen, and a net dipole moment. The other drawings either show linear geometry or canceling dipoles, which do not represent water's polarity.