Question

Describe the molecular structure of water, and explain why it is a polar molecule.

Answer 1

Molecular structure
- A water molecule (H2O) has one oxygen atom bonded to two hydrogen atoms. Each O–H bond is a single covalent bond formed by overlap of an oxygen sp3-type orbital with a hydrogen 1s orbital.
- Oxygen also carries two nonbonding lone pairs of electrons, so there are four regions of electron density around oxygen. By VSEPR theory the electron-pair geometry is roughly tetrahedral, but because only two of the four positions are occupied by atoms the molecular shape is bent (angular).
- Typical numbers: O–H bond length ≈ 0.958 Å, H–O–H bond angle ≈ 104.5° (less than the 109.5° tetrahedral angle because lone pairs repel bonding pairs more strongly).

Why water is polar
- Electronegativity: oxygen (≈3.44 on the Pauling scale) is much more electronegative than hydrogen (≈2.20). Each O–H bond is therefore polar (a polar covalent bond): electron density is pulled toward oxygen, giving oxygen a partial negative charge (δ–) and each hydrogen a partial positive charge (δ+).
- Geometry and vector sum of bond dipoles: each O–H bond has a dipole pointing toward the oxygen. Because the molecule is bent rather than linear, the two bond dipoles do not cancel; their vector sum produces a net dipole moment directed from the hydrogen side toward the oxygen atom. Water’s net dipole moment is about 1.85 Debye.
- Consequence: the unequal distribution of charge makes water a polar molecule, which underlies properties such as strong hydrogen bonding, high boiling point, and high dielectric constant.