How Electronegativity and Shape Make Water Polar

Electronegativity describes an atom's pull on electrons

In a covalent bond, atoms share electrons. When one atom attracts the shared electrons more strongly, the sharing is unequal and the bond is polar covalent. Oxygen is more electronegative than hydrogen, so each O–H bond places more electron density near oxygen.

This unequal distribution creates partial charges rather than full ionic charges:

$$\mathrm{H}^{\delta+}-\mathrm{O}^{\delta-}-\mathrm{H}^{\delta+}.$$

Molecular shape decides whether bond dipoles cancel

Bond polarity alone does not determine whether an entire molecule is polar. The directions of all bond dipoles must be combined with the molecule's geometry.

Water has a bent shape, so its two O–H bond dipoles point in directions that do not cancel. The molecule therefore has a net dipole: the oxygen side is partially negative and the hydrogen side is partially positive.

Carbon dioxide provides a useful contrast. Each C=O bond is polar, but the molecule is linear and symmetric. Its equal, opposite bond dipoles cancel, making the molecule nonpolar overall.

Polarity enables hydrogen bonding

The partially positive hydrogen of one water molecule is attracted to the partially negative oxygen region of another. This intermolecular attraction is a hydrogen bond. A hydrogen bond is weaker than an O–H covalent bond, but many hydrogen bonds together strongly influence water's behavior.

Hydrogen bonding contributes to water's cohesion, high heat capacity, and relatively high heat of vaporization. In liquid water, individual hydrogen bonds continually break and reform as molecules move.

Keep four ideas separate

• Electronegativity is an atomic tendency.
• Bond polarity comes from unequal electron sharing.
• Molecular polarity depends on bond dipoles and molecular shape.
• Hydrogen bonding is an attraction between suitable partial-charge regions on neighboring molecules.

Keeping these levels separate prevents the common mistake of calling a molecule “electronegative” when the intended property is polarity.