Explain Why Water Is a Polar Molecule: The 105° Bent Shape

Water is polar because oxygen and hydrogen share electrons unevenly, and the molecule’s bent shape keeps the bond dipoles from cancelling. This leaves a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

Both parts of this explanation matter. Polar bonds do not guarantee that an entire molecule is polar. The arrangement of the bonds determines whether their effects cancel.

Unequal sharing in O–H bonds

A covalent bond forms when atoms share electrons. In each O–H bond, oxygen attracts the shared electrons more strongly than hydrogen does. This attraction is called electronegativity.

As a result, electrons spend more time near the oxygen. The oxygen end of each bond has a partial negative charge, written δ−, while the hydrogen end has a partial positive charge, written δ+. These are partial charges, not full ionic charges: the atoms still share electrons. The Royal Society of Chemistry discusses this kind of unequal sharing in its bonding resources.

Each O–H bond has a dipole, a separation of charge along the bond. This tells us that the bonds are polar. To determine whether the whole water molecule is polar, we also need to consider its shape.

How water’s shape prevents cancellation

A water molecule is bent, with an H–O–H bond angle of about 105°. The oxygen has two lone pairs of electrons, which contribute to the molecule’s bent geometry. The angle is close to 105°, not the 180° angle of a straight molecule, as described by Encyclopaedia Britannica.

Imagine each O–H bond dipole as an arrow pointing from a hydrogen toward the oxygen. Since the bonds form an angle, the arrows do not point in opposite directions. Their sideways components partly cancel, while the components toward the oxygen add together. The molecule has a net dipole: its oxygen side is partially negative and its hydrogen side is partially positive. Khan Academy explains the same general method of combining bond polarity with molecular shape.

If water were linear, its two identical bond dipoles would point in opposite directions and cancel. Its overall shape is essential to its polarity.

One comparison: carbon dioxide

Carbon dioxide shows why polar bonds do not always make a polar molecule. Each C=O bond is polar because oxygen attracts electrons more strongly than carbon. But CO₂ is linear, so its two bond dipoles point in opposite directions and cancel. The molecule has no net dipole.

Water also has polar bonds, but its bent shape leaves a net dipole. This comparison shows why a molecule’s geometry must be considered along with the polarity of its bonds.

What water’s polarity means

Water’s partial charges attract other charges and polar molecules. For example, the oxygen side of water can be attracted to positive ions, while the hydrogen side can be attracted to negative ions. These attractions help water dissolve many ionic substances. Water also interacts readily with many polar molecules.

Oil and water separate because many oil molecules are nonpolar and do not interact with water in the same way as ions and polar molecules do. Water molecules attract one another through their partial charges, including through hydrogen bonding. This interaction contributes to water’s relatively high boiling point for a molecule of its size.

These properties depend on more than bond polarity. Oxygen attracts the shared electrons more strongly, making each O–H bond polar. The bent shape prevents the bond dipoles from cancelling, so the molecule has a net dipole. That net dipole helps explain how water interacts with other substances and with itself.

Key Takeaways

• Water is polar because its O–H bonds are polar and its bent shape prevents their dipoles from cancelling.
• Oxygen attracts the shared electrons more strongly than hydrogen, giving oxygen a partial negative charge and the hydrogens partial positive charges.
• Water’s H–O–H angle is about 105°. Its two lone pairs contribute to its bent geometry.
• The molecule’s net dipole helps explain why water interacts with ions and other polar molecules.
• Carbon dioxide has polar bonds but no net dipole because its linear shape makes the bond dipoles cancel.