Why Is Water Considered a Polar Molecule?
Water is polar because oxygen attracts the shared electrons in each O–H bond more strongly than hydrogen does. This uneven sharing gives oxygen a partial negative charge (δ−) and each hydrogen a partial positive charge (δ+). Water’s bent shape prevents the two bond dipoles from canceling, so the molecule has a net dipole.
Both parts of this explanation matter. Uneven electron sharing makes each O–H bond polar. The molecule’s shape determines whether those bond dipoles cancel or combine.
How water’s bonds become polar
In a covalent bond, atoms share electrons. They do not always share them equally: an atom with greater electronegativity attracts the shared electrons more strongly. Oxygen is more electronegative than hydrogen, so the electron cloud in each O–H bond shifts toward oxygen LibreTexts Chemistry.
This creates partial charges, not full ionic charges. The oxygen end of the molecule is δ−, while the hydrogen ends are δ+. A bond with this unequal distribution of electrons is called polar covalent.
Polar bonds do not always make a molecule polar. The arrangement of those bonds matters too. If their dipoles point in directions that cancel, the molecule can have no net dipole.
Why water’s shape matters
An oxygen atom in a water molecule has two bonds to hydrogen and two lone pairs of electrons. These four electron domains arrange themselves roughly tetrahedrally. Lone pairs occupy space and repel the bonding pairs, so the two O–H bonds form a bent shape instead of a straight line.
The O–H bond dipoles point in different directions, but they do not oppose each other exactly. Their effects combine into a net dipole toward the oxygen side. As a result, the partial charges remain separated across the molecule as a whole.
Carbon dioxide provides a useful contrast. Its two C=O bonds are polar, but the molecule is linear. The bond dipoles point in opposite directions and cancel, leaving carbon dioxide with no net dipole LibreTexts Chemistry.
One example: why water dissolves salt
Water’s partial charges help explain how it interacts with dissolved ions. The δ− oxygen end is attracted to positively charged ions, while the δ+ hydrogen ends are attracted to negatively charged ions. Water molecules can surround the ions and help keep them dispersed in solution.
This interaction is a practical effect of water’s polarity. The same charge distribution lets water molecules attract one another through hydrogen bonds. These attractions contribute to water’s cohesion and surface tension, as well as its relatively high boiling point for a molecule of its size USGS Water Science School.
Water’s polarity does not mean every substance dissolves in it. Water interacts well with many ionic and polar substances, while nonpolar substances such as oils generally do not mix readily with it.
A simple way to judge molecular polarity
To assess whether a molecule is polar, check its bonds and its shape:
- Check whether the bonds are polar. Compare how strongly the bonded atoms attract shared electrons.
- Check whether the bond dipoles cancel. Consider the molecule’s three-dimensional shape and the direction of each dipole.
For water, the O–H bonds are polar, and the bent shape prevents cancellation, so the molecule is polar. For carbon dioxide, the polar bonds are arranged linearly and cancel, so the molecule is nonpolar.
Key Takeaways
- Oxygen attracts the shared electrons in water’s O–H bonds more strongly than hydrogen does.
- This creates a partial negative charge on oxygen and partial positive charges on the hydrogens.
- Water’s bent shape keeps the O–H bond dipoles from canceling, giving the molecule a net dipole.
- Water’s polarity helps it interact with ions and polar molecules and contributes to attractions between water molecules.
- Molecular polarity depends on both bond polarity and molecular shape.
