What is the bond angle in a water molecule?

The bond angle in a water molecule is about 104.5°. This is the angle formed by the two hydrogen–oxygen bonds, with the oxygen atom at the vertex. Because the molecule is bent rather than straight, water has an uneven distribution of electrical charge and is polar.

Around oxygen are four regions of electron density: two bonding pairs shared with hydrogen and two lone pairs. These regions arrange themselves approximately tetrahedrally, but lone pairs repel more strongly than bonding pairs. Their repulsion pushes the hydrogen atoms closer together, reducing the angle from the ideal tetrahedral angle of 109.5° to 104.5°.

The angle helps explain several properties of water, including its polarity and its ability to form hydrogen bonds. A common mix-up is choosing 109.5°, which is the characteristic tetrahedral angle in molecules such as methane. Methane has four bonding pairs around carbon, whereas water has two bonding pairs and two lone pairs around oxygen. The H–O–H angle can vary slightly with molecular environment, but 104.5° is the standard canonical value.