Which of the following shows the bond angle between the two O–H bonds in water as approximately 104.5 degrees?

• A. 90 degrees
• B. 120 degrees
• C. 104.5 degrees
• D. 180 degrees

Answer: (C)

The bond angle in water is determined by how electron pairs arrange themselves around the oxygen atom. Oxygen has four electron domains: two O–H bonding pairs and two lone pairs. These four domains adopt a roughly tetrahedral arrangement, but the lone pairs take up more space and push the bonding pairs closer together. This repulsion reduces the H–O–H angle from the ideal 109.5 degrees of a perfect tetrahedron to about 104.5 degrees. So the observed angle is around 104.5 degrees. The other angle values correspond to geometries that don’t apply here: 90 and 180 would imply different, more open or linear structures, while 120 would imply a planar trigonal arrangement. Water’s bent shape comes from the two lone pairs causing the angle to shrink to roughly 104.5 degrees.