What is the bond angle between the two O–H bonds in a water molecule?

What is the bond angle between the two O–H bonds in a water molecule?

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

Answer: (A)

Water has four electron domains around the oxygen: two bonding pairs (the O–H bonds) and two lone pairs. In the electron-domain picture, these four regions adopt a tetrahedral arrangement, which would give an ideal angle of about 109.5 degrees between bonds if only bond pairs were present. But lone pairs take more space and repel bonding pairs more strongly, squeezing the O–H bonds closer together. The result is a bent molecule with the bond angle between the two O–H bonds of about 104.5 degrees. That’s why the angle isn’t 90, 120, or 180 degrees—the geometry is determined by four electron domains with significant lone-pair repulsion, yielding a compressed, bent shape.

Water has four electron domains around the oxygen: two bonding pairs (the O–H bonds) and two lone pairs. In the electron-domain picture, these four regions adopt a tetrahedral arrangement, which would give an ideal angle of about 109.5 degrees between bonds if only bond pairs were present. But lone pairs take more space and repel bonding pairs more strongly, squeezing the O–H bonds closer together. The result is a bent molecule with the bond angle between the two O–H bonds of about 104.5 degrees. That’s why the angle isn’t 90, 120, or 180 degrees—the geometry is determined by four electron domains with significant lone-pair repulsion, yielding a compressed, bent shape.