The predicted bond angle in a water molecule H2O is approximately 104.5 degrees. This is due to the presence of two lone pairs of electrons on the oxygen atom, which repel the bonding pairs of electrons, causing the H-O-H bond angle to be smaller than the ideal tetrahedral angle of 109.5 degrees.In an ammonia molecule NH3 , there is one lone pair of electrons on the nitrogen atom and three bonding pairs of electrons. The lone pair of electrons repels the bonding pairs, resulting in a bond angle of approximately 107.3 degrees, which is also smaller than the ideal tetrahedral angle of 109.5 degrees.In summary, the bond angle in a water molecule 104.5 degrees is smaller than the bond angle in an ammonia molecule 107.3 degrees due to the difference in the number of lone pairs of electrons on the central atom.