In a molecule of ammonia NH3 , the central atom is nitrogen, which is surrounded by three hydrogen atoms and has one lone pair of electrons. The electron pair geometry of NH3 is tetrahedral, but due to the presence of the lone pair, the molecular geometry is trigonal pyramidal.The predicted bond angle in a perfect tetrahedral geometry is 109.5. However, in ammonia, the lone pair of electrons occupies more space than the bonding pairs, causing a slight repulsion between the bonding pairs and a decrease in the bond angle. As a result, the bond angle in ammonia N-H-N is slightly less than 109.5, approximately 107.