Abstract The theoretical calculation method was employed to interpret the stepwise decreasing of bond dissociation energies of NH3. The calculated results showed a decrease of the bond dissociation energies and an increase of the bond lengths with the decreasing number of binding hydrogen atoms. With the increasing number of binding hydrogen atoms to nitrogen atom, the increasing promotion of 2s electrons (N) to 2p orbitals (N) gives rise to less screening effect, the more electron transfer of 1s electrons (H) to 2p orbital (N) increases electrostatic interaction between nitrogen atom and hydrogen atoms, and the enlargement of the angle between unpaired electron (p orbitals) or lone pair electrons with N-H bond decreases repulsion. The three factors cause stepwise decrease of bond dissociation energies of NH3.
