NH2-,NH3, and NH4+ have H-N-H bond angles of 105, 107, and 109. Explain this variation in bond angles
The variation in bond angles among NH2-, NH3, and NH4+ is mainly due to the number of lone pairs of electrons on the central nitrogen atom and the repulsion between these lone pairs and the bonding pairs.
NH2- has one lone pair of electrons on the central nitrogen atom and two bonding pairs. The lone pair takes up more space around the nitrogen atom compared to the bonding pairs, creating stronger repulsion. This repulsion pushes the bonding pairs closer together, resulting in a smaller bond angle of 105 degrees.
NH3 has one lone pair of electrons and three bonding pairs around the central nitrogen atom. The lone pair still exerts repulsion, but it is balanced out by the bonding pairs. The repulsion between the lone pair and the bonding pairs causes the bonding pairs to spread out slightly, resulting in a larger bond angle of 107 degrees compared to NH2-.
NH4+ has no lone pairs of electrons on the central nitrogen atom, only four bonding pairs. Without any repulsion from lone pairs, the bonding pairs can spread out even more, leading to the largest bond angle of 109 degrees among the three compounds.
In summary, the variation in bond angles among NH2-, NH3, and NH4+ can be explained by the repulsion between lone pairs and bonding pairs. As the number of lone pairs increases, the bond angle decreases due to stronger repulsion.
Look at the Lewis electron dot structures. You may need to draw them out to see this. I may be able to find some sites that would show the Lewis structure if you can't draw them out. At any rate, the difference in bond angles is due to the unpaired electrons (the lone pairs). Lone pair electrons take up more space than shared electrons and they "push" on the opposite site so those elements opposite to the lone pairs are closer together; hence the angel is less.
NH4^+ has no long pairs so it is the closest to the tetrahedral angle of 109o