<p>NH2(-) doesn't start out with 180, it's got tetrahedral electron geometry(two atoms, two lone pairs) so it "starts out" with 109.5, which is decreased by the two lone pairs. Its VSEPR geometry is bent (or angled, whatever you want to call it).</p>
<p>NH3 has a similar effect, but it's only got one lone pair, so its angle is slightly larger than NH2(-). Its VSEPR geometry is trigonal pyramidal.</p>
<p>NH4+ is tetrahedral with NO lone pairs (hence the positive charge), so it will obviously have the largest angle of 109.5.</p>
<p>edit: afruff got to it first.</p>