Polyatomic Ions

The carbonate CO₃^2- and nitrate ions NO₃^- are trigonal planar as they both have three bonding regions of electron density surrounding the central atom.The shape of a polyatomic ion is predicted in exactly the same way as for a molecule: count the bonding regions and lone pairs around the central atom, then apply electron pair repulsion theory. The charge on the ion must be included when counting the electrons.The ammonium ion, NH₄⁺, has four bonding pairs and no lone pairs, so it is tetrahedral with bond angles of 109.5°. The sulfate ion, SO₄^2-, has four bonding regions around sulfur and is also tetrahedral.The oxonium ion, H₃O⁺, has three bonding pairs and one lone pair on oxygen, so it is trigonal pyramidal like ammonia (about 107°).The amide ion, NH₂^-, has two bonding pairs and two lone pairs on nitrogen, so it is non-linear like water (about 104.5°).Worked example: the shape of NH₂^-. Nitrogen has 5 outer electrons, the two hydrogen atoms add 1 each and the negative charge adds 1, giving 8 electrons, or 4 pairs. Two pairs are bonding pairs, so the other two are lone pairs. Two bonding pairs and two lone pairs give a non-linear shape.