The shapes of polyatomic ions are predicted in the same way as molecules, by counting the regions of electron density around the central atom. A double bond counts as one region.The carbonate ion CO32- and the nitrate ion NO3- each have three bonding regions and no lone pairs on the central atom. They are trigonal planar with bond angles of 120°.The ammonium ion NH4+ has four bonding pairs around nitrogen, one of them a dative bond. The four bonds are identical, so the ion is tetrahedral with bond angles of 109.5°.The sulfate ion SO42- also has four bonding regions around sulfur, so it is tetrahedral with bond angles of 109.5°, like CH4.