Question 6.17: Use VSEPR theory to predict the shape of the polyatomic ion ...

STEP 1   Draw the Lewis structure. The polyatomic ion $NO_{3}^{-}$ contains three electron groups (two single bonds between the central N atom and O atoms, and one double bond between N and O). Note that the double bond can be drawn to any of the O atoms.

STEP 2   Arrange the electron groups around the central atom to minimize repulsion. To minimize repulsion, three electron groups would have a trigonal planar geometry.

STEP 3   Use the atoms bonded to the central atom to determine the shape. Because $NO_{3}^{-}$ has three bonded atoms, it has a trigonal planar shape.