An NO3- ion, or nitrate, has a trigonal planar molecular geometry. Trigonal planar is a molecular geometry model with one atom at the center and three ligand atoms at the corners of a triangle, all on a one-dimensional plane. In trigonal planar models, where all three ligands are identical, all bond angles are 120 degrees. As explained by ChemGuide, the ligands try to arrange themselves as far apart as possible.
Nitrate contains one central nitrogen atom surrounded by three identically bonded oxygen atoms.
VSEPR theory is a model used in chemistry to determine the geometry of individual molecules. When applying VESPR theory, the AXE method of electron counting is often used. The A represents the central atom, the X represents the number of atoms bonded to A and E represents the number of lone electron pairs surrounding the central atom. The sum of X and E is the steric number. The central nitrogen atom in nitrate has three X ligands due to the three bonded oxygen atoms. Because nitrogen has no lone electron pairs, E is equal to zero. Calculating X plus E, nitrate has a steric number of 3. Nitrate can also be described as having a molecular arrangement of "AX3." With a steric number of 3, and no lone electron pairs, nitrate is described as having a trigonal planar geometry.