The nitrate ion (\(\text{NO}_3^-\)) is a common polyatomic ion found in fertilizers and various biological processes. To determine the bond angle within this structure, we must first analyze its connectivity and electron distribution. The ion’s overall three-dimensional shape results directly from how its atoms and electrons arrange themselves to achieve the most stable configuration.
Building the Nitrate Ion Structure (Lewis Structures and Resonance)
Determining the structure starts by counting the total valence electrons. The central nitrogen atom contributes five electrons, and the three oxygen atoms contribute six each, totaling 23 electrons. The negative one charge adds one electron, bringing the total to 24 valence electrons. Nitrogen is the least electronegative atom, so it is placed in the center, bonded to the three oxygen atoms.
A standard Lewis structure shows one nitrogen-oxygen double bond and two single bonds. This depiction is inaccurate because it suggests one bond is shorter and stronger than the others. All three nitrogen-oxygen bonds are measured to be the same length and strength. This equality is explained by resonance, where the actual structure is a hybrid of three possible Lewis structures. The double-bond character is delocalized and shared equally among all three bonds.
Predicting Molecular Shape Using VSEPR Theory
The three-dimensional shape of the nitrate ion is predicted using the Valence Shell Electron Pair Repulsion (VSEPR) theory. VSEPR theory states that electron groups around a central atom arrange themselves to maximize separation and minimize repulsion. In the nitrate ion, the central nitrogen atom is surrounded by three groups of bonding electrons, one connecting to each oxygen atom.
When applying VSEPR theory, a double bond is treated as a single electron domain. Therefore, the central nitrogen atom has three electron domains corresponding to the three oxygen atoms. Crucially, the central nitrogen atom has no lone pairs of electrons remaining. With three equivalent electron domains and no lone pairs, the electrons maximize their separation by arranging symmetrically in a flat plane. This arrangement places the three oxygen atoms at the corners of a triangle around the central nitrogen.
The Resulting Bond Angle and Molecular Geometry
The spatial arrangement maximizing the distance between three electron domains in a single plane results in a specific bond angle. The three oxygen atoms position themselves at the vertices of an equilateral triangle around the nitrogen atom. This geometry is known as the trigonal planar molecular geometry.
In a perfect trigonal planar arrangement, the angle between any two adjacent bonds is exactly \(120^\circ\). This angle provides the largest possible separation for three domains, minimizing repulsive forces. Since resonance makes the three nitrogen-oxygen bonds chemically equivalent, the repulsions are perfectly balanced. This results in the ideal \(120^\circ\) angle for each \(\text{O-N-O}\) connection. The nitrate ion is a highly symmetrical, flat, triangular molecule.