The VSEPR (Valence Shell Electron Pair Repulsion) theory helps us understand the three-dimensional arrangement of atoms in a molecule based on the repulsion between electron pairs. For the nitrate ion (NO3–), we start by determining the total number of valence electrons.
Nitrogen (N) has 5 valence electrons, and each oxygen (O) has 6, giving us a total of:
- 1 nitrogen = 5 electrons
- 3 oxygens = 3 × 6 = 18 electrons
- Plus, 1 extra electron for the negative charge = 1 electron
Total = 5 + 18 + 1 = 24 valence electrons.
In NO3–, nitrogen is the central atom surrounded by three oxygen atoms. We place one single bond between the nitrogen and each oxygen atom, using up 6 electrons (2 for each bond). To count the remaining electrons, we have:
- Total electrons used for bonds: 6
- Remaining electrons = 24 – 6 = 18 electrons.
We then complete the octet for each oxygen atom by adding lone pairs. A single bond is formed with nitrogen, and to ensure that each oxygen achieves an octet, we distribute the remaining electrons as lone pairs on the oxygen atoms.
However, one of the oxygen atoms can form a double bond with nitrogen, which reduces the number of lone pairs needed. The resonance structures of NO3– show that the bonds are equivalent, and the molecule can be represented as:
- One nitrogen-oxygen double bond and two nitrogen-oxygen single bonds with a formal charge distributed such that the nitrogen has no formal charge, and one of the oxygens has a negative charge.
According to VSEPR theory, the electron pair geometry around the nitrogen atom in NO3– is trigonal planar because of three regions of electron density (the three bonds to oxygen) arranged to minimize repulsion. The bond angles are approximately 120 degrees.
To visualize this geometry, you can draw nitrogen at the center, with three oxygen atoms spread out around it in a flat triangle, reflecting the trigonal planar shape.