The Lewis structure of NO3– (nitrate ion) can be drawn by following these steps:
- Count the total number of valence electrons: Nitrogen (N) has 5 valence electrons, and each oxygen (O) has 6 valence electrons. Since there are three oxygen atoms, the total number of valence electrons is 5 + (3 × 6) = 23. However, since the molecule has a negative charge, we add one more electron, making it 24 valence electrons in total.
- Place the least electronegative atom in the center: Nitrogen is less electronegative than oxygen, so it is placed in the center.
- Connect the atoms with single bonds: Draw single bonds between the nitrogen atom and each of the three oxygen atoms. This uses up 6 electrons (3 bonds × 2 electrons).
- Distribute the remaining electrons: After forming the single bonds, 18 electrons remain. These electrons are placed as lone pairs on the oxygen atoms to satisfy the octet rule. Each oxygen atom will have 6 electrons in lone pairs, and the nitrogen atom will have 2 electrons in lone pairs.
- Check for octet rule compliance: Each oxygen atom has 8 electrons (2 in bonds and 6 in lone pairs), and the nitrogen atom also has 8 electrons (6 in bonds and 2 in lone pairs).
- Assign formal charges: The formal charge on nitrogen is 0, and the formal charge on each oxygen atom is -1. The overall charge of the ion is -1, which matches the given charge.
The geometry of the NO3– ion is trigonal planar. This is because the central nitrogen atom is surrounded by three regions of electron density (the three oxygen atoms), and there are no lone pairs on the nitrogen atom. The bond angles are approximately 120 degrees, which is characteristic of a trigonal planar geometry.