The central nitrogen atom in the nitrate ion (NO3-) has four bond pairs and zero lone pairs.
Understanding Bond Pairs and Lone Pairs in the Nitrate Ion (NO3-)
The nitrate ion (NO3-) is a polyatomic anion where a central nitrogen atom is bonded to three oxygen atoms, and the entire ion carries a negative one charge. The bonding within the nitrate ion is best described by resonance structures, meaning the actual electron distribution is an average of several contributing Lewis structures.
Bond Pairs on the Central Nitrogen Atom
In any of the major resonance structures of the nitrate ion, the central nitrogen atom forms one double bond and two single bonds with the three oxygen atoms.
- Each single bond involves two shared electrons, constituting one bond pair.
- The double bond involves four shared electrons, which constitutes two bond pairs.
Therefore, the total number of bond pairs directly associated with the central nitrogen atom is calculated as:
1 (from one single bond) + 1 (from another single bond) + 2 (from the double bond) = 4 bond pairs.
This count aligns with the understanding that nitrogen in the nitrate ion has four bond pairs. Some chemical models describe the formation of these bonds, particularly one of the N-O single bonds, as a coordinate covalent bond (dative bond) where an oxygen atom donates both electrons to the nitrogen. This mechanism contributes to nitrogen's ability to participate in four shared electron pairs while adhering to or extending its octet.
Lone Pairs on the Central Nitrogen Atom
In the nitrate ion, the central nitrogen atom has zero lone pairs. Nitrogen utilizes all its valence electrons to form bonds with the oxygen atoms. Its formal charge of +1 in the nitrate ion is consistent with this electron distribution, where it does not retain any non-bonding electron pairs.
Summary of Electron Pairs on Central Nitrogen in NO3-
For clear understanding, here's a summary of the electron pair count for the central nitrogen atom:
| Feature | Count |
|---|---|
| Bond Pairs | 4 |
| Lone Pairs | 0 |
It's important to note that while the central nitrogen atom has no lone pairs, the terminal oxygen atoms in the nitrate ion do possess lone pairs. For example, in a resonance structure where one oxygen is double-bonded to nitrogen, that oxygen typically has two lone pairs. The two singly-bonded oxygens each typically have three lone pairs, contributing to the overall stability and charge of the ion.
Resonance and Delocalization
The actual structure of the nitrate ion is a resonance hybrid, meaning the electrons in the double bond are delocalized over all three N-O bonds. This delocalization results in all three N-O bonds being equivalent in length and strength, intermediate between a single and a double bond. This delocalized system effectively contributes to the four shared electron pairs around the central nitrogen, giving it a trigonal planar geometry.
