What Is The Nitrite Ion? (Chemistry Specs)
The nitrite ion is NO₂⁻: one nitrogen atom bonded to two oxygen atoms, with an overall charge of −1. It has 18 valence electrons and a bent shape. Two equivalent resonance contributors explain why its N–O bonds are equivalent in the actual ion. Nitrite is also the conjugate base of nitrous acid, HNO₂.
If a formula, Lewis structure, or reaction uses “nitrite,” a small mix-up can change the chemistry. Nitrite is easy to confuse with nitrate, but they have different formulas, charges, and shapes. I use a short sequence to keep the details straight: count electrons, draw resonance contributors, check the shape, then connect the ion to its acid.
Nitrite Ion Overview: Formula, Charge, and Chemical Identity
The nitrite ion, written NO₂⁻, contains one nitrogen atom and two oxygen atoms. Its superscript minus sign means the whole group has one extra electron compared with the neutral atoms. That charge belongs to the ion as a whole; it is not a label for just one oxygen in the real structure.
Nitrite is a polyatomic ion: a group of bonded atoms that carries a net charge. In a formula or equation, keep its atoms together and show the charge when the ion appears by itself. For example, sodium nitrite is written NaNO₂. The sodium ion has a +1 charge, which balances the nitrite ion’s −1 charge in the formula.
Count the valence electrons
Valence electrons are the outer electrons used to describe bonding. Nitrogen contributes five, each of the two oxygen atoms contributes six, and the −1 charge adds one more:
| Source | Electron count |
|---|---|
| Nitrogen | 5 |
| Two oxygen atoms | 12 |
| One extra electron from the charge | 1 |
| Total | 18 |
This total is a useful check when drawing a Lewis structure. If a proposed structure does not account for 18 electrons, revisit the count before deciding on its bonds or shape.
Nitrogen has an oxidation state of +3 in nitrite, while each oxygen has an oxidation state of −2. Oxidation state is a bookkeeping value assigned by chemical rules; it is not the same as the ion’s formal charge or its overall −1 charge. The values add up: +3 + (2 × −2) = −1.
Lewis Structures, Resonance, and Molecular Geometry
A Lewis structure is a map of bonds and lone electron pairs. For nitrite, the two main Lewis contributors each show one N=O double bond, one N–O single bond, and a lone pair on nitrogen. The double bond can be drawn to either oxygen, so neither drawing alone fully represents the ion.
You can write the two contributors in a compact form as:
- [O=N–O⁻]
- [⁻O–N=O]
In each contributor, the singly bonded oxygen carries a formal charge of −1. Formal charge is an electron-counting tool for one atom within a Lewis structure. Nitrogen and the double-bonded oxygen each have a formal charge of zero in these contributors. The charges sum to −1, as they must.
Why resonance matters
Resonance means more than one valid Lewis structure is needed to represent the same arrangement of atoms. The contributors are not two structures that rapidly switch back and forth. Instead, the real nitrite ion is a resonance hybrid, with electron density shared across both N–O bonds.
As a result, the two N–O bonds are equivalent in the resonance hybrid. In the simple resonance model, each has a bond order of about 1.5, between a single and a double bond. That number is a useful model-based description, not a claim that either bond literally changes length back and forth.
Shape and bond angle
VSEPR theory predicts shape by counting regions of electron density around a central atom. Around nitrite’s nitrogen, there are three regions: two N–O bonding regions and one lone pair. These regions have trigonal-planar electron-domain geometry, but the atoms alone form a bent, or V-shaped, molecular geometry.
The O–N–O angle is approximately 115°. It is less than the ideal 120° for three evenly spaced electron regions because the lone pair affects the arrangement. The angle is approximate; measured values can depend on the conditions and method used.
| Feature | Nitrite ion |
|---|---|
| Formula and overall charge | NO₂⁻; −1 |
| Total valence electrons | 18 |
| Main resonance contributors | Two, with the N=O placement alternating |
| Electron-domain geometry at N | Trigonal planar |
| Shape based on atom positions | Bent |
| Approximate O–N–O angle | 115° |
A common homework error is to draw only one N=O placement and call it the complete structure. Show both contributors, or label the drawing as one resonance contributor, to avoid implying that the two oxygen atoms have different bonds in the actual ion.
Key Chemical Properties and Acid–Base Relationships
Nitrous acid, HNO₂, is the conjugate acid of nitrite. A conjugate acid is the form produced when a base accepts a proton, H⁺. In this pair, nitrite accepts a proton to form nitrous acid; nitrous acid can lose a proton to form nitrite.
NO₂⁻ + H⁺ ⇌ HNO₂
The pKa of nitrous acid is approximately 3.3 at 25 °C. The pKa is a measure used to describe acid strength: a lower pKa generally means a stronger acid. The quoted value is approximate and can vary somewhat with measurement conditions.
In water, the balance between HNO₂ and NO₂⁻ depends on pH. Around the pKa, meaningful amounts of both forms may be present. Below the pKa, the protonated form is favored; above it, the deprotonated form is favored. This is a general acid–base guide, not a substitute for a calculation using the actual solution conditions.
A practical check when reading an equation is to track both atoms and charge. In the protonation equation above, the atoms match on each side, and the charges also balance: −1 plus +1 gives a neutral product.
Applications, Reactions, and Safety Considerations
Nitrite appears in chemical formulas, reaction equations, and analytical chemistry. Recognizing its formula and charge helps you identify the species being discussed, balance equations, and distinguish it from other nitrogen–oxygen ions. Its acid–base relationship also matters when a reaction changes the solution’s pH.
When nitrite is acidified, it can form nitrous acid. Nitrous acid is not stable under all conditions and may take part in further reactions. The products depend on the conditions and other substances present, so do not assume that every acidified nitrite solution follows one simple reaction.
Nitrite salts also require sensible chemical handling. Sodium nitrite, for example, is hazardous if swallowed. Follow the product label and the relevant safety data sheet for the specific substance, use, and concentration. Do not treat the −1 ion charge as a measure of a compound’s safety; hazards depend on the compound and exposure.
A worked identification check
Suppose a problem gives NO₂⁻ and asks for its Lewis structure and shape. I would check it in this order:
- Count 18 valence electrons, including the one added for the negative charge.
- Draw both resonance contributors, placing the double bond on alternate oxygen atoms.
- Confirm the formal charges add to −1 in each contributor.
- Count two bonding regions and one lone pair around nitrogen.
- Name the shape as bent, with trigonal-planar electron-domain geometry.
This sequence catches the most common errors without treating a single Lewis drawing as the full structure.
| If you see… | Identify it as… | Key distinction |
|---|---|---|
| NO₂⁻ | Nitrite ion | Two oxygen atoms; −1 charge; bent |
| NO₃⁻ | Nitrate ion | Three oxygen atoms; −1 charge; trigonal planar around N |
| HNO₂ | Nitrous acid | Conjugate acid of nitrite |
| NaNO₂ | Sodium nitrite | Sodium cation paired with nitrite anion |
Nitrite and nitrate are not interchangeable. Nitrate has three oxygen atoms, not two. Its nitrogen oxidation state is +5, compared with +3 in nitrite. Checking the subscript and charge before using either name prevents a basic but consequential formula error.
Conclusion: The Fast Nitrite Check
Nitrite is NO₂⁻, with 18 valence electrons, nitrogen at oxidation state +3, and a bent shape. Its two equivalent N–O bonds are explained by two resonance contributors. When you meet the ion in a formula or problem, check its charge, electron count, shape, and relationship to HNO₂ before moving on.
Frequently asked questions
What is the formula of the nitrite ion?
The formula is NO₂⁻. It contains one nitrogen atom and two oxygen atoms and has an overall charge of −1.
How many valence electrons does NO₂⁻ have?
It has 18 valence electrons: five from nitrogen, twelve from the two oxygen atoms, and one added because of the −1 charge.
What is the charge on nitrite?
The nitrite ion has a net charge of −1. In a Lewis contributor, that formal charge is shown on one oxygen, but resonance makes the two oxygen positions equivalent in the hybrid.
What is the Lewis structure of nitrite?
Draw two resonance contributors. Each has one N=O bond, one N–O bond, and a lone pair on nitrogen; the double bond alternates between the oxygen atoms.
Does nitrite have one single bond and one double bond?
That describes either individual Lewis contributor, not the complete ion. The resonance hybrid has two equivalent N–O bonds, often described by an approximate bond order of 1.5.
What is the shape of the nitrite ion?
Nitrite is bent around nitrogen. Its electron-domain geometry is trigonal planar because nitrogen has two bonding regions and one lone pair.
What is the O–N–O bond angle in nitrite?
It is approximately 115°. The value is approximate and may vary with measurement conditions.
What is the oxidation state of nitrogen in nitrite?
Nitrogen has oxidation state +3. Each oxygen has oxidation state −2, giving a total charge of −1 for NO₂⁻.
What is the conjugate acid of nitrite?
The conjugate acid is nitrous acid, HNO₂. Nitrite accepts a proton, H⁺, to form HNO₂.
How is nitrite different from nitrate?
Nitrite is NO₂⁻, while nitrate is NO₃⁻. Both have a −1 charge, but they differ in oxygen count, geometry, and nitrogen oxidation state.
What is the pKa of nitrous acid?
Its pKa is about 3.3 at 25 °C. The value is approximate and can shift slightly with measurement conditions.
Is nitrite the same as sodium nitrite?
No. Nitrite is the NO₂⁻ ion. Sodium nitrite, NaNO₂, is a compound made of sodium ions and nitrite ions.
(This article was written by one of our staff writers, Nathan Sterling. Visit our Meet the Team page.)
