Understanding Hybridisation of NO₃⁻: Nitrate Ion

Nitrate Ion (NO₃⁻) is also known as nitrate. It is a polyatomic ion. The nitrogen here is central atom which is bonded to three oxygen atoms. It’s an excellent example of sp² hybridisation in inorganic chemistry.

Let us understand how hybridisation happens in NO₃⁻. Read on to learn how it leads to its bonding and molecular shape.

What is the Hybridisation of NO₃⁻?

Nitrate Ion consists of one nitrogen atom and three oxygen atoms. Each of the oxygen atoms is bonded to nitrogen atom and it also undergoes resonance, hence leading to π bonds. In all, the molecule also contains negative charge. In order to form these bonds and satisfy the octet rule, nitrogen undergoes sp² hybridisation.

Using the Hybridisation Formula

We can determine the hybridisation of nitrate ion using the simple formula:

formula

Step-by-step calculation:

Valence electrons of central atom (N): 5
Monovalent atoms (O): 3
Negative charge: 1
Positive charge: 0

formula

Interpretation:

3 σ bonds + 0 lone pairs → 3 regions of electron density → sp² hybridisation

For ions with resonance, the simple formula gives an approximate value. Electron domain counting is more reliable.

Breakdown of NO₃⁻ Hybridisation

The nitrate ion has equal bond lengths between N–O bonds due to resonance. The ion is also known for its trigonal planar shape.

Here is a complete understanding of its hybridisation.

Electronic Configuration of Nitrogen

The atomic number of nitrogen is 7.

The ground state of nitrogen:

1s² 2s² 2p³

Only three unpaired electrons → sufficient to form required bonds but not enough for delocalised π system explanation.

Excited state configuration:

1s² 2s¹ 2p_x¹ 2p_y¹ 2p_z²

One electron from the 2s orbital is promoted to the empty 2p_z orbital to allow hybridisation.

Four regions for electron density → enough to form 3 σ bonds, and still have one unhybridised p orbital left for resonance.

Formation of Hybrid Orbitals

sp² hybridisation occurs when 1 s orbital and 2 p orbitals mix.

The result:

3 sp² hybrid orbitals on the nitrogen atom
The remaining 1 unhybridised p orbital stays available for π bonding in the resonance structures

Bond Formation in Nitrate Ion

Each nitrogen atom uses:

3 sp² orbitals to form σ bonds with oxygen
The one unhybridised p orbital overlaps sideways with p orbital of oxygen atom to form 1 π resonant delocalised bond

Result:

3 N–O σ bonds
1 π bond, which is delocalised between the three N–O bonds
Hybridisation type: sp²
Bond angle: 120°
Geometry: Trigonal planar

Lewis structure and resonance representation of Nitrate Ion Ball and stick model showing geometry and bonding of Nitrate Ion

Geometry and bonding of Nitrate Ion

Details At A Glance

Property	Details
Molecule	Nitrate Ion (NO₃⁻)
Hybridisation	sp²
Geometry	Trigonal planar
Bond angle	120°
Bonding	3 σ bonds (N–O), 1 π bond (delocalised)
Unhybridised orbitals	1 (on nitrogen for π bonding)
Nitrogen valency satisfied?	Yes, by forming 3 bonds and having resonance

Formal Charge in NO₃⁻

Formal charge = Valence electrons − (Lone pair electrons + ½ × Bonding electrons)

Nitrogen (N) – central atom

Valence electrons: 5
Lone pairs: 0
Bonding electrons: 8

Formal charge = 5 − (0 + ½ × 8) = 5 − 4 = +1

Oxygen (O) – doubly bonded

Valence electrons: 6
Lone pairs: 2
Bonding electrons: 4

Formal charge = 6 − (4 + ½ × 4) = 6 − 6 = 0

Oxygen (O) – singly bonded (each)

Valence electrons: 6
Lone pairs: 3
Bonding electrons: 2

Formal charge = 6 − (6 + ½ × 2) = 6 − 7 = −1

Thus, atoms in NO₃⁻ carry formal charges of Nitrogen (+1), one oxygen (0) and two oxygens (−1), resulting in an overall charge of −1.

Summing Up

The nitrogen in NO₃⁻ forms 3 bonds with oxygen and shares a delocalised π bond. sp² hybridisation leads to a trigonal planar shape and 120° bond angles. The negative charge of the ion is distributed due to resonance.