Molecular orbital (MO) diagram for N2, N2+, N22-, N22+, N2-, and their bond order

If you are looking for the molecular orbital (MO) diagram of nitrogen (N₂) then immediately stop scrolling. The good news is- you have hit the right spot.

This is by far the most comprehensive article you will find on the web, teaching you how to draw the Molecular orbital diagram of N₂, what is its bond order and magnetic properties, etc.

Additionally, you will also find the MO diagrams of N₂⁺, N₂^–, N₂²⁺, and N₂^2- all in this article.

How to draw the molecular orbital (MO) diagram of N₂ with its bond order?

As per the molecular orbital theory (MOT) of chemical bonding, after bond formation, the individual atomic orbitals cease to exist. Rather, the atomic orbitals of constituent atoms combine to form a unique set of molecular orbitals (MOs).

The electrons of the participant atoms are thus held in these MOs, belonging to the entire molecule in unison.

The linear combination of atomic orbitals (LCAO) produces two types of molecular orbitals:

• Bonding molecular orbitals
• Anti-bonding molecular orbitals

The number of MOs produced is exactly equal to the number of atomic orbitals coming together.

A bonding molecular orbital is formed by the linear combination of two AOs in the same phase.

Contrarily, an antibonding molecular orbital is produced by the linear combination of two AOs in the opposite phase, counteracting the cohesive forces of the combining nuclei.

This is why, a bonding MO always lies at a lower energy (greater stability) than the parent AOs while an antibonding MO occupies an energy level higher than that of parent AOs (higher instability).

The electrons are filled in these MOs following the three simple rules:

1. Aufbau Principle: Electrons first occupy the lower energy orbitals followed by their placement in the higher energy molecular orbitals.
2. Hund’s Rule: The incoming electrons are singly filled in the degenerate MOs before pairing occurs.
3. Pauli Exclusion Principle: Two electrons placed in the same MO exhibit an opposite spin (clockwise and anticlockwise).

The different numbers of electrons present in the bonding and/or antibonding MOs of a molecule are represented in the form of an energy level diagram called the molecular orbital (MO) diagram.

The MO diagram in turn helps in predicting other useful properties of molecules such as their bond order, bond stability, magnetic behavior, etc.

Nitrogen (N₂) is a homonuclear diatomic molecule. Two identical N-atoms combine to form an N₂ molecule.

We can easily draw the Molecular orbital diagram of N₂ following the steps given below.

Steps for drawing the molecular orbital (MO) diagram of N₂ with its bond order

1. Write down the electronic configuration of N₂ atoms

N₂ is composed of two nitrogen (N) atoms.

The electronic configuration of each N-atom is 1s² 2s² 2p_x¹ 2p_y¹ 2p_z¹.

Usually, only the valence electrons are displayed in the MO diagram of a molecule, therefore, it is important to note that each N-atom contains 5 valence electrons.

2 N-atoms combined make a total of 14 electrons and 10 valence electrons in the N₂ molecule.  

2. Determine whether the molecule is homonuclear or heteronuclear

N₂ is a neutral molecule. It is homonuclear as it is formed by two atoms of the same element.

As per the rule of LCAO, the two 1s atomic orbitals of nitrogen overlap to produce two molecular orbitals i.e., a bonding molecular orbital (σ1s) and an antibonding molecular orbital (σ*1s).

Similarly, two 2s atomic orbitals combine to form two MOs, σ2s and σ*2s. Finally, the three 2p atomic orbitals from each N-atom overlap to produce six MOs including three bonding MOs (π2p_x, π2p_y, and σ2p_z) and three anti-bonding MOs (π*2p_x, π*2p_y, and σ*2p_z).

The MOs discussed above are located on the MO diagram in an increasing energy order.

3. Fill the molecular orbitals of N₂ with electrons following the energy and bonding principles

A total of 4 electrons are present in the 1s atomic orbitals of two nitrogen atoms. Therefore, as per the Aufbau principle, the first two electrons go in the lowest energy σ1s MO, and the remaining two are accommodated in σ*1s.

Similarly, the 4 electrons in the 2s atomic orbitals of nitrogen, are uniformly distributed between σ2s and σ*2s molecular orbitals of N₂.

In contrast, there are a total of 3 + 3 = 6 electrons in the 2p atomic orbitals of two N-atoms.

2 out of the 6 electrons are first singly filled in the π2p_x and π2p_y MOs of N₂ (Hund’s rule). These are then paired up in an opposite spin (Pauli Exclusion Principle).

The remaining 2 electrons (6 – 4 = 2) are finally situated as an electron pair in the σ2p_z molecular orbital of N₂. In this way, the 2p antibonding molecular orbitals of N₂ stay entirely unoccupied. 

As a result, we have successfully drawn the Molecular orbital diagram of N₂ as shown below.

Is N₂ diamagnetic or paramagnetic?

N₂ is a diamagnetic molecule as there are no unpaired electrons in its molecular orbital diagram.

Diamagnetic substances possess no permanent dipole moment value; therefore they get repelled by an external magnetic field.

Bond order of N₂

The bond order formula is:

∴ Bond order = (N_b –N_a)/2

• N_b = Electrons present in the bonding MOs (Bonding electrons).

∴ Electrons in σ1s + σ2s + π2p_x + π2p_y + σ2p_z = 2 + 2 + 2 + 2 + 2 = 10

• N_a= Electrons present in the anti-bonding MOs (Anti-bonding electrons).

∴ Electrons in σ*1s + σ*2s = 2 + 2 = 4

⇒ Bond order of N₂ = (10 – 4)/2 = 6/2 = 3. 

A bond order of 3 implies that there is a triple covalent bond (N≡N) between two N-atoms in the N₂ molecule.

A positive and significantly high bond order value also shows that the nitrogen molecule is extremely stable and it thus exists abundantly in Nature (76% of air is N₂).

MO diagrams and bond orders of N₂⁺, N₂^–, N₂²⁺ and N₂^2-  

N₂⁺ represents a cation of nitrogen. It is a molecular ion formed by the loss of 1 valence electron from the nitrogen molecule.

This valence electron is removed from one of the three half-filled 2p orbitals of an individual N-atom. Upon N₂⁺ formation, the σ2p_z MO gets unpaired. As a result, N₂⁺ is paramagnetic and its Molecular orbital diagram is as shown below:

∴ Bond order of N₂⁺ = (N_b –N_a)/2 = (9 – 4)/2 = 2.5  

MO electronic configuration of N₂⁺: (σ1s²) (σ^*1s²) (σ2s²) (σ^*2s²) (π2p_x²) (π2p_y²) (σ2p_z¹)

⇒ Paramagnetic

Conversely, N₂²⁺ is formed by removing two electrons, one from a 2p AO of each N-atom. Therefore, both the electrons present in the σ2p_z of N₂ are removed resulting in the N₂²⁺ Molecular orbital diagram.

∴ Bond order of N₂²⁺ = (N_b –N_a)/2 = (8 – 4)/2 = 2

MO electronic configuration of N₂²⁺: (σ1s²) (σ^*1s²) (σ2s²) (σ^*2s²) (π2p_x²) (π2p_y²)

⇒ Diamagnetic  

Contrarily, N₂^– is a negatively charged anion. 1 extra valence electron is gained by a N-atom. This extra electron singly fills π*2p_x in the Molecular orbital diagram of N₂^–.

∴ Bond order of N₂^– = (N_b –N_a)/2 = (10 – 5)/2 = 2.5

MO electronic configuration of N₂^–: (σ1s²) (σ^*1s²) (σ2s²) (σ^*2s²) (π2p_x²) (π2p_y²) (σ2p_z²) (π*2p_x¹)

⇒ Paramagnetic

Finally, N₂^2- is formed when 2 extra valence electrons are gained in the 2p AOs, one by each parent N-atom. Thus, as per Hund’s rule, these 2 additional valence electrons singly occupy the π*2p_x and π*2p_y molecular orbitals in the Molecular orbital diagram of N₂^2-, again a paramagnetic substance.

 ∴ Bond order of N₂^2- = (N_b –N_a)/2 = (10 –6)/2 = 2

MO electronic configuration of N₂^2-: (σ1s²) (σ^*1s²) (σ2s²) (σ^*2s²) (π2p_x²) (π2p_y²) (σ2p_z²) (π*2p_x¹) (π*2p_y¹)

⇒ Paramagnetic

Stability and bond lengths of N₂, N₂⁺, N₂^–, N₂²⁺ and N₂^2-  

For the nitrogen family, the bond order increases as follows:

N₂²⁺ = N₂^2- < N₂^– = N₂⁺ < N₂

Generally, bond order also signifies the strength of a bond. However, other factors such as the pattern in which electrons are filled in the respective bonding and antibonding MOs, may also affect overall bond strength and molecular stability.

Among the five species discussed above, the bond orders of N₂²⁺ and N₂^2- are equal. Similarly, N₂^– and N₂⁺ possess an identical bond order value. Still, N₂^2- is less stable than N₂²⁺.

N₂^2- contains unpaired electrons in the high-energy antibonding MOs (π*2p_x and π*2p_y) which leads to an overall destabilizing effect. However, no such electrons are present in the MO diagram of N₂²⁺ therefore, it is comparatively more stable.

A greater number of bonding electrons have a stabilizing effect while a greater number of antibonding electrons have a destabilizing effect.

Similarly, N₂⁺ is relatively more stable than N₂^– considering a higher number of antibonding electrons in the latter.

Thus, on account of the factors discussed above, the N₂ family is arranged in an ascending order of bond strength, energy, and stability as follows:

N₂^2- < N₂²⁺ < N₂^– < N₂⁺ < N₂

On the other hand, bond order is inversely proportional to bond strength. The greater the bond strength, the closer the bonded atoms are to each other, i.e., a shorter bond length.

Therefore, the suspected bond lengths of the nitrogen family follow the reverse order:

N₂^2- > N₂²⁺ > N₂^– > N₂⁺ > N₂

Also read:

• Molecular orbital diagram (MO) for Ne2, Ne2+, Ne22+, and Bond order
• Molecular orbital diagram (MO) for C2, C2-, C2+, C22+, C22-, and Bond order
• Molecular orbital diagram (MO) for He2+, He2, He22+, He22-, He2-, and Bond order
• Molecular orbital diagram (MO) for H2, H2-, H2+, H22-, H22+, and Bond order
• Molecular orbital diagram (MO) for Li2, Li2+, Li2-, Li22-, Li22+, and Bond order
• Molecular orbital diagram (MO) for Be2, Be2+, Be22-, Be2-, Be22+, and Bond order
• Molecular orbital diagram (MO) for B2, B2+, B22-, B2-, B22+, and Bond order
• Molecular orbital diagram (MO) for O2+, O2-, O22+, O22-, O2, and Bond order
• Molecular orbital diagram (MO) for F2, F2+, F2-, F22+, F22-, and Bond order
• Molecular orbital diagram (MO) for NF+, NF, NF-, and Bond order
• Molecular orbital diagram (MO) for NO, NO+, NO-, and Bond order
• H2O Molecular orbital diagram (MO), Bond order
• HF Molecular orbital diagram (MO), Bond order

FAQ

Summary

• Nitrogen (N₂) is a homonuclear diatomic molecule. Two identical N-atoms combine to form N₂.
• The MO electronic configuration of N₂ is (σ1s²)(σ^*1s²)(σ2s²)(σ^*2s²)(π2p_x²)(π2p_y²)(σ2p_z²).
• The absence of any unpaired electrons in the Molecular orbital diagram of N₂ denotes it is a diamagnetic molecule.  
• The bond order of N₂ is 3 which means there is ideally a triple covalent bond between two N-atoms in the N₂ molecule.
• N₂⁺, N₂^–, N₂²⁺, and N₂^2- are molecular ions formed by the loss or gain of electrons in the valence shell atomic orbitals of individual N-atoms.
• The bond order follows the ascending pattern: N₂^2- = N₂²⁺ < N₂^– = N₂⁺ < N₂ i.e., 2, 2.5, and 3 respectively.
• N₂⁺, N₂^–, and N₂^2- are paramagnetic while N₂²⁺ is diamagnetic in nature.