Molecular orbital (MO) diagram for NO, NO+, NO-, and their bond order

You may find drawing the molecular orbital (MO) diagram of nitrogen monoxide (NO) complicated at first glance, but it will no longer be the case by the end of this article. 

In this article, you will find the easiest way of drawing the Molecular orbital diagrams of NO, NO⁺, and NO^–. Using these MO diagrams, we will also teach you to calculate the bond orders of these molecular species and predict their bond length, bond strength, and magnetic properties in relation to one another.

So what are you waiting for? Dive into the article and start reading!

How to draw the molecular orbital (MO) diagram of NO 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 (BMO) is formed by the linear combination of two AOs in the same phase.

Contrarily, an antibonding molecular orbital (ABMO) 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 displayed schematically on an energy level diagram called the molecular orbital (MO) diagram.

NO is a heteronuclear diatomic molecule i.e., a molecule containing two atoms from more than one type of element, in this case, nitrogen (N) and oxygen (O).

Follow the steps given below and draw the molecular orbital diagram of NO with us.

Steps for drawing the molecular orbital (MO) diagram of NO with its bond order

1. Write down the electronic configuration of NO atoms

NO is made up of one atom of nitrogen (N) and one oxygen (O) atom.

The electronic configuration of a N-atom is 1s² 2s² 2p³.

The electronic configuration of an O-atom is 1s² 2s² 2p⁴.

7 electrons of nitrogen and 8 electrons of oxygen make a total of 7 + 8 = 15 electrons available to be filled in the Molecular orbital diagram of the NO molecule.

Usually, only the valence electrons are displayed in the MO diagram of a molecule, therefore, it is important to note that there are 5 + 6 = 11 valence electrons in the NO molecular orbital diagram.

2. Determine whether the molecule is homonuclear or heteronuclear

As discussed above, NO is a heteronuclear molecule. This implies that in the Molecular orbital diagram of NO, the individual atomic orbitals of the N-atom and the O-atom do not lie at the same energy level.

Oxygen is a more electronegative element than nitrogen. Conversely, it bears a higher effective nuclear charge than the latter. Therefore, the AOs of the O-atom lie at a relatively lower energy level than the corresponding AOs of the N-atom, as shown below.  

As per the rule of LCAO, the 1s atomic orbital of the O-atom overlaps with the 1s atomic orbital of the N-atom 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 six 2p atomic orbitals, three from each of the two atoms, combine 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.

The 2p bonding MOs lie closer to the atomic orbitals of the O-atom while the 2p antibonding MOs lie closer to the atomic orbitals of the N-atom, in terms of energy.

3. Fill the molecular orbitals of NO with electrons following the energy and bonding principles

A total of 4 electrons are present in the 1s atomic orbitals of the N-atom and the O-atom. 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, are uniformly distributed between σ2s and σ*2s molecular orbitals of NO.

Next, there are a total of 7 electrons in the 2p atomic orbitals of the N-atom and the O-atom. The first two electrons singly fill the π2p_x and π2p_y MOs (as per Hund’s rule) and then pairing occurs in an opposite spin (as per Pauli Exclusion Principle).

The next two electrons are subsequently filled as an electron pair in π2p_z MO while the last electron singly occupies the higher energy π*2p_x antibonding Molecular orbitals, as shown below.

As per the correct molecular orbital diagram of NO successfully drawn above, the MO electronic configuration of NO is (σ1s²) (σ*1s²) (σ2s²) (σ*2s²) (π2p_x²) (π2p_y²) (σ2p_z²) (π*2p_x¹).

Is NO diamagnetic or paramagnetic?

The presence of an unpaired electron in the pi antibonding molecular orbital of NO (π^*2p_x) confirms the paramagnetic nature of nitrogen monoxide.

Possessing unpaired electrons, paramagnetic substances when exposed to an external magnetic field, experience a magnetic pull and behave like magnets themselves.  

Bond order of NO

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 + π*2p_x = 2 + 2 + 1 = 5

⇒ Bond order of NO = (10 – 5)/2 = 5/2 = 2.5. 

Bond order > 0 implies that NO is a stable molecule.

Interestingly, the bond order value equal to 2.5 denotes that the strength of an NO bond is intermediate to that of the N≡N triple bond and an O=O double covalent bond.

MO diagrams and bond orders of NO⁺ and NO^–

NO⁺ represents the nitrosonium ion. It is formed when the O-atom loses a valence electron to change into the O⁺ ion or the N-atom loses an electron to change into N⁺. In either case, this makes a total of 15-1 = 14 electrons available to be filled in the MO diagram of NO⁺.

Thus, the electron present in the highest energy π*2p_x MO of NO is removed to transform the NO molecular orbital diagram into the NO⁺ molecular orbital diagram, as shown below.

The absence of any unpaired electron in the above diagram means NO⁺ is a diamagnetic molecular species, unlike NO.

∴ Bond order of NO⁺ = (N_b –N_a)/2 = (10 –4)/2 = 3  

Molecular orbital electronic configuration of NO⁺: (σ1s²) (σ^*1s²) (σ2s²) (σ^*2s²) (π2p_x²) (π2p_y²) (σ2p_z²).

⇒ Diamagnetic

In contrast, NO^– is a negatively charged molecular ion, produced as a result of the strongly electronegative O-atom gaining an extra valence electron.

This makes a total of 15 + 1 = 16 electrons available to be filled in the Molecular orbital diagram of NO^–. This extra electron is accommodated in the π*2p_y molecular orbital, as shown below.

The presence of two unpaired electrons in the above Molecular orbital diagram suggests the paramagnetic nature of NO^–. 

∴ Bond order of NO^– = (N_b –N_a)/2 = (10 –6)/2 = 2

Molecular orbital electronic configuration of NO^–: (σ1s²) (σ^*1s²) (σ2s²) (σ^*2s²) (π2p_x²) (π2p_y²) (σ2p_z²) (π*2p_x¹) (π*2p_y¹).

⇒ Paramagnetic  

The bond order also signifies the strength and stability of a bond. Therefore, for the NO family, the bond strength increases in the order:

NO^– < NO < NO⁺

However, an increasing bond order in turn means a decreasing bond length. Therefore, the bond lengths of the NO family follow the reverse order:

NO^– > NO > NO⁺

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 N2, N2+, N22-, N22+, N2-, 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
• H2O Molecular orbital diagram (MO), Bond order
• HF Molecular orbital diagram (MO), Bond order
• Molecular orbital diagram (MO) for O2+, O2-, O22+, O22-, O2, and Bond order

FAQ

Summary

• Nitrogen monoxide (NO) is a heteronuclear diatomic molecule, containing two atoms from two different elements.
• The Molecular orbital electronic configuration of NO is (σ1s²)(σ^*1s²)(σ2s²)(σ^*2s²) (π2p_x²)(π2p_y²) (σ2p_z²)(π^*2p_x¹)
• The presence of an unpaired electron in the π2p antibonding Molecular orbital of NO means it is paramagnetic in nature.
• The bond order of NO is 2.5, which implies that it is a stable molecule.
• NO⁺ and NO^– are molecular ions formed by the loss or gain of electrons in the valence shell atomic orbitals of constituent atoms.
• The bond order follows the ascending pattern: NO^– < NO < NO⁺ e., 2, 2.5, and 3 respectively.
• NO^– is paramagnetic while NO⁺ is a diamagnetic molecular species.