Molecular orbital (MO) diagram for O2 , O2+, O2-, O22+, O22-, and their bond order

If you are curious to know the chemistry behind chemical reactions and bond formation, then this article is for you.

In this article, we will discuss the formation of oxygen (O₂) molecules under the light of the molecular orbital theory (MOT) – a game changer in the world of chemistry.

Additionally, we will teach you how to draw the molecular orbital diagram of O₂, what is its bond order, whether it is paramagnetic or diamagnetic in nature, etc. We will also talk about the related ions (O₂⁺, O₂^–, O₂²⁺, and O₂^2-).

So for this super interesting information, continue reading!

How to draw the molecular orbital (MO) diagram of O₂ 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, stability, magnetic behavior, etc.

Oxygen (O₂) is a homonuclear diatomic molecule. Two O-atoms combine to form an O₂ molecule.

We can easily draw the MO diagram of O₂ following the steps given below.

Steps for drawing the molecular orbital (MO) diagram of O₂ with its bond order

1. Write down the electronic configuration of O₂ atoms

O₂ consists of two oxygen (O) atoms.

The electronic configuration of each O-atom is 1s² 2s² 2p_x¹ 2p_y¹ 2p_z².

Usually, only the valence electrons are displayed in the MO diagram of a molecule, therefore, it is important to note that each O-atom contains 6 valence electrons.

2. Determine whether the molecule is homonuclear or heteronuclear

O₂ 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 oxygen overlap to form 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 O-atom combine to form six MOs including three bonding MOs (σ2p_z, π2p_x, and  π2p_y) 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 O₂ with electrons following the energy and bonding principles

A total of 4 electrons are present in the 1s atomic orbitals of two oxygen 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, are uniformly distributed between σ2s and σ*2s molecular orbitals of O₂.

In contrast, there are a total of 4 + 4 = 8 electrons in the 2p atomic orbitals of two O-atoms. Thus, the first two electrons are accommodated in σ2p_z while the next two are placed in π2p_x and π2p_y and consequently paired up.

2 + 2 + 2 = 6 leaves behind 8 – 6 = 2 valence electrons which are finally placed in the high energy antibonding MOs (π*2p_x and π*2p_y), one in each.

This successfully completes the molecular orbital diagram of O₂ as shown below.

Hence, the molecular orbital electronic configuration of O₂ is:

(σ1s²)(σ^*1s²)(σ2s²)(σ^*2s²)(σ 2p_z²)(π2p_x²)(π2p_y²)(π^*2p_x¹)(π^*2p_y¹)

Is O₂ diamagnetic or paramagnetic?

The presence of 2 unpaired electrons in the antibonding molecular orbitals of O₂ (π^*2p_x and π^*2p_y) confirms the paramagnetic nature of oxygen.

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

Bond order of O₂

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 + σ2p_z + π2p_x + π2p_y = 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 + π*2p_y = 2 + 2 + 1 + 1 = 6

⇒ Bond order of O₂ = (10 – 6)/2 = 4/2 = 2.

Thus, there is a double covalent bond (O=O) between the two oxygen atoms in O₂.

MO diagrams and bond orders of O₂⁺, O₂^–, O₂²⁺ and O₂^2- 

O₂⁺ represents a cation of oxygen. It is a molecular ion formed by the loss of 1 valence electron from the oxygen molecule.

This valence electron is removed from the least stable, highest energy atomic orbital of an O-atom i.e., 2p_z.  As a result, the MO diagram of oxygen is transformed as shown below:

∴ Bond order of O₂⁺ = (N_b –N_a)/2 = (10 – 5)/2 = 2.5

⇒ Paramagnetic

Similarly, O₂²⁺ is formed by removing two electrons, one from a 2p AO of each O-atom. Therefore, both the unpaired electrons (π^*2p_x¹ and π^*2p_y¹) are removed from the O₂ MO diagram to produce the O₂²⁺ MO diagram.

∴ Bond order of O₂²⁺ = (N_b –N_a)/2 = (10 – 4)/2 = 3

⇒ Diamagnetic

Contrarily, O₂^– is a negatively charged anion. 1 extra valence electron is gained by an O-atom. This extra electron is accommodated in the antibonding MO such that π*2p_x gets paired up.

∴ Bond order of O₂^– = (N_b –N_a)/2 = (10 – 7)/2 = 1.5

⇒ Paramagnetic

Finally, O₂^2- is formed when 2 extra valence electrons are gained, one by each parent O-atom. Consequently, both the antibonding MOs get paired up (π*2p_x + π*2p_y). This produces a diamagnetic molecular ion with a bond order of:

Bond order of O₂^2- = (N_b –N_a)/2 = (10 – 8)/2 = 1

The bond order signifies the strength of a bond. Therefore, for the oxygen family, the bond strength increases in the order:

O₂^2- < O₂^– < O₂ < O₂⁺ < O₂²⁺

However, an increasing bond order in turn means a decreasing bond length. Therefore, the bond lengths of the oxygen family follow the order:

O₂^2- > O₂^– > O₂ > O₂⁺ > O₂²⁺

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
• 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

• Oxygen (O₂) is a homonuclear diatomic molecule. Two identical O-atoms combine to form O₂.
• As per MOT, 10 atomic orbitals of individual oxygen atoms combine to form 10 molecular orbitals including 5 bonding and 5 antibonding MOs.
• The Molecular orbital electronic configuration of O₂ is (σ1s²)(σ^*1s²)(σ2s²)(σ^*2s²) (σ 2p_z²) (π2p_x²)(π2p_y²)(π^*2p_x¹) (π^*2p_y¹).
• The presence of 2 unpaired electrons in the π2p antibonding MOs of O₂ means it is paramagnetic in nature.
• The bond order of O₂ is 2 which means there is a double covalent bond between two O-atoms in the O₂
• O₂⁺, O₂^–, O₂²⁺, and O₂^2- are molecular ions formed by the loss or gain of electrons in the valence shell atomic orbitals of individual O-atoms.
• The bond order follows the ascending pattern: O₂^2- < O₂^– < O₂ < O₂⁺ < O₂²⁺ e., 1, 1.5, 2, 2.5, and 3 respectively.
• O₂⁺ and O₂^– are both paramagnetic while O₂²⁺ and O₂^2- are diamagnetic in nature.