Molecular orbital (MO) diagram for Li2, Li2+, Li2-, Li22-, Li22+, and their bond order

Do you want to learn how to draw the molecular orbital (MO) diagram of Li₂ and calculate its bond order? Then, this article is for you.

In this article, you will find a step-by-step guide on drawing the molecular orbital (MO) diagram of Li₂, calculating its bond order, and predicting its magnetic properties. We have also included some extremely valuable information on the MO diagrams and bond orders of Li₂⁺, Li₂^–, Li₂²⁺, and Li₂^2-.

So without any further delay, dive into the article and start reading! Happy learning.

How to draw the molecular orbital (MO) diagram of Li₂ 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.

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

Li₂ is a homonuclear diatomic molecule i.e., a molecule containing two identical atoms from the same element, in this case, lithium (Li). 

Drawing the MO diagram of Li₂ is a super easy task if you follow the steps given below.

Steps for drawing the molecular orbital (MO) diagram of Li₂ with its bond order

1. Write down the electronic configuration of Li₂ atoms

Li₂ comprises two identical lithium (Li) atoms.

The electronic configuration of each Li-atom is 1s² 2s¹.

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

2 Li-atoms together make a total of 2(2) = 4 electrons and 2(1) = 2 valence electrons to be filled in the Molecular orbital diagram of Li₂.

2. Determine whether the molecule is homonuclear or heteronuclear

Li₂ 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 1s atomic orbitals of two lithium atoms 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 Li-atom 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, as shown below.

3. Fill the molecular orbitals of Li₂ with electrons following the energy and bonding principles

A total of 4 electrons are present in the 1s atomic orbitals of two lithium 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.

The 2 electrons present in the 2s atomic orbitals of lithium are then placed in the σ2s molecular orbital, following Hund’s rule and Pauli Exclusion principle.

As all 6 electrons are already consumed and there are no more electrons to be filled in this MO diagram, therefore the 2s sigma antibonding as well as all the 2p bonding and antibonding MOs of Li₂ stay unoccupied.

This successfully completes the Li₂ Molecular orbital diagram shown below.

Is Li₂ diamagnetic or paramagnetic?

The absence of any unpaired electrons in the Molecular orbital diagram of Li₂ reveals its diamagnetic nature.

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

Bond order of Li₂

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 = 2 + 2 = 4

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

∴ Electrons in σ*1s = 2

⇒ Bond order of Li₂ = (4 – 2)/2 = 2/2 = 1.

Bond order > 0 predicts the stability of the Li₂ molecule. Moreover, a bond order of 1 means that there is a single covalent bond between two Li-atoms.

MO diagrams and bond orders of Li₂⁺, Li₂^–, Li₂²⁺ and Li₂^2-  

Li₂⁺ represents a cation of Li₂, carrying a positive 1 charge which means it is formed by the loss of 1 valence electron from one of the two neutral Li-atoms.

This valence electron is removed from the singly filled 2s atomic orbital of a lithium atom.  

This makes a total of 6 -1 = 5 electrons available to be filled in the Molecular orbital diagram of Li₂⁺. Thus, the electrons present in σ2s MO get unpaired, as shown below.

The presence of an unpaired electron means Li₂⁺ is paramagnetic, unlike Li₂.

∴ Bond order of Li₂⁺ = (N_b –N_a)/2 = (3-2)/2 = 0.5   

MO electronic configuration of Li₂⁺: (σ1s²) (σ^*1s²) (σ2s¹)

⇒ Paramagnetic

Conversely, Li₂²⁺ is formed by removing two electrons. 6 – 2 = 4 electrons are available to be filled in the MO diagram.

Therefore, both the electrons present in σ2s MO of Li₂ are removed, producing the Li₂²⁺ Molecular orbital diagram, containing no unpaired electron.

∴ Bond order of Li₂²⁺ = (N_b –N_a)/2 = (2 – 2)/2 = 0

MO electronic configuration of Li₂²⁺: (σ1s²) (σ^*1s²)

⇒ Diamagnetic

A bond order of zero denotes that Li₂²⁺ is only a hypothetical molecular ion and it does not exist in real life.

Li₂^– is a negatively charged ion (anion). 1 extra valence electron is gained by a Li-atom.

This makes a total of 6 + 1 = 7 electrons available to be filled in the Molecular orbital diagram of Li₂^–. As per the Aufbau principle, this extra valence electron is placed in the 2s sigma antibonding molecular orbital (σ*2s) as shown in the MO diagram of Li₂^– drawn below.

The presence of an unpaired electron predicts the paramagnetic nature of the anion.   

∴ Bond order of Li₂^– = (N_b –N_a)/2 = (4– 3)/2 = 0.5  

MO electronic configuration of Li₂^–: (σ1s²) (σ^*1s²) (σ2s²) (σ^*2s¹)

⇒ Paramagnetic

Finally, Li₂^2- is formed when 2 extra valence electrons are gained in the 2p AOs, one by each parent Li-atom.

6 + 2 = 8 electrons available to be filled in the Molecular orbital diagram of Li₂^2-. Thus, these two electrons are accommodated as an electron pair in σ*2s in the molecular orbital diagram as shown below.

The absence of any unpaired electron makes Li₂^2- a diamagnetic molecular ion.

 ∴ Bond order of Li₂^2- = (N_b –N_a)/2 = (4 –4)/2 = 0

MO electronic configuration of Li₂^2-: (σ1s²) (σ^*1s²) (σ2s²) (σ^*2s²)

⇒ Diamagnetic

Again, a bond order of zero implies that Li₂^2- just like Li₂²⁺ is extremely unstable and thus non-existent.

Bond stability and bond lengths of Li₂, Li₂⁺, Li₂^–, Li₂²⁺ and Li₂^2-  

To sum up the information discussed above, for the Li₂ family, the bond order increases as follows:

Li₂²⁺ = Li₂^2- < Li₂⁺ = Li₂^– < Li₂

However, you must keep in mind that the strength of a bond also depends on the relative placement of electrons in the bonding and antibonding MOs, in addition to the bond order value.

Hence, although the bond order of Li₂⁺ and Li₂^– is equal, similarly that of Li₂²⁺ and Li₂^2- is equal, still, Li₂⁺ is more stable as compared to Li₂^–; similarly, Li₂²⁺ is more stable than Li₂^2-, due to a greater number of antibonding electrons in the respective anion as opposed to the cation.

Therefore, the stability of the Li₂ family increases in the order shown below:

Li₂^2- < Li₂²⁺ < Li₂^– < Li₂⁺ < Li₂

On the other hand, bond length is inversely proportional to bond strength, so for the Li₂ family, the bond lengths decrease in the following order:  

Li₂^2- > Li₂²⁺ > Li₂^– > Li₂⁺ > Li₂

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 O2+, O2-, O22+, O22-, O2, 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

• Li₂ is a homonuclear diatomic molecule. Two identical lithium atoms combine to form Li₂.
• The MO electronic configuration of Li₂ is (σ1s²) (σ^*1s²) (σ2s²).
• The absence of any unpaired electrons in the Molecular orbital diagram of Li₂ denotes it is a diamagnetic molecule. 
• The bond order of Li₂ is 1, which means there is a single covalent bond between two lithium atoms in Li₂.
• Li₂⁺, Li₂^–, Li₂²⁺, and Li₂^2- are molecular ions formed by the loss or gain of electrons in the valence shell atomic orbitals of individual Li-atoms.
• The bond order follows the ascending pattern: Li₂²⁺ = Li₂^2- < Li₂⁺ = Li₂^– < Li₂ i.e., 0, 0.5, and 1 respectively.
• Li₂⁺ and Li₂^– are both paramagnetic while Li₂²⁺ and Li₂^2- are diamagnetic molecular ions.