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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 Li22-.

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

Name of molecule

Di-Lithium 

Chemical formula

Li2

Electronic configuration

1s2 2s1

Molecular orbital electronic configuration

(σ1s2)(σ*1s2)(σ2s2)

Number of electrons in bonding MOs

4

Number of electrons in anti-bonding MOs

2

Bond order

1

Paramagnetic or Diamagnetic?

Diamagnetic 

 

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.

formation of bonding and antibonding Molecular orbital diagram (MO) for Li2

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 (π2px, π2py, and σ2pz) and three anti-bonding MOs (π*2px, π*2py, and σ*2pz).

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

Energy level diagram for Li2

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.

Molecular orbital diagram (MO) of Lithium (Li2) and it's bond order

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 formula for Li2

∴ Bond order = (Nb –Na)/2

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

∴ Electrons in σ1s + σ2s = 2 + 2 = 4

  • Na= 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 Li22-  

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.

Li2+ Molecular orbital diagram (MO) and Bond order

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

∴ Bond order of Li2+ = (Nb –Na)/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+ = (Nb –Na)/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.

Li22+ Molecular orbital diagram (MO) and Bond order

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 = (Nb –Na)/2 = (4– 3)/2 = 0.5  

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

Paramagnetic

Li2- Molecular orbital diagram (MO) and Bond order

Finally, Li22- 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 Li22-. 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 Li22- a diamagnetic molecular ion.

 ∴ Bond order of Li22- = (Nb –Na)/2 = (4 –4)/2 = 0

MO electronic configuration of Li22-: (σ1s2) (σ*1s2) (σ2s2) (σ*2s2)

Diamagnetic

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

Li22- Molecular orbital diagram (MO) and Bond order

Bond stability and bond lengths of Li2, Li2+, Li2, Li22+ and Li22-  

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

Li22+ = Li22- < 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 Li22- is equal, still, Li2+ is more stable as compared to Li2; similarly, Li22+ is more stable than Li22-, 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:

Li22- < 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:  

Li22- > Li22+ > Li2> Li2+ > Li2

Also read:

FAQ

How to draw the MOT diagram of Li2?

The molecular orbital (MO) diagram of Li2 is shown below.

MO diagram of Li2

The MO electronic configuration of Li2 is (σ1s2) (σ*1s2) (σ2s2).

The bond order of Li2 is calculated as follows:

∴ (Nb – Na)/2

∴ (4 – 2)/2

 = 1

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

What is the molecular orbital configuration of Li2?

The molecular orbital electronic configuration of Li2 is (σ1s2) (σ*1s2) (σ2s2).

Is Li2 diamagnetic or paramagnetic?

Li2 is diamagnetic as there are no unpaired electrons in any of its molecular orbitals.

What is the bonding order of Li2?

The bond order of Li2 is 1 which means there is a single covalent bond between two Li-atoms in Li2.

Which one is more stable according to MOT, Li2+ or Li22-, and why? 

The stability of a molecule or molecular ion is directly related to its bond order. The higher the bond order value, the greater the stability.

The bond order of Li2+ is 0.5 while that of Li22- is 0. Therefore, Li2+ is more stable as compared to Li22- as per the molecular orbital theory.

Give the electron configuration for the ions Li2+ and Li2 in molecular orbital terms. Compare the Li-Li bond order in these ions with the bond order in Li2

The MO electronic configuration of Li2+ is (σ1s2) (σ*1s2) (σ2s1) while that of Li2 is (σ1s2) (σ*1s2) (σ2s2) (σ*2s1). There is 1 less electron in the sigma bonding molecular orbital of Li2+ as compared to Li2.

bond order of Li2+ vs Li2-

(4-3)/2 = 0.5

However, the Li-Li bond order in Li2 is 1.

Bond order (Li2) > [Bond order (Li2+) = Bond order of (Li2)].  

The MO diagrams for which of the following molecules or molecular ions are similar?

  • A)  Be2 and Li22-
  • B)  Be2 and Li2
  • C)  Be2 and Li2+
  • D) Be2 and Li22+

Option A gives the correct answer. There are a total of 8 electrons to be filled in the Molecular orbital diagrams of both Be2 and Li22-.

The MO electronic configuration of each of Be2 and Li22- is (σ1s2) (σ*1s2) (σ2s2) (σ*2s2). Hence, the Molecular orbital diagrams of Be2 and Li22- look the same, as shown below.

Molecular orbital diagram of Be2 vs Li22-

How many unpaired electrons are there in the Li-atom and in the Li2 molecule?

The electronic configuration of a Li-atom is 1s2 2s1. Therefore, there is 1 unpaired electron in the Li-atom.

In comparison, the MO electronic configuration of the Li2 molecule is (σ1s2) (σ*1s2) (σ2s2) which means there are no unpaired electrons present in it.

Which of the following options gives the correct order of stability of the species  Li2, Li2, and Li2+?

  • A) Li2< Li2+ < Li2
  • B) Li2 < Li2 < Li2+
  • C) Li2 < Li2 < Li2+
  • D) Li2 < Li2+ < Li2

Option A gives the correct answer. The strength of a bond is directly related to its bond order value. Out of the three species given above, Li2 is the most stable, possessing a higher bond order value i.e., 1 as compared to both Li2 and Li2+.

Although the bond order of Li2+ is equal to that of Li2 i.e., 0.5 still Li2+ is considered slightly more stable as  Li2 contains a single unpaired electron in its 2s sigma antibonding MO.

The greater the number of electrons in the higher energy antibonding MOs, the lower the stability. 

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 Li22- 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+ = Li22- < Li2+ = Li2 < Li2 i.e., 0, 0.5, and 1 respectively.
  • Li2+ and Li2 are both paramagnetic while Li22+ and Li22- are diamagnetic molecular ions. 
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Vishal Goyal author of topblogtenz.com

Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/

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