Molecular orbital (MO) diagram for Be2, Be2+, Be22-, Be2-, Be22+, and their bond order

Do you know how to draw the molecular orbital (MO) diagram of Be₂? Probably this is what you are here to learn. So what are you waiting for? Dive into the article and start reading!

In this article, we have covered everything related to the Molecular orbital diagram of Be₂ including how to draw it, what is its MO electronic configuration, bond order, magnetic properties, etc.

Additionally, we will also teach you how to draw the Molecular orbital diagrams of related ions i.e., Be₂⁺, Be₂^–, Be₂²⁺, and Be₂^2-, calculating the bond order for each.

How to draw the molecular orbital (MO) diagram of Be₂ 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 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.

Be₂ is a homonuclear diatomic molecule i.e., a molecule comprising two identical atoms from the same element i.e. beryllium (Be).

Drawing the MO diagram of Be₂ is a super easy task if you follow the steps given below.

Steps for drawing the molecular orbital (MO) diagram of Be₂ with its bond order

1. Write down the electronic configuration of Be₂ atoms

Be₂ comprises two identical beryllium (Be) atoms.

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

Usually, only the valence electrons are displayed in the MO diagram of a molecule, therefore, it is important to note that each Be-atom contains 2 valence electrons only.

2 Be-atoms together make a total of 2(4) = 8 electrons and 2(2) = 4 valence electrons to be filled in the Molecular orbital diagram of Be₂.

2. Determine whether the molecule is homonuclear or heteronuclear

Be₂ 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 beryllium 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 Be-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).

You must note that the two p-orbitals overlapping end to end produce sigma bonding (σ2p_z) and antibonding (σ*2p_z) MOs. Contrarily, the other four p-orbitals overlapping side by side in sets of two, produce the pi bonding (π2p_x and π2p_y ) and antibonding (π*2p_x and π*2p_y) MOs.

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

3. Fill the molecular orbitals of Be₂ with electrons following the energy and bonding principles

A total of 4 electrons are present in the 1s atomic orbitals of two beryllium 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 beryllium, are uniformly distributed between σ2s and σ*2s molecular orbitals of Be₂.

As all 8 electrons are already consumed and there are no more electrons to be filled in this MO diagram, therefore the 2p bonding and antibonding MOs of Be₂ stay unoccupied.

This successfully completes the Be₂ Molecular orbital diagram shown below.

Is Be₂ diamagnetic or paramagnetic?

The absence of any unpaired electrons in the Molecular orbital diagram of Be₂ reveals its diamagnetic nature.

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

Bond order of Be₂

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 + σ*2s = 2 + 2 = 4

⇒ Bond order of Be₂ = (4 – 4)/2 = 0/2 = 0.

A bond order of zero implies that there is no chemical bond between two Be-atoms. Isn’t it a bit strange?

Well, it is, but do not get confused because Be₂ is a hypothetical (imaginary) molecule and scientists create such hypothetical situations all the time to see how atoms react and new products are formed (or in this case not formed).  

However, the non-viability of Be₂ does not mean its related ions (Be₂⁺, Be₂^–, Be₂²⁺, and Be₂^2-) can also not be formed. Let’s see how, drawing their MO diagrams and calculating their bond order values.

MO diagrams and bond orders of Be₂⁺, Be₂^–, Be₂²⁺ and Be₂^2-  

Be₂⁺ represents a cation, carrying a positive 1 charge which means it is formed by the loss of 1 valence electron from one of the two neutral Be-atoms.

This valence electron is removed from the 2s atomic orbital of a beryllium atom.

This makes 8-1 = 7 total electrons available to be filled in the Molecular orbital diagram of Be₂⁺. Thus, the electrons present in the 2s sigma antibonding MO (σ*2s) get unpaired, as shown below.

The presence of an unpaired electron in σ*2s means Be₂⁺ is paramagnetic, unlike Be₂.

∴ Bond order of Be₂⁺ = (N_b –N_a)/2 = (4-3)/2 = 0.5   

MO electronic configuration of Be₂⁺: (σ1s²) (σ^*1s²) (σ2s²) (σ^*2s¹)

⇒ Paramagnetic

Conversely, Be₂²⁺ is formed by removing two electrons from Be₂, i.e., one electron lost by each parent Be-atom. 8 – 2 = 6 electrons are now available to be filled in the MO diagram.

Therefore, both the electrons present in σ*2s MO of Be₂ are removed, producing the Be₂²⁺ Molecular orbital diagram, containing no unpaired electron.

∴ Bond order of Be₂²⁺ = (N_b –N_a)/2 = (4 – 2)/2 = 1

MO electronic configuration of Be₂²⁺: (σ1s²) (σ^*1s²) (σ2s²)

⇒ Diamagnetic 

A bond order of 1 means there is a clear possibility of Be₂²⁺ formation via a Be⁺-Be⁺ single covalent bond.

Be₂^– is a negatively charged ion. 1 extra valence electron is gained by a Be-atom.

8 + 1 = 9 electrons available to be filled in the MO diagram. As per the Aufbau principle, this extra valence electron is placed in the high-energy π2p_x molecular orbital as shown in the Molecular orbital diagram of Be₂^– drawn below.

The presence of an unpaired electron predicts the paramagnetic nature of the Be₂^– ion.

∴ Bond order of Be₂^– = (N_b –N_a)/2 = (5– 4)/2 = 0.5  

MO electronic configuration of Be₂^–: (σ1s²) (σ^*1s²) (σ2s²) (σ^*2s²) (π2p_x¹)

⇒ Paramagnetic

Finally, Be₂^2- is formed when 2 extra valence electrons are gained in the 2p AOs, one by each parent Be-atom.

8 + 2 = 10 electrons available to be filled in the MO diagram. Thus, these two electrons singly occupy the π2p_x and π2p_y MOs of Be₂^2-, following Hund’s rule.

Two unpaired electrons in the MO diagram drawn below denote the paramagnetic nature of Be₂^2-.

 ∴ Bond order of Be₂^2- = (N_b –N_a)/2 = (6 –4)/2 = 1

MO electronic configuration of Be₂^2-: (σ1s²) (σ^*1s²) (σ2s²) (σ^*2s²) (π2p_x¹) (π2p_y¹)

⇒ Paramagnetic

Again a bond order of 1 reveals an equal possibility of both the formation of Be₂²⁺ and Be₂^2- ions via a single covalent bond between the constituent beryllium ions.

Hence, for the Be₂ family, the bond order increases as follows:

Be₂ < Be₂⁺ = Be₂^– < Be₂²⁺ = Be₂^2-

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

• Be₂ is a homonuclear diatomic molecule. Two identical beryllium atoms combine to form Be₂.
• The MO electronic configuration of Be₂ is (σ1s²) (σ^*1s²) (σ2s²) (σ^*2s²).
• The absence of any unpaired electrons in the Molecular orbital diagram of Be₂ denotes it is a diamagnetic molecule. 
• The bond order of Be₂ is zero, which means it is only a hypothetical molecule and does not exist in real life.
• Be₂⁺, Be₂^–, Be₂²⁺, and Be₂^2- are molecular ions formed by the loss or gain of electrons in the valence shell atomic orbitals of individual Be-atoms.
• The bond order follows the ascending pattern: Be₂ < Be₂⁺ = Be₂^– < Be₂²⁺ = Be₂^2- i.e., 0, 0.5, and 1 respectively.
• Be₂⁺, Be₂^– and Be₂^2- are all paramagnetic while Be₂²⁺ is a diamagnetic molecular ion.