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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 Be22-, calculating the bond order for each.

Name of molecule

Di-Beryllium

Chemical formula

Be2

Electronic configuration

1s2 2s2

Molecular orbital electronic configuration

(σ1s2)(σ*1s2)(σ2s2)(σ*2s2)

Number of electrons in bonding MOs

4

Number of electrons in anti-bonding MOs

4

Bond order

0

Paramagnetic or Diamagnetic?

Diamagnetic 

 

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.

formation of bonding and antibonding Molecular orbital diagram (MO) for Be2

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

You must note that the two p-orbitals overlapping end to end produce sigma bonding (σ2pz) and antibonding (σ*2pz) MOs. Contrarily, the other four p-orbitals overlapping side by side in sets of two, produce the pi bonding (π2px and π2py ) and antibonding (π*2px and π*2py) MOs.

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

Energy level diagram of Be2

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.

Molecular orbital diagram (MO) of Beryllium (Be2) and it's bond order

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 formula for Be2

∴ 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 + 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 Be22-) 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 Be22-  

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+ = (Nb –Na)/2 = (4-3)/2 = 0.5   

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

Paramagnetic

Be2+ Molecular orbital diagram (MO) and Bond order

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+ = (Nb –Na)/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.

Be22+ Molecular orbital diagram (MO) and Bond order

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 π2px 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 = (Nb –Na)/2 = (5– 4)/2 = 0.5  

MO electronic configuration of Be2: (σ1s2) (σ*1s2) (σ2s2) (σ*2s2) (π2px1)

Paramagnetic

Be2- Molecular orbital diagram (MO) and Bond order

Finally, Be22- 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 π2px and π2py MOs of Be22-, following Hund’s rule.

Two unpaired electrons in the MO diagram drawn below denote the paramagnetic nature of Be22-.

 ∴ Bond order of Be22- = (Nb –Na)/2 = (6 –4)/2 = 1

MO electronic configuration of Be22-: (σ1s2) (σ*1s2) (σ2s2) (σ*2s2) (π2px1) (π2py1)

Paramagnetic

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

Be22- Molecular orbital diagram (MO) and Bond order

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

Be2 < Be2+ = Be2 < Be22+ = Be22-

Also read:

FAQ

How to draw the Molecular orbital diagram of Be2

The MO diagram of Be2 is shown below.

MO diagram of Be2

The MO electronic configuration of Be2 is (σ1s2) (σ*1s2) (σ2s2) (σ*2s2).

The absence of any unpaired electron in the above Molecular orbital diagram reveals the diamagnetic character of Be2.

The bond order of Be2 is:

∴ (Nb-Na)/2 = (4-4)/2 = 0

Using the MO diagram of Be2 and its bond order, answer the following questions:

  • a) Is Be2 stable? 
  • b) Is Be2 a diamagnetic substance? 
  • c) What is the outer (valence) electron configuration of Be2+?

As per the Molecular orbital diagram of Be2, its bond order is calculated as follows:

∴ (Nb-Na)/2 = (4-4)/2 = 0

  • a) A bond order of zero implies that Be2 is not a stable molecule.
  • b) Yes. The absence of any unpaired electrons in the Be2 MO diagram means it is a diamagnetic substance.
  • c) Be2 is converted into Be2+ by the loss of 1 valence electron. This electron is removed from the 2s pi antibonding MO. Thus, the MO electronic configuration of Be2+ is (σ1s2) (σ*1s2) (σ2s2) (σ*2s1).

Use molecular orbital theory to explain why the Be2 molecule does not exist.

As per the molecular orbital diagram of Be2, there are an equal number of electrons in the bonding and antibonding MOs of Be2.

Therefore, the bond order of Be2 is zero, which means it is only a hypothetical molecule, non-existent in real life.

Which of the following molecules does not exist?

  • A) Li2 and He2
  • B) Li2 and B2
  • C) Li2 and Be2
  • D) Be2 and He2

Option D gives the correct answer. As per the following MO diagrams, the bond orders of both Be2 and He2 are zero, therefore these are non-existent molecules.

MO diagram of Be2 vs He2

Contrarily, the bond orders of Li2 and B2 are 1 hence they exist.

Which of the following is paramagnetic?

  • A)  Be2
  • B)  Be22+
  • C)  Be22-
  • D) All of the above  

Option C gives the correct answer. Be22- is paramagnetic containing two unpaired electrons in its 2p pi bonding MOs (π2px and π2py).

MO electronic configuration of Be22-: (σ1s2) (σ*1s2) (σ2s2) (σ*2s2) (π2px1) (π2py1)

In contrast, both Be2 and Be22+ are diamagnetic as all their electrons are paired up in the respective MOs.

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

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

How is the Molecular orbital diagram of Be2+ different from that of Be2

There is a single unpaired electron in the σ*2s MO of Be2+. However, Be2 contains all paired electrons.

Molecular orbital diagram of Be2 vs Be2+

How is the Molecular orbital diagram of Be2 different from that of B2? Predict their bond orders. 

There are a total of 10 electrons to be filled in the MO diagram of B2 as opposed to the 8 electrons available for the Be2 MO diagram.

Therefore, B2 contains two unpaired electrons in its pi-bonding MOs. It is paramagnetic in nature as opposed to Be2, which is a diamagnetic molecule.

The bond order of B2 is 1 while that of Be2 is zero.

MO diagram of Be2 vs B2

What is the bond order of Be22+ and Be22-? Do you expect these molecules to exist in the gas phase?

The bond order of both Be22+ and Be22- is 1. Yes, these molecular ions exist in the gas phase.

Which of the following statements is correct?

  • A) Be2 is more stable than Be2 which is more stable than Be22+
  • B) Be2 is more stable than either of Be22+ or Be22-
  • C) Be2+ is more stable than Be2 which is more stable than Be2
  • D) Be22+ and Be22- are both more stable than Be2

Option D provides the correct answer.

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.

Both Be22+ and Be22- (bond order 1) are thus more stable than Be2 (bond order 0).

Be2 is also less stable than both Be+ and Be (bond order 0.5) thus, all the other options are incorrect. Comparing Be2+ and Be2, the latter is relatively less stable on account of its higher number of antibonding electrons.  Similarly, Be22+ is comparatively more stable than Be22-.

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 Be22- 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+ = Be22- i.e., 0, 0.5, and 1 respectively.
  • Be2+, Be2 and Be22- are all paramagnetic while Be22+ is a diamagnetic molecular ion.
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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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