Molecular orbital (MO) diagram for HF, and it's bond order
Did you start to draw the molecular orbital (MO) diagram of hydrogen fluoride (HF) but were confused mid-way? Well, we have got you covered.
In this article, you will find a super easy, step-by-step guideline for drawing the MO diagram of HF, calculating its bond order, and predicting its bond stability and magnetism.
So, let us start. We wish you a super fun learning experience!
How to draw the molecular orbital (MO) diagram of HF 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:
Aufbau Principle: Electrons first occupy the lower energy orbitals followed by their placement in the higher energy molecular orbitals.
Hund’s Rule: The incoming electrons are singly filled in the degenerate MOs before pairing occurs.
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.
HF is a heteronuclear diatomic molecule i.e., a molecule containing two atoms from more than one type of element, in this case, hydrogen (H) and fluorine (F).
Follow the simple steps given below and draw the molecular orbital diagram of HF with us.
Steps for drawing the molecular orbital (MO) diagram of HF with its bond order
1. Write down the electronic configuration of HF atoms
HF comprises a hydrogen (H) atom and a fluorine (F) atom.
The electronic configuration of an H-atom is 1s1.
The electronic configuration of a F-atom is 1s2 2s2 2p5.
9 electrons of fluorine and 1 hydrogen electron make a total of 9 + 1 = 10 electrons available to be filled in the Molecular orbital diagram of HF.
2. Determine whether the molecule is homonuclear or heteronuclear
As discussed above, HF is a heteronuclear molecule. This implies that the individual AOs of the H-atom and the F-atom will occupy different energy levels in the Molecular orbital diagram of HF.
The 1s atomic orbital of fluorine lies at the lowest energy level, followed by its 2s atomic orbital.
However, it is important to note that the 1s and 2s atomic orbitals of fluorine do not interact with the 1s AO of the H-atom, at all. Therefore, these retain their positions as non-bonding MOs (called 1σ and 2σ) in the HF MO diagram.
Non-bonding electrons stay localized to a single atom and do not participate in bonding.
Among the valence orbitals, the 2p AOs of the F-atom are situated at a much lower energy level as compared to the 1s AO of the H-atom. This is because fluorine is more electronegative than hydrogen; conversely, it possesses a higher effective nuclear charge.
However, among the three 2p AOs of fluorine, only the 2pz AO has the proper symmetry and adequate energy to overlap with the 1s AO of hydrogen.
A linear combination of two AOs produces two MOs i.e., a bonding MO (σ2pz, also sometimes referred to as 3σ) and an antibonding MO (σ*2pz, also referred to as 4σ*).
In contrast, the 2px and 2py AOs of the F-atom yield two degenerate non-bonding molecular orbitals (together called 1π), as shown below.
3. Fill the molecular orbitals of HF with electrons following the energy and bonding principles
The electrons present in the 1s and 2s AOs of fluorine remain at the same energy level, filled in the 1σ and 2σ non-bonding MOs.
However, out of a total of 6 electrons present in the 1s AO of H-atom and 2p AOs of F-atom, the electrons occupy positions in the HF molecular orbitals in an increasing energy order (Aufbau principle).
The first two electrons are placed as an electron pair in the sigma bonding MO (σ2pz). The next two electrons singly occupy the non-bonding MOs (Hund’s rule) and are consequently paired up (Pauli Exclusion Principle).
The sigma antibonding MO (4σ*) stays unoccupied. Thus, successfully completing the Molecular orbital diagram of HF, as shown below.
As per the diagram shown above, the Molecular orbital electronic configuration of HF is (1σ2) (2σ2) (3σ2) (1π4).
Is HF diamagnetic or paramagnetic?
The absence of unpaired electrons in the Molecular orbital diagram of hydrogen fluoride (HF) suggests its diamagnetic nature.
Diamagnetic substances possess no permanent dipole moment value; therefore they get repelled by an external magnetic field.
Bond order of HF
The bond order formula is:
∴ Bond order = (Nb –Na)/2
Nb = Electrons present in the bonding MOs (Bonding electrons).
∴ Electrons in σ2pz = 2
Na= Electrons present in the anti-bonding MOs (Anti-bonding electrons).
∴ Electrons in σ*2pz= 0
⇒ Bond order of HF = (2 – 0)/2 = 2/2 = 1.
Bond order > 0 implies that HF is astable molecule.
Additionally, a bond order value of 1 means that there is ideally a single covalent bond between the H-atom and the F-atom in the HF molecule.
The molecular orbital (MO) diagram of HF is shown below:
The 1s AO of the H-atom overlaps with the 2pz AO of the F-atom to produce a bonding and an antibonding MO. All the other AOs of the F-atom stay in a non-bonding state.
The Molecular orbital electronic configuration of HF is (1σ2) (2σ2) (3σ2) (1π4).
The absence of any unpaired electrons in the HF MO diagram suggests the diamagnetic nature of hydrogen fluoride.
The bond order of HF is 1. Thus, it is a stable molecule, containing an H-F single covalent bond between the constituent atoms.
Which is the highest occupied molecular orbital (HOMO) in HF?
The non-bonding MOs, 1π (2px and 2py) are the highest occupied molecular orbitals (HOMO) in HF.
Which is the lowest unoccupied molecular orbital (LUMO) in HF?
The antibonding MO i.e., 4π* is the lowest unoccupied molecular orbital (LUMO) in HF.
How is the MO diagram of HF different from that of H2?
HF is a heteronuclear diatomic molecule while H2 is a homonuclear diatomic molecule.
Therefore, in the MO diagram of H2, the 1s AOs of the two H-atoms both lie at the same energy. Two AOs combine to form a bonding (σ1s) and an antibonding MO (σ*1s).
Contrarily, in HF, the different types of AOs of the constituent atoms occupy different energy levels. The 1s AO of the H-atom overlaps sufficiently only with the 2pz AO of the F-atom to produce a bonding and an antibonding MO.
There are some non-bonding MOs in HF, unlike H2.
However, both H2 and HF are diamagnetic molecules, possessing a bond order of 1, as can be determined from their MO diagrams shown below.
How is the MO diagram of HF different from that of F2?
F2 unlike HF is a homonuclear diatomic molecule. The corresponding 1s, 2s, and 2p AOs of two identical F-atoms lie at the same energy level.
The linear combination of these corresponding AOs results in a total of 5 bonding and 5 antibonding MOs in the F2 MO diagram.
In contrast, there is only 1 bonding and 1 antibonding MO in the MO diagram of HF (a heteronuclear diatomic molecule). HF also contains 4 non-bonding MOs.
Why is HCl a stronger acid than HF? Explain using the molecular orbital theory (MOT).
The placement of electrons in the MO diagram of HCl is quite similar to that of HF. However, the F-atom is much more electronegative than the Cl-atom.
It is because of this reason, that the energy gap between the completely filled (1π) non-bonding MO and the empty (4σ*) ABMO of HCl is less than the energy gap between these two MOs in HF.
Upon excitation, electrons can jump from 1π to 4σ* relatively easily in HCl. Therefore, HCl is a better electrolyte and also a stronger acid, as the H-Cl bond can break more easily to liberate H+ ions in an aqueous solution.
Summary
Hydrogen fluoride (HF) is a heteronuclear diatomic molecule, containing two atoms from two different elements.
The MO electronic configuration of HF is (1σ2) (2σ2) (3σ2) (1π4).
The absence of any unpaired electron in the Molecular orbital diagram of HF reveals its diamagnetic nature.
The bond order of HF is 1, which means it is a stable molecule and there is a single covalent bond between the H-atom and the F-atom in the HF molecule.
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