MO Diagram of heteronuclear diatomic Molecules - Chemical Bonding & Molecular Structures

MO Diagram of heteronuclear diatomic Molecules - Chemical Bonding & Molecular Structures - Inorganic

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The molecules having two atoms of two different elements are known as heteronuclear diatomic molecules.
1. Difference in Electronegativity: So the first is Difference in Electronegativity, lets consider an example of HCL molecule. As hydrogen is highly electropositive electron is not having that much attraction towards the nucleus and thus it revolves in a very large orbital. It is still in the 1s orbital but the size of orbital is very large. On the other hand Chlorine is highly electronegative so 1s orbital electrons have more attraction towards the nucleus thus they revolve around very close to nucleus. This means in Chlorine size of 1s orbital is very small as compared to H1s orbital or we can say the energy of the election is very very low and stability is very high. And thats why to remove 1s election from Chlorine we need very high energy. Similarly other orbitals also shrink and finally 3p orbital is now having equivalent size of that of H1s orbital. In other words now energy of H1s orbital is matching with Energy of 3p orbital of Chlorine atom. Which is the basic condition for atomic orbitals to make molecular orbitals. So in case of heteronuclear diatomic molecules instead of similar orbitals from two different atoms, orbitals of similar energy overlaps. Which can be different also like in HCl 1s orbital of hydrogen and 3p orbital of Chlorine overlap. This difference of electronegativity is also is also expressed in terms of electron cloud. This happens after overlap of atomic orbitals. That means when two atoms are having different electronegativity. the electrons feel different forces of attraction towards different nucleus. The attraction is high between electron and more electronegative atom as compared to more electro positive atom. This generates an unsymmetrical electron cloud density between two atoms.
As you can see from the image here oxygen have more electron clouds as compared to carbon atom so, it becomes partially negative while carbon becomes partially positive. This increases the ionic character and reduces covalent nature of molecule. If there is huge difference in electronegativity, the electrons are strongly pulled by one of the atoms and it can be said that the electron spends most of its time in orbital of one atom then the bond becomes ionic.
This means if the electrons are more attracted towards oxygen, the bonding molecular orbital will have energy very close to the energy of oxygen atom. On the other hand the energy of anti bonding molecular orbital will be close to carbon or more electropositive atom.
2. Reduced Covalent nature: As per the liner combination of atomic orbitals when two atomic orbitals overlap, they produce two molecules orbitals, one is bonding and another in antibonding. In case of homonucler diatomic molecule the energy separation of bonding and anti bonding molecular orbitals are same from the energy of atomic orbitals. But in case of heteronucler diatomic molecules as one atom being more electronegative pulls the electrons towards itself. Thus in molecular orbitals, the bonding electrons are more closer to the energy of electronegative atom while the anti bonding orbitals resembles to electropositive atom.
The difference between energy of atomic orbitals and molecular orbital is known as Exchange Energy denoted by delta E and as you can see from the diagram, that the exchange energy is always lower in case of heteronuclear diatomic molecule.
In heteronuclear diatomic molecules, the exchange energy is reduced because the atomic orbitals are not of same energy level. The wave functions of bonding and anti bonding molecular orbitals can be written as
{ \Psi }_{ bonding } = x{ \Psi }_{ A } + y{ \Psi }_{ B }
\Psi _{ antibonding } = y\Psi _{ A } - x\Psi _{ B }
Where x and y are the coefficients of atomic orbitals of A and B respectively. In Homonuclear diatomic molecule x = y as both atoms are same. But in case of heteronuclear diatomic molecule x ≠ y and ratio y/x is a measure of polarity of the molecule.
Lets take first example of Hydrochloric acid. Here hydrogen is highly electropositive and Chlorine is highly electronegative. So the energy of 1s orbital of hydrogen becomes equivalent of that of 3p orbitals of Chlorine and they can now overlap. The overlap is with only one of the 3p orbital which can be considered as the z axis or the overlap axis. The other two 3p orbitals, being perpendicular to the bond axis, remain non-bonding.
In the diagram all other orbitals are not shown, only overlapping orbitals are shown here. When these two orbitals overlap they form sigma bonding and sigma star anti bonding molecular orbital. Here 1s orbital of hydrogen is having 1 unpaired electron and 3pz orbital also have an unpaired electron, so two electrons needs to be filled in molecular orbitals which will be accommodated in sigma molecular orbitals. And electronic configuration of HCl molecule will be KL, 3s2, 𝛔2, 3px2, 3py2