Molecular Orbital Diagram of B2, C2, and N2 Molecules.

From the periodic table as we have already discussed the Molecular orbital diagrams of diatomic molecules of 1st two periods starting from Hydrogen to Neon. Only three atoms are remaining and they are Boron, carbon and Nitrogen.
These three atoms are very special as in this case there is mixing of 2s and 2p orbitals takes place. This is also known as hybridisation of atomic orbital.
So why mixing of atomic orbitals takes place? From boron the number of electrons are now added in 2p orbitals, and it is very important to see that in these atoms the energy gap between 2s and 2p orbitals is very low. As shown in the table the energy difference between 2s and 2p orbitals are very low around 400 to 600 KJ/mole in case of B, C and N. But it is very high in Oxygen and F around 1500 to 2000. So as the energy of 2s and 2p orbital are almost similar, mixing of these orbitals is possible.
Another important factor in mixing of atomic orbital is symmetry, This means for mixing of orbitals, the direction or symmetry of orbitals should match. e.g. in case of two p orbitals if the lobes are in different directions, they will have different symmetry, and if lobes are in same direction they will have same symmetry. In case of s and p orbitals symmetry axis exists and because of these two favourable conditions mixing of 2s and 2p orbitals takes place.
So if we take two atoms with hybridised orbitals or mixed 2s and 2p orbitals and try to arrange for the maximum possible overlap. We can find that the one lobe of these orbitals can overlap across the overlap and form a sigma bond, here the overlap will be maximum so energy of this orbital will be low. This is sigma 2s orbital in our molecular orbital diagram. Then there is possibility of lateral overlap of two lobes which are at 109 degrees to this lobe, they can overlap to form pi molecular orbitals, indicated by pi 2px and pi 2py orbitals in diagram. Both the orbitals will have the same energy and presented on same level.
Now last lobe which is in the opposite direction have very less chances of overlap. Also there are three small lobes of anti bonding orbitals of already formed molecular orbitals which will try to repeal the orbital overlap. So the energy gf this orbital increases and it goes beyond the pi orbitals. This is again axial overlap so it is sigma bond but the energy is very high. This is indicated by sigma 2pz orbital in our diagram,
This mains after mixing the energy of pi 2p and sigma 2p orbitals are now changed. Now lets try to make the molecular orbital diagram of Boron with this concept of mixing of orbitals.
Now in case of boron as there are 5 electrons with electronic configuration 1s2, 2s2 and 2p1
So 2s orbital and 2p orbitals being almost similar energy can be mixed or hybridised to create 3 sp2 hybrid orbitals of same energy with single electron in each while one p orbital remains empty.
So if we draw the molecular orbital diagram of boron molecule. First 1s orbitals will not overlap being inner shel orbitals creating non bonding situation. Now as there is mixing of 2s and 2p orbitals takes place, one sigma bond is formed by axial overlap of one lobe of sp2 hybridised orbital, here represented by sigma 2s and sigma star 2s orbitals, then there will be two pi orbitals by lateral overlap of two lobes, making pi 2p and pi star 2p orbitals and lastly there will be one sigma bond represented by sigma 2pz and sigma star 2pz orbitals.
Now if we try to fill the electrons, in pi 2p orbitals, 2 unpaired electrons can be accommodated. So electronic configuration becomes. KK,
Now as there are 2 unpaired electrons, the boron diatomic molecule should be paramagnetic. And its bond order calculated from the formula bond order = no of electrons in bonding - no of electrons in anti bonding divide by 2. It gives 1. This means there is one week pi bond is formed between two boron atoms.
Similarly we can work for diatomic carbon molecule, In this case carbon have 6 electrons with electronic configuration 1s2 2s2 and 2p2. As orbitals of 2s and 2p have similar energy they will me themselves or hybridise, as one s orbital and 3 p orbitals are involved, it is called sp3 hybridisation.
Now similar molecular orbital digram can be constructed where the energy of pi 2p orbital is lower than the sigma 2pz orbitals. And if we try to fill the electrons, it seems that the pi 2p orbitals have all paired electrons with electronic configuration K..
Now as all the electrons are paired up, the molecule should be diamagnetic and bond order calculated from same formula comes to be 2. This means there are two pi bonds are formed in diatomic carbon molecule.
Now finally lets discuss the formation of diatomic nitrogen molecule. Here Nitrogen have 7 electrons with electronic configuration 1s2 2s2 and 2p3. Here also 2s and 2p orbital mix themselves and generated sp3 hybridised orbitals.
So similar molecular orbital diagram can be constructed with lower pi 2p orbitals than sigma 2pz orbitals. Now

[@priyanjanaroy2181]

Very very excellent explanation ... And very good animations 😍😍😍

[@Edmerls]

Glad you liked it

[@kaushikpramanik1394]

@@Edmerls where are you from?

[@weineltituskalo]

Sir you're a champion 🎉

[@failtosucceed-i5c]

Great Explanation...Thanks

[@rajatgorh3111]

Thank you so much sir ❤😊

[@RajeshKumarKarma]

Well lecture

[@techchemistry787]

As you mentioned that as we move from boron to nitrogen bond will be stronger and bond length decreases Also, bond energy decreases But I have read that Stronger the bond greater the bond energy Because bond energy is the amount of energy required to break a bond So plz justify that how bond energy decreases as we move from boron to nitrogen

[@jyothibarki9935]

Bond enthalpy means the amount of enery released when one mole of covalent bond is formed . It means amount of enery stored in bonds between atoms in a molecule the higher the bond enthalpy the more enery needed to break the bond and And the stronger the bond

[@ahsanhasibpranto4356]

What is kk and why using at the first?? can u explain sir??

[@priyankathakare5708]

KK means that he is not showing the first orbitals as they are filled.

[@talhashahbaz5552]

Welldone

[@anjalimaurya8133]

Sir is mixing required ? Can't we make the orbital diagrams without mixing them?

[@Edmerls]

Yes, Its important, Mixing gives hybrid orbitals of same energy.

[@anjalimaurya8133]

@@Edmerls Ok sir thanks a lot 🙏

[@beautyofswat1591]

Why pi orbital are more stable than sigma in nitrogen

[@shakilahmad214]

Sir on which app or software we draw orbital diagram and these structure as write you????

[@krutibashdhamudia7529]

Helpful

[@Edmerls]

Glad you think so! Thanks

[@krutibashdhamudia7529]

@@Edmerls sir...can you provide the..mo diagram for hetero atomic molecules like...co,no,no+,no-

[@Vitaminprotein_07]

Confusing

[@chimwemwegerald8745]

Bond order is 2

[@riteshverma9544]

Thanks sir

[@Edmerls]

So nice of you

[@RajeshKumarKarma]

Well done

[@RajeshKumarKarma]

I want utand your lecture

[@bineetayadav2881]

Hindi me bataiye