We start studying one of the most seminal theory in chemistry with this post – THE HYBRIDIZATION THEORY. This theory was a leap forward as it did not consider atomic orbitals as they are, but proposed that they mix together and form new hybrid orbitals.
Need for the concept of hybridization.
The concept of hybridization was mainly needed as –
- Failure of VBT to explain valencies for some elements –
No. of unpaired electrons in valence shells
Beryllium , Be
inert element as both s orbitals are filled (Zero valent)
(has two unpaired electrons / valencies)
BeCl2 , BeF2
Boron , B
1s2 2s2 2px1
Monovalent as only one unpaired electron is present.
(has three unpaired electrons/ valencies)
Carbon , C
1s2 2s2 2px1 2py1
Divalent as two unpaired electrons are present
(has four unpaired electrons / valencies)
As seen in the above table, based on VBT theory beryllium should be inert, boron should be monovalent and carbon should be divalent. However, in reality we see that this is not true. Beryllium forms compounds like beryllium dichloride (BeCl2) and beryllium difluoride(BeF2) indicating that it has two unpaired electrons , which form bonds with with chlorine and fluorine atoms respectively.
Similar observations were made for boron and carbon too. Physicists then had found out with experimentation , that carbon had two sub shells in its valence shell with slightly different energy.So, carbon should be bivalent with only 2 unpaired electrons in its valence sub shell. If these compounds are forming more bonds than the number of unpaired electrons in their valence shells , there must be some phenomenon which VBT was not taking into consideration.
- The bond angles and geometries of some molecules could not be explained based on earlier theories.
Ammonia molecule has Nitrogen atom at its centre.
N (7) ⇒ 1s2 2s2 2px1 2py1 2pz1.
According to VBT , the three 2p orbitals of N should overlap with three 1s orbitals of H to form ammonia molecule. However, we know that the three p – orbitals are perpendicular(90º )to each other .So, the bond angle in ammonia molecule should be 90º . However, the bond angle is found to be 107º . This observation could not be explained by earlier theories.
The Man Behind the theory.
It was Linus Pauling who introduced the concept of hybridization , in 1931. Remember him ? Yes ! He is the same chemist , who developed the Pauling scale for electronegativity ! (See post 46). He is regarded as one of the two great scientists of all times! He is the only person to be awarded two unshared Nobel Prizes!!
The element carbon was studied extensively as it formed the basis of all life – Carbohydrates, fats, amino acids etc all has carbon in it! As stated earlier , the physicists of that time proposed that carbon had two valence sub shells(with slightly different energies) and the outer valence sub shell had only 2 electrons and thus, carbon should be bivalent. This proposition was based on recorded experimental data.However, the chemists knew that carbon was tetravalent as it formed 4 bonds and had tetrahedral geometry.This discrepancy needed to be resolved .Linus Pauling’s hybridization theory was the answer to this conflict.
In 1928, he first came up with some mathematical calculations to believe that his idea of hybridization was even worth working on.He quoted,
“It was so complicated that I thought people won’t believe it.And perhaps I don’t believe it, either. Anybody could see that the quantum mechanics must lead to the tetrahedral carbon atom, because we have it. But the equations were so complicated that I never could be sure that I could present the arguments in such a way that they would be convincing to anybody.”
One night , while working on this complex problem , he had an important insight. It occurred to him that if he could somehow combine the two sub shells into one valence shells , he could solve the problem. So, leaving behind some complex mathematical calculations , he simply combined the two wave functions (we have already studied about wave functions in quantum mechanics posts). This is how hybridization theory was developed.
“One day, late in the day…I had an idea. It was the basic idea of hybrid orbitals. I was trying to understand why the carbon atom is tetrahedral, forms bonds directed towards the four corners of a tetrahedron. Even as early as 1924, I had made a model of methane, in which I said the four outer electrons of the carbon are in orbits directed towards the corners of a tetrahedron…. When quantum mechanics came along, a result was confirmed that had been accepted earlier, that the four outer electrons in the carbon are of two different kinds…. I thought ‘the basic principles of quantum mechanics permit us to combine these functions from the Schrödinger equation in another way.’ And I said to myself: ‘Let’s suppose that I look just at the distribution in various directions, and not worry about the difference in the radial distribution for those.’ This permitted rather simple calculations to be made in a straightforward manner. The first result I got was that the best bonds that the carbon atom can form are directed towards the four corners of a tetrahedron. So, in 1931, I had a simple theory of the tetrahedral carbon atom and an explanation of a great bit of organic chemistry.”Linus Pauling
March 27, 1964
He kept working for hours. Using the same basic approach, he found he could apply this theory to other molecules too !
“I was so excited and happy, I think I stayed up all night, making, writing out, solving the equations, which were so simple that I could solve them in a few minutes,” he remembered. “Solve one equation, get the answer, then solve another equation about the structure of octahedral complexes such as the ferrocyanide ion in potassium ferrocyanide, or square planar complexes such as in tetrachloroplatinate ion, and various other problems. I just kept getting more and more euphorious as time went by.”
Let us begin discussing this theory in our next post. Till then ,
Be a perpetual student of life and keep learning !
References and further reading –
Image source –
- By Library of Congress – http://www.notablebiographies.com/Ni-Pe/Pauling-Linus.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=17802529