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Hybridization (Bonding Orbtials)- Reasoning behind LDS & VSEPR
Carbon can combines with hydrogen, one of possible compounds is CH4 (called methane). Determine below the LDS, geometry and bond angles of this compound.
As you can see from the above LDS you made above, you get four identical bonds between the C and H. Let us try to understand why.
As you know, bonds are sharing of two valence electrons, one from each atom in the bond. Write out the electron configuration of carbon below.
Hydrogen has an electron configuration of 1s1. So you would expect one hydrogen could share electrons with each orbital of carbon that only has one electron in it.
If that is the case, you would only be able to bond 2 hydrogens with carbon. So that can't be correct.
Maybe, we could take one of the two electrons in the 2s subenergy level and promote it to the unfilled p orbital. If we can do that, we would have 4 orbtials (one s and 3 p) that have one electrons in them. Great, now 4 hydrogens can bond with the carbon (one hydrogen for each of the orbitals in carbon that have one electron in it).WE ARE NOW HAPPY SINCE WE HAVE 4 BONDS IN CARBON LIKE OUR LEWIS DOT STRUCTURE EXPECTED, RIGHT?
Yes, but we still have a problem. The LDS has four identical bonds. We also have 4 bonds, but are they identical? Look at them. Are there any difference? YES
3 of the bonds bring one of carbons p orbital electrons to be shared with hydrogen's s orbital electron. However, there is one bond that carbon uses an s orbital electron to share with hydrogen's s orbital electron. So these are NOT four identical bonds.
THEREFORE, WE CAN NOT USE ATOMIC ORBITALS TO EXPLAIN BONDING.
We create a new theory, called HYBRIDIZATION.
When two atoms come together to share electrons, there are hybridized orbitals or bonding orbitals. They are totally different than atomic orbitals.
Without getting into the specifics of the theory, you can use the following procedures to determine the type of hybridized orbital an atom has when it bonds.
1st year Rules for Determining Hybridization (Always on Central Atom only)
1.Count the number of outside atoms that central atom is bonding with.
2.Count the number of lone pairs on central atom
3.Add together to get number.
4.This number is number of Hybridized orbital
Central atom is bonded to 4 outside atoms and no lone pair so hybridization is sp3 where there are 4 of them.
Here is a website that shows how atomic orbitals become hybridized orbitals. [Animated Website (mhhe) for Hybridized Orbitals]