orbital hybridization???

[rthmjohn]

My friend gave me a brief explanation on this topic, but I still don't fully grasp the concept. I understand that it has to do with electrons switching orbitals in order for an atom to bond with another, but beyond that, I do not understand... Can someone explain?

[woelen]

In order to understand this you need some quantum mechanics. Electrons, when confined to a limited space, can only exist in discrete states. The electrons around atoms are confined in space (by means of a potential, the positively charged nucleus binds the electron to the atom).

 

Because electrons only are allowed to be in discrete states, it is possible to count all states, starting from a lowest-energy state and going upwards. In doing so, one has to introduce the concept of shells, and inside each shell, there is a number of different states. The shell, closest to the nucleus only allows 2 states, the shell, next to it, allows 8 states, the shell above that (nr. 3) allows 18 states. Two states are paired, electrons in the same pair of states are said to be in the same orbital. Shell number 2 for instance has four orbitals, one at a lower level, called the s orbital and three at the same higher level, called p orbitals. Shell three also has d-orbitals (there are 5 of these beasts).

 

Now, if atoms are combined to form molecules, then it is possible that orbitals of different levels inside the same shell combine. E.g. a very common form of combining is that the s-orbital and the three p-orbitals combine into four orbitals of the same energy level and each of them looking the same. So, a carbon atom, which normally has a single s orbital in shell 1 (which is fully occupied with 2 electrons) and a single s orbital in shell 2, and three p orbitals in shell 2 (which are only partially filled, as there are only 6 electrons in carbon), can be transformed, such that at shell 2 it has 4 orbitals, which all are the same. The s-orbital and the three p-orbitals 'merge' and blend into four similar orbitals. These orbitals are called hybridized orbitals and are written as sp3 (one s and 3 p's are merged). In a molecule, like methane, all 4 hydrogen atoms are bound to the C-atom in the same way. The C-atom uses a sp3 orbital and the H-atom uses an s-orbital for the chemical bond. If no hybridization would occur, then the molecule CH4 would have one atom, which would have another type of bonding than the other 3, because it would be bonded through the C-atom's s-orbital and the other three through the p-orbitals.

 

Hybridization also can occur between 2 p-orbitals and one s-orbital. In that case the C-atom has one p-orbital, and three sp2 orbitals. In a molecule like H2C=CH2, the H-atoms are bonded through an sp2-hybridized orbital of the C-atom and an s-orbital from the H-atom. One of the bonds between the two C-atoms also it through sp2 orbitals (from both C-atoms) and the other bond is through the remaining p-orbitals. So, in this molecule, the two bonds between the C-atoms actually are different!

 

In more general terms, hybridization can occur between any orbitals and this occurs, as soon as that is energetically more favorable. Most of the chemical compounds we know are due to some form of hybridization of orbitals. In order to have complete understanding of this, have a look at the concepts of quantum mechanics and molecular orbital theory. These, however, certainly are not the easiest concepts to grasp!

[The Thing]

Consider this:

Let's take methane as an example, and see what happens if the C-H bonds are created without hybridization.

1. the 2p and 2s atomic orbitals will lead to two different C-H bonds, namely the overlapping of the Carbon's 2s and Hydrogen's 1s, and the overlapping of the Carbon's 2p and Hydrgoen's 1s. This can't be, because it is determined that methane has 4 identical C-H bonds.

2. The 2p orbitals of carbon are perpendicular, thus creating 90 degree angles with the hydrogen atoms. However, the shape of a methane molecule is tetrahedral, and thus the angles between bonds are 109.5 degrees.

So, either the entire model of the atom is wrong, or the atoms didn't bond with unhybridized orbitals and used something other than their native orbitals. Hence the hybridization, and in this case the 3 2p orbitals and the 1 2s orbital merge, and this is the sp3 hybridization. There are also sp2, sp, dsp3 and other hybridizations, but the principle is essentially the same.

There are two kinds of covalent bonds, the sigma and the pi bond. Sigma bonds are created when hybridized orbitals overlap, such as all of the bonds in a methane molecule. Pi bonds are created when unhybridized parellel orbitals share electrons above and below a sigma bond (in a double bond). Thus, a double bond contains one sigma and one pi bond.

To understand the sigma and pi bonds better, look at a ethylene molecule, and you'll see the sigma bond of C=C bond in ethylene is a result of a sp2 hybridization, and each carbon atom has one unhybridized 2p orbital. To visualize this, the hybrid orbitals of the carbon atoms can be seen as a trigonal planar shape, and two ends of the "triangle" are sigma bonding with the 1s orbitals of the hydrogen, and the other end is sigma bonding with the other C atom. The unhybridized orbitals provide a pi bond, thus accounting for the double bond of C=C.

[rthmjohn]

woelen... I understand basic quantum mechanics and electron configuration but I just didn't quite understand hybridization. Thanks for both of your guys' help though. I get the picture now. Does hybridization have anything to do with sigma and pi bonds?

[The Thing]

Yes. That was what I wrote. Sigma bonds are created when hybridized orbitals overlap, such as all of the bonds in a methane molecule. Pi bonds are created when unhybridized parellel orbitals share electrons above and below a sigma bond (in a double bond). Thus, a double bond contains one sigma and one pi bond.

[insane_alien]

basically its all about when you've just got over the shock that electrons don't orbit in nice little circles but complex shapes that seem to make no sense, they give you another shock in that those complex shapes can form into even more complex and nonsensical shapes. fun...

[The Thing]

Oh yea. Then you get another pleasant shock as you realize that the new orbitals (the non-circular weird shapes that electrons orbit the nucleus in) don't even contain the electrons, but are merely a space where the electron is "likely" to be found.

[rthmjohn]

Maybe after several years of calculus I'll try to interpret Schroedinger's equation and draw out the orbitals for fun