INTEC Chemistry Blog

Follow up notes on hybridization – CHM412

Posted on: March 7, 2014

When atoms share electrons to bond, the first bond that forms between them is a sigma bond. So single bonds are sigma (symbol = σ) bonds.

Sometimes, further bonding can occur between the same two atoms.  If this happens, they can then form a pi (symbol = π) bond. The σ and the π bond are collectively known as a double bond. To form this π bond, two electrons (in addition the two e- already in the sigma bond) will need to be shared.

It is possible for even more electrons to be shared between the same two atoms. If this happens a second pi bond will form, and all these bonds are known collectively as a triple bond.

multiple bonding

A sigma bond is formed when the valance orbitals of one atom, directly overlap, head-on, with the valence orbitals of a different atom

When carbon, oxygen and nitrogen  bond covalently to other atoms, they hybridize. Note: some atoms do not hybridize, but for this course we focus strongly on C,N,O which do hybridize.

You may notice the atoms, C,N and O have for their valence shell, the n=2 shell, involving the 2s orbital, the 2p(x) orbital, 2p(y) orbital and the 2p(z) orbital. Remember an orbital can only hold 2 e- and when this happens, they must have opposite spin!

Hybridization (mixing) of an atoms internal orbitals allows better overlap between atoms, hence stronger bonds form, hence more energy is released when they form, hence the resulting species is more stable than it would be if no hybridization takes place.

  • If an s orbital hybridizes (mixes) with a p orbital on the same atom, you get an sp orbital.
  • If an s orbital hybridizes (mixes) with two lots of p orbital on the same atom, you get an sp2  orbitals.
  • If an s orbital hybridizes (mixes) with three lots of p orbital on the same atom, you get an sp3  orbitals.

The diagram below is to try and help illustrate that. It shows a 2p(x) orbital of one atom (e.g. a carbon atom) bonding/overlapping with an s orbital of another atom (e.g. a hydrogen atom).

non-hybrid overlap and hybrid overlap - illustration

Summary: Sigma bonds (single bonds) form by head on overlap of the atomic orbitals of different atoms

What about pi bonds?
Pi bonds are formed from NON-hybridized p orbitals overlapping in a sideways manner.

The electron density in the atoms is a pi bond is formed by the sideways overlap of non-hybridized orbitals. This is illustrated below (note the preceding sigma bond between the C atoms is not shown for the sake of clarity, but in reality, it is there!)

pi bond formation done

A very useful statement:

The shortest distance between two points is a straight line. The e- in a sigma bond essentially lie in a straight line (with a bit of bulging) between the attracting nuclei. So the e- are at the shortest distance that’s possible between to two attracting nuclei, hence they are attracted (i.e. bonded) as strongly as they can be, and so are ‘tied’ up, and not so easily available for reaction.

The electrons in pi bond are NOT directly in between the attracting nuclei, so the distance of the attracting nuclei to the electrons in a pi bond is greater than it is for sigma bond.

HENCE, the e- in a pi bond are attracted less strongly than a corresponding sigma bond, so the e- in the pi bond are MORE reactive than the electrons in the sigma bond.

The result of this is that pi bonds are more reactive than sigma bonds, so pi bonds will react first. This helps you identify some reactive sites in molecules, and so you can begin to ‘feel’ how a chemical can react.

Last note:

Skeletal diagrams show C-C bonds and bonds to heteroatoms (e.g. C,N,O,F,Cl,Br,I). Bonds between C and H are usually left off – especially if they are not involved in a discussion or a reaction.

Get familiar with using skeletal sketches of molecules. They will be used on the exam paper.

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