Hybridisation, sigma and pi bonds

Last time

Before we start: last time, in Covalent bonding, shapes and electronegativity, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Why nitrogen gas is so unreactive

Nitrogen makes up almost four fifths of the air we breathe, and yet it hardly reacts at all. The reason is the bond holding two nitrogen atoms together, a triple bond that takes 944 kilojoules per mole to break. To understand that triple bond, and the double bond in alkenes, we need to see how orbitals overlap.

Sigma bonds

A sigma bond is formed by head-on overlap of two atomic orbitals, along the line joining the two nuclei. It can be s with s, s with p, or p with p end to end. The electron density is concentrated between the two nuclei. Every single bond is one sigma bond. It is the first bond between any two atoms.

Pi bonds

A pi bond is formed by sideways overlap of two p orbitals, parallel to each other, above and below the line between the nuclei. The electron density is in two regions, above and below the bond axis. There is none on the axis itself. A pi bond is weaker than a sigma bond, since the overlap is less effective, and it is more exposed, so more reactive. A double bond is one sigma and one pi. A triple bond is one sigma and two pi.

Counting bonds

Nitrogen, N triple bond N, has one sigma and two pi bonds. Ethene, C two H four, has five sigma bonds and one pi bond. The four C–H and the C–C sigma, plus one pi from the double bond. Propene has eight sigma and one pi. Ethyne has three sigma and two pi. Rule: every bond has one sigma. Extra bonds in double or triple bonds are pi.