Describe how sigma and pi bonds form and describe how single and double bonds differ.

The sigma bond results due to the overlapping of orbitals along an inter-nuclearaxis.  In other words if two s orbitals are directly overlapping, then the bond formed is linear between the two nuclear centres thus forming a sigma bond. This also happens with the end to end overlapping of p orbitals The pi bond will result from the overlapping of parallel or sideways p orbitals. In other words when a sigma bond is formed, this will bring two parallel p orbitals closer together allowing them to overlap sideways (or laterally) to form a region of electron density that isn't between the nucli, but are just out of it. A pi bond can only form after a sigma bond has already formed and not vice verca. Single bonds only use sigma bonds while double bonds have one sigma bond and one pi bond. Pi bonds never form in single bonds but in double and triple bonds.