[harkkam]

I was reading that a higher bond order makes that bond shorter. That a double bond will be shorter than a single bond.

But why does the extra pair of electrons that is shared in C=C vs a C-C move the carbon molecules together.

Wouldn't the presence of more electrons in the same space produce a repulsion effect or does the nucleus of each atom try and move closer to the extra electron it gets due to a second bond that forms. It has to be to overcome the repulsion of the electrons.

[sanderol]

Wouldn't the presence of more electrons in the same space produce a repulsion effect or does the nucleus of each atom try and move closer to the extra electron it gets due to a second bond that forms. It has to be to overcome the repulsion of the electrons.

Molecules form since the attraction(nuclei-electron) overcomes the repulsion (electron-electron + nuclei-nuclei). This fact can be applied to double bonds as well, but now there is more electron density so this means more attraction making the bond shorter/stronger.
When the nuclei will be put closer together the repulsive forces will eventually become more important.