For our purposes, there are just two types of bonding in molecules.
Any bond involving s orbitals (s, sp, sp2, or sp3) is a sigma bond (more commonly referred to as a single bond).
Have you ever pushed two water droplets together to make a bigger water droplet? That’s not unilke what’s going on when a sigma bond forms – it’s an end to end joining of two electron clouds to make a bigger cloud.
By contrast, Pi bonds involve overlap of two p orbitals. Here, since they don’t face each other, the bonding isn’t end-to-end, but side-on-side.
One consequence of this difference is that sigma bonds can rotate freely, but pi bonds cannot. For an analogy, look at this 1×1 Lego block. If you joined two of them together, they could still easily rotate along the axis where they’re joined.
By contrast, if you join two 1×2 lego blocks together (using both holes), they’re confined to one axis. You can’t rotate the area where they’re joined without breaking the bond.
(Sorry about the pictures, I’ll have to get better ones next time I visit my nephews).
A different analogy for looking at sigma and pi bonds: Imagine hammering two boards together with a nail. With a single nail, the boards are free to rotate, right? but if you add a second nail, the boards become completely fixed in place.
Next Tip: What fluorine has in common with Gordon Gekko.
Thanks for reading – James