Reason for double bonds being sigma/pi

[Adrienne Chan 1G]

Lavelle gave an explanation in lecture about why double bonds had to be one sigma and one pi, but I still didn't understand. Could someone please explain it in a different manner?

[Bethany Yang 2E]

The very simplistic way I think of it, is a sigma bond is a single bond. So when there is a double bond, or triple bond something else needs to fill it. So for a double bond, you can always start out with the sigma bond (single) and then however many more bonds you need will be the pi bonds. Therefore, a double bond has a sigma and a pi bond. In a triple bond, you would start with the single (sigma) and then the rest would each be pi bonds. Pi bonds are basically used in multiple bonds.

[Pranav Daggubati 3C]

One of the pairs of electrons bonds linearly and the other can no longer bond linearly so they bond in the characteristic "over and under" pi bonds of two electron probabilities.