Hi Mam,

For this question doesn't ethyne have only single bonds so it doesn't have any PI bonds?

Draw the Lewis structure of ethyne, C₂H₂, first:

H–C≡C–H

Each carbon must make four bonds to satisfy its tetravalency. In ethyne, each carbon is already bonded to one hydrogen, so each carbon still needs three more bonds. Therefore, the two carbon atoms must be connected by a triple bond.

Now remember:

A triple bond consists of one sigma bond and two pi bonds.

So, in one molecule of ethyne, there are 2 pi bonds.