Double Bonds and Triple Bonds

[Nathan Mariano 2G]

According to the information we learned in lecture, double bonds consist of one sigma bond and one pi bond and that triple bonds consist of one sigma bond and two pi bonds. Why can't double bonds and triple bonds consist of solely sigma bonds or solely pi bonds?

[lukezhang2C]

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[Angela Cong 3C]

the sigma bond is the first bond between the two elements and thus any bond created after that can only pair side by side, creating the different pi bonds. The triple bond has the same concept where the electrons pair side by side.