While learning about Carbon and its Compounds in a class 10 coaching, I suggested a structure for some isomers. It contained a connection of two consecutive carbons connected by double bonds. The teacher said that it can be counted as one, but is not a stable compound. A recent research on C18 carbon carbon chain allotrope involved alternating triple bonds. My question is, why is consecutive connection of carbon atoms via double bonds not stable, whereas an alternating triple Bond chain is found to be existing? Also explain some physics behind it