Question

c=c-c,here only one double bond can exist between 3 carbon atoms or c=c-c-c ,here too only one double bond .why not like this c=c=c or c=c=c-c

Answer 1

There are few compounds that contains such unusual composition. like C₃O₂ which is called as carbon suboxide which has following formula

O=C=C=C=O

But generally such compounds are not very common because around each small C-atom, the electron cloud become so high that they repel each other and molecule becomes unstable.