Recently , when I was reading about molecular orbital theory I came by molecule c2 (dicarbon) it was told that there are two π-bonds b/w the two carbon and no sigma bond ,so please any one will explain it to me ?

It comes from application of MO theory. There is a diagram that shows the way in which molecular orbitals are populated for dicarbon in this link: http://www.chem1.com/acad/webtext/chembond/cb08.html

 

Essential, because the bonding pi orbitals are lower in energy than the sigma orbitals, they end up being populated first. As there are only 4 valence electrons available to carbon, the end result is that the bonding pi orbitals are filled and the sigma ones are empty, meaning that only the pi molecular orbitals contribute to the bonding between the two carbon atoms.

OK got it and thanks for the link it has illustrated more than I wanted but can u also explain it without the use of molecular orbital theory ,I mean in simple language

 

And thanks

MO theory is really the only way I can explain it. If you aren't familiar with it, perhaps get a hold of a chemistry text book and read up. If you go through the rest of the link I gave may also help. I can have a go at simplifying it when I get home, but you would probably be much better off just reading around yourself.

OK , I know mo theory but just want it to know it in more basis language but that link was also enough so thanks