Re: why is C2 an unstable molecule?

Apparently the C2 molecule has been studied. It has been made, and its 
fleeting existence has been investigated, probably spectroscopically.   
It has a bond energy of 620 kJ/mol and a bond length of 1.31A, These 
values are close to 614 kJ/mol and 1.34A, the average values found for a 
C=C double bond in organic compounds.

The word "stable" is a relative one. In this case, the question that 
should be asked is "Why is C2 not as stable as other forms of carbon?"
Carbon likes to make four total bonds.  In the C2 molecule each carbon 
makes only two bonds.  This is probably the reason that C2 is 
not "stable"; the other forms of carbon: diamond, graphite, charcoal, 
C60, nanotubes, etc. in which carbon makes four bonds are more stable. C2 
molecules spontaneously hooks up with other C2 molecules to form one of 
these more stabe forms of carbon.