Chemistry
Valence bond theory predicts a quadruple bond as the only way to satisfy the octet rule for carbon. However, molecular orbital theory shows that there are two sets of paired electrons in the sigma system (one bonding, one antibonding), and two sets of paired electrons in a degenerate pi bonding set of orbitals. This adds up to give a bond order of 2, meaning that there exists a double bond between the two carbons in a C2 molecule. This is surprising because the MO diagram of diatomic carbon would show that there are two pi bonds and no sigma bonds.
Bond dissociation energies of B2, C2, and N2 show increasing BDE, indicating single, double, and triple bonds, respectively.
C2 is a component of carbon vapor. One paper estimates that carbon vapor is around 28% diatomic, but theoretically this depends on the temperature and pressure.
Read more about this topic: Diatomic Carbon
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