Question

why the 2 carbons are not make tetra bond ...4 th bond

Answer 1

because of instability because it will require so much amount of energy to do so. and after that also it will be very hard to remain in tetra bond and confisiously they will break the bond.

Answer 2

From a Lewis point of view, there is no reason why carbon can't form a quadruple bond satisfying the Octet Rule and leaving no electrons for further bonding. But it implies that C₂ is a perfectly stable molecule, like N₂, and that just isn't the case.

If we go on to the valence-bond model, in which bonds result from the overlap of atomic orbitals, we see a better explanation: carbon cannot form a quadruple bond because it doesn't have enough atomic orbitals pointing in the right directions. sp hybridization leaves two p orbitals over, while sp₂ hybridization leaves one p orbital.

Valence-bond theory predicts two possible bonding states for C₂:

1. double bond with all electrons paired, :C=C: (as seen in Alkene)

2. a triple bond with two unpaired electrons, .C(Triple)C. (As seen in Alkyne)

These are called resonance structures, and both must be considered as partial representations of the real situation.