The c-c sigma bond in acetylene is formed by the overlap of which two orbitals.

When carbon atom tends to form single bonds then its hybridization is , when carbon atom tends to form double bond then its hybridization is  and when a carbon atom is attached to a triple bond or with two double bonds then its hydridization is sp.

For example, in HCN molecule there is a triple existing between the carbon and nitrogen atom.

So, hybridization of carbon in this molecules is sp. Moreover, nitrogen atom is also attached via triple bond and it also has a lone pair of electrons. Hence, the hybridization of nitrogen atom is also sp.

Thus, we can conclude that s and p type of orbitals overlap to form the sigma bond between C and N in H−C≡N:

Explanation:

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