What is the shape of the $\ce{C2H2}$ molecule?

The fact that is has a triple covalent bond between the 2 carbon atoms and 2 other bonding pairs between the hydrogen and carbon atoms leads me to believe that it could be linear shape.

However, I'm not sure if the triple bond counts as a single electron bonding pair or 3. If it counts as 3 bonding pairs then the molecule would have 5 overall, and therefore be a trigonal bipyramidal shape.

So is it octohedral or linear? Pretty sure it's linear


1 Answer 1


According to VSEPR theory, each carbon has 2 electron "pairs". These electron pairs are not what you would typically think of as electron pairs because VSEPR theory considers double bonds and triple bonds to still be a single "pair".

So considering each carbon atom, the shape is $AX_2E_0$ ($A$ is the carbon atom, $X$ is a bonding pair of electrons, and $E$ is a non-bonding pair (lone pair) of electrons).

$AX_2E_0$ corresponds directly to a linear molecule.

To directly answer your question, when trying to determine the shape of a molecule, consider double and triple bonds as only a single electron bonding pair.

  • $\begingroup$ I am not an expert in VSEPR theory, but is it commonly used to explain the geometry of molecules like acetylene? I only know it for cases where you have one center atom surrounded by different substituents, and all examples on the linked Wikipedia page seem to be like that. $\endgroup$ Nov 30, 2021 at 16:09

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