Why is the acetylene molecule linear?

1 Answer
Aug 6, 2017

Because God wanted it that way......

Explanation:

The acetylene molecule features a linear HCCH vector in which participate ALL the valence electrons possessed by hydrogen (1 electron) and carbon (4 electrons) participate. So we have 10 electron to distribute....and these all end up residing in bonding orbitals.

To borrow from valence bond representation, 2 electrons from carbon participate in the linear σCC and the 2×CH bonds; the remaining electrons required for the 2× terminal σCH bonds come from the hydrogen, i.e. .H; i.e. we form 3×sp hybrid bonds for each carbon centre.

The two electrons remaining on carbon are perceived to be unhybridized, and these are formally pz and py orbitals. These overlap in a π interaction above and below the plane....

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The electron density BETWEEN the carbon nuclei, negate nucleus, nucleus repulsion, and allows a very short CC bond length of approx. 1.2×1010m. This is to be compared to the CC bond lengths of 1.35×1010m, and 1.54×1010m observed for ethylene and ethane, H2C=CH2, and H3CCH3 respectively. Dinitrogen, NN, another sp interaction also has a short bond of 1.10×1010m.