Why is the acetylene molecule linear?

1 Answer
Aug 6, 2017

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

Explanation:

The acetylene molecule features a linear H-C-C-HHCCH 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 sigma-C-CσCC and the 2xxC-H2×CH bonds; the remaining electrons required for the 2xx2× terminal sigma-C-HσCH bonds come from the hydrogen, i.e. dotH.H; i.e. we form 3xxsp3×sp hybrid bonds for each carbon centre.

The two electrons remaining on carbon are perceived to be unhybridized, and these are formally p_zpz and p_ypy orbitals. These overlap in a piπ 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 C-CCC bond length of approx. 1.2xx10^-10*m1.2×1010m. This is to be compared to the C-CCC bond lengths of 1.35xx10^-10*m1.35×1010m, and 1.54xx10^-10*m1.54×1010m observed for "ethylene"ethylene and "ethane"ethane, H_2C=CH_2H2C=CH2, and H_3C-CH_3H3CCH3 respectively. Dinitrogen, N-=NNN, another spsp interaction also has a short bond of 1.10xx10^-10*m1.10×1010m.