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I'm asked the following:

Explain why the structure $\ce{H-C=C-H}$ doesn't make sense chemically.

Not only am I just looking for a check up of my answer, but am I making any sense in what I'm saying here? Am I mixing up or misunderstanding concepts here?

My answer was:

The hydrogen and carbon atom form a covalent bond. Hydrogen has only 1 electron in its valence shell, so it 'needs' 1 of carbon's electrons to complete it. Now there are 3 electrons left to take in the carbon atom. But this isn't enough to make a double bond with the other carbon atom: we're one electron short.

Does this sound about right?

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    $\begingroup$ There are three electrons left on each carbon. Your assumption will lead you astray. $\endgroup$ – matt_black Apr 20 '16 at 15:42
  • $\begingroup$ Could you elaborate a bit on this? Does a single bond in a structure have a 'higher priority' than a double bond? $\endgroup$ – Apeiron Apr 20 '16 at 15:47
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    $\begingroup$ My comment has nothing to do with "priority" it is about the the total number of electrons in play for bonding. $\endgroup$ – matt_black Apr 20 '16 at 15:49
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    $\begingroup$ @Aperion We are just counting the total electrons available for bonding. The order you count them in doesn't matter so you are free to deal with simpler bonds first and then see what is available for the rest. But you have to count the total correctly or you will not get a sensible answer. $\endgroup$ – matt_black Apr 20 '16 at 15:55
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    $\begingroup$ @Apeiron You can also always write up a self-answer. That is encouraged on Stack Exchange ;) $\endgroup$ – Jan Apr 20 '16 at 16:47
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Your analysis was a good start, but then it dropped. You need to consider four electrons on each carbon atom. Each of the two carbons can then form a single bond to one hydrogen atom using one of its electrons. Each of the carbon atoms then uses two of the remaining electrons to form the double bond.

Thus, each of the carbon atoms has one extraneous electrons; the structure would be a diradical structure. It should better be represented e.g. like this: $\ce{H-C^.=C^.-H}$.

It is possible for these two radicals to be alongside each other — see for example the structurally strongly related dioxygen molecule which is a triplet (diradical) in ground state and not adequately described in simple Lewis structure formalisms. However, it is highly unlikely for two carbons without additional constraints to form a diradical like that; forming a triple bond ($\ce{H-C#C-H}$) would be much better energetically (for carbon!).

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  • $\begingroup$ Shine light on acetylene $\to$ get $\ce{H-C=C-H}$ $\to$ profit? $\endgroup$ – orthocresol♦ Feb 24 '17 at 0:42
  • $\begingroup$ @orthocresol If only relaxation were not a thing ;P $\endgroup$ – Jan Feb 24 '17 at 0:44

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