Bond length

[DrRoyal Pains]

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I am working on a problem and it ask's which has the shortest bond length. I marked a C≡C bond as the answer, but it was wrong and they said that a C-H bond is shorter?

 

[ROCOCO8]

Is it bc H is a smaller atom? Electrons are closer to its nucleus?

 

[DrRoyal Pains]

That could be the reason .I know bond length decreases as the size of the atom decreases, but I also know what a triple bond pulls the two atoms involved in the bond closer than double bond, which pulls the atoms closer together than a single bond. Am I missing something here, or was the problem wrong?

 

[Adamsswag]

Well I believe you're thinking of the comparison between C=C and C≡C and with that, yes, your triple bond is shorter since we're comparing the bonds between identical atoms in both the double bond and triple bond structures. However, when comparing C≡C with C--H, we have two different atoms in the two structures and therefore you must look at the size of the atoms. Basically what I'm trying to say is (1) examine the size of the atoms and if they're the same size (hence the same atom) then move on to (2) compare the length between single, double, and triple bonds. Hope this helps.

 

[blackberryman]

So say it was C-H and C-F then how would it be?

 

[DrRoyal Pains]

Well I believe you're thinking of the comparison between C=C and C≡C and with that, yes, your triple bond is shorter since we're comparing the bonds between identical atoms in both the double bond and triple bond structures. However, when comparing C≡C with C--H, we have two different atoms in the two structures and therefore you must look at the size of the atoms. Basically what I'm trying to say is (1) examine the size of the atoms and if they're the same size (hence the same atom) then move on to (2) compare the length between single, double, and triple bonds. Hope this helps.

Gotcha. Thanks. TO blackberry, F is gigantic compared to H, so the C-H bond is shorter than the C-F bond. Just remember that smaller atoms make smaller bonds, and larger atoms make larger bonds.