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In a covalent bond, there are several forces acting together to determine the bond length.

Electron-electron repulsion increases the bond length.

Proton-proton repulsion increases the bond length.

Proton-electron attraction decreases the bond length.

When I try to understand that a triple bond is shorter than a single bond, I have a hard time trying to comprehend how big a role does electron-electron repulsion play in the bond length. Since a triple bond has three pairs of shared electrons, I would reason that the repulsion is stronger than a single bond and therefore the bond length should be longer. Clearly this is not the case. The proton- electron attraction is also ruled out. As a single bonded c-o has the same number of protons as a triple bonded c-o. Triply bonded c-o however, has a shorter bond length of 113 pm while single bonded c-o is 143 pm. While I can rote memorize this fact, it will be really appreciated if there is a rational explanation.

Dan L
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  • Bonding means lowering energy of valence electrons and that effectively means electron attraction. – Poutnik Sep 17 '21 at 04:35
  • Try to approach your both hands, with fingers stretched. When your thumbs are touching one another, it is similar to a single bond : the distance between the centers of the hands is large. When the thumbs and the forefingers are touching one another, this looks like a double bond : the distance between the centers of your hands is smaller. When the thumbs, the forefingers and any other third fingers are touching one another, it looks like a triple bond : the distance between the centers of your hands is still smaller. – Maurice Sep 17 '21 at 07:56
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    The logic you follow is correct. The three assertions are somewhat correct as well. The point is that chemical bond does not relies on classical electrostatic. And not even the existence of H does. – Alchimista Sep 17 '21 at 14:34
  • @Mithoron that article is a bit advance for me to comprehend at my current level, but it will be kept on my reading list. – Dan L Sep 21 '21 at 04:54

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