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Name: Margaret
Status: educator
Grade: 9-12
Location: DC
Country: N/A
Date: March 2006

Question:
Why are double bonds shorter than single bonds? Is it because there are more electrons to be shared by the two atoms, so the nuclei are more attracted? I would expect the bonds to be longer because there are more electrons and the electrons would repel each other more.



Replies:
The general rule of thumb is "longer bond -- weaker bond; shorter bond -- stronger bond". And that correlation applies to most all circumstances. You are correct that there is increased repulsion between the electrons forming multiple bonds, but the strength of multiple bond formation out-weighs the energy debt due to repulsion. So for example, going from single, to double, to triple bonds in carbon the average bond energy varies as: 348, 682, 962. If there were no energy "debt" due to repulsion one might expect the trend to be 348x1=348, 348x2=696, 248x3=1044 (all energies in kJ/mol). In reality it's a bit more complicated than that because the types of the chemical bonds change -- they are not just balls and springs -- but that does not minimalize your perceptive observation.

Vince Calder


Margaret,

The short answer is that the atoms have to be closer together for more electronic orbitals to overlap to form a second bond--yes, there is more sharing of electrons between the atoms.

Below is a much more complete and detailed answer.

Single bonds are formed by head-to-head overlap of orbitals. For example, H2: Each hydrogen only has one electron in a spherical s-orbital. These s-orbitals overlap to form a sigma (single) bond. In the next example, let us think about ethene (C2H4) Here, the carbons are double bonded to each other, and each carbon is single bonded to two hydrogen atoms. The carbon atoms are both sp2 hybridized--in other words, their s and 2 of their p-orbitals have mixed together to make three identical orbitals. The left over unhybridized orbital is a p-orbital.

The sp2 orbitals overlap with the Hydrogen's s orbitals to make single bonds. There is also head-to-head overlap of orbitals from each carbon to form the single bond.

Now we can answer the question: what makes the double bond. The extra p-orbitals from each carbon (the ones that didn't hybridize) overlap to form a pi bond (the second bond between the carbons).

A double bond is composed of both the sigma bond (from head-to-head overlap) and a pi bond (from overlapping p-orbitals).

Now for the real answer to your original question: the double bond is shorter than the single bond because the atoms have to be close enough together for those unhybridized p-orbitals to overlap.

Hope this helps.

--Michelle Weinberger



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