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Consider nitric oxide, which has a real bond order of 2.5, contrary to what this UC Davis rendering might imply:

enter image description here

What's the best way to visualize a molecule with a fractional bond order? What exactly is "half a bond"? The electron spends half its time participating in bonding and the other half of the time as a lone electron on nitrogen?

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    $\begingroup$ What exactly is a "bond"? $\endgroup$ – LordStryker May 22 '14 at 19:27
  • $\begingroup$ Pairing of two electrons, one from either atom? $\endgroup$ – Dissenter May 22 '14 at 19:28
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    $\begingroup$ Electrons are waves, not necessarily just particles. Electron density is shared between two atoms which gives rise to attractive forces, not 'one electron from either atom to form a bond'. (I'm not trying to act superior by saying this). The Lewis structure is an approximation and can be grossly incorrect. It is commonly misinterpreted and can lead to problems such as defining what a 'half-bond' is among other things. In order to understand what is going on here, we need to re-think about what electrons and bonds are. $\endgroup$ – LordStryker May 22 '14 at 19:47
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    $\begingroup$ Are you going to explain? $\endgroup$ – Dissenter May 22 '14 at 20:11
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    $\begingroup$ A "half a bond" does not exist because the Lewis concept of dots being electrons is unphysical. Therefore, it is difficult to answer your question. Bonds are attractive forces that arise from electron density surrounding atoms in molecules (and between molecules). $\endgroup$ – LordStryker May 22 '14 at 20:28
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Maybe some examples will help. In the picture below, I've drawn some resonance structures for butadiene and acrolein. What do they tell us? They suggest that the central "single" bond should have some double bond character, it's more than a single bond, but less than a double bond. By the same token, the "double"

enter image description here

bonds at the ends of the molecules should both have some single bond character. These are examples of fractional bond order. The real world consequences of fractional bond order is a higher barrier to rotation about the central "single" bond in these two molecules than we might have expected, and a lower barrier to rotation in the pi bonds than we might have expected.

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  • $\begingroup$ Interesting comment about the rotation! $\endgroup$ – Dissenter May 22 '14 at 22:29
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    $\begingroup$ It might be helpful to know that your molecule is a hybrid of all these structures. It is not that these structures exist separately in equilibrium. You have to superimpose all of the structures to understand the molecule. $\endgroup$ – evil999man May 23 '14 at 3:31

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