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In an exam, I have been asked about the number of lone pairs in the molecule NO, the answer has been given as 2. According to Wikipedia, there is a structure relating a three electron bond as a third bond, but doesn't it violate the octet of oxygen?

I kind of understand that the octet rule is old and has been disproven, I haven't found any explanation for this phenomenon, the only other fact that supports it (which I have found) is the bond order from MOT, but I don't understand it.

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    $\begingroup$ Are you primarily asking about the bond order or about the number of lone pairs? You can correlate bond order (a concept we made up) to something real, the bond length. If you compare the bond lengths of $\ce{NO+, NO*, NO-}$, and maybe also look at isoelectronic species (e.g. $\ce{O2, N2, CO}$), you can get a sense of what it going on without having to rely on bonding theories you don't yet understand. $\endgroup$
    – Karsten
    Apr 21, 2023 at 12:41
  • $\begingroup$ I have assumed both the bond order and number of lone pairs to be correlated, but I am not sure about it, then what do we say about the number of lone pairs in NO molecule, and I have edited the question $\endgroup$
    – Seeker
    Apr 21, 2023 at 13:27

1 Answer 1

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Here is the most common way of writing the Lewis structure of NO, a molecule with an uneven number of valence electrons:

enter image description here

Source: http://www.chem.ucla.edu/~harding/IGOC/N/nitric_oxide01.png

From that image, you would say that there are two lone pairs on the oxygen atom (that is the typical binding pattern) and one lone pair and an unpaired electron on the nitrogen.

However, the Lewis structure does not reflect that the bond is a bit shorter than the N=O double bond found in other molecules and ions (and a bit longer than the triple bond).

If you take a look at the MO-diagram, you will see three pairs of electrons designated as bonding, and a single electron as antibonding (if we call the electrons in the 2s orbitals non-bonding for now).

enter image description here

Source: https://chemistry.stackexchange.com/a/15317

There is no good way to draw this as a Lewis structure. It seems like Wikipedia tried something without explaining it:

enter image description here

Source: https://commons.wikimedia.org/wiki/File:Nitric_oxide.svg

This depiction, however, is confusing. There should never be three electrons in an orbital.

You could invent a new way of drawing Lewis structures, showing a triple bond plus an electron that is designated as antibonding. It would look like this (but it should be emphasized that this is a non-standard depiction):

enter image description here

However, if you take this as the Lewis structure, you would count two lone pairs. Hopefully, this discussion shows you that lone pairs are not well-defined if you can't draw a Lewis structure that captures the molecule well.

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