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I would draw the electron dot diagram structure for chlorine monoxide as follows:

Lewis diagram for ClO by OP

The correct answer is apparently with chlorine containing one unpaired electron:

Lewis diagram for ClO by OP's answer key

How would I know that? In the first example, all the formal charges are zero. But in the second, the charges for oxygen and chlorine are −1 and +1, respectively.

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  • $\begingroup$ 1. Lewis structures are models that help us predict things and understand things about ions and molecules. 2. In some problems there is no "right" answer, or be.tter to say, there are several answers, neither of which are really "right". I like your reasoning about formal charge, by the way. I would ask an additional question: What about CLO can we learn, predict, or understand better with one of those structures vs the other? $\endgroup$ Commented Jul 27, 2014 at 20:26
  • $\begingroup$ intermolecular forces - would they be higher with the second due to the formal charge? $\endgroup$
    – aaazalea
    Commented Jul 27, 2014 at 20:32
  • $\begingroup$ Maybe they would be weaker. The dipole moment of CO is .1D or so since C has a -1 and O a +1 formal charge. One might expect a C-O triple bond to be quite polar but it isn't. $\endgroup$ Commented Jul 27, 2014 at 20:34

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In fact, the unpaired electron belongs neither to Cl nor to O, but to a molecular orbital unequally spread over both atoms. The Lewis structure is simply not a complete representation of what is going on. And for what it's worth Wikipedia — Chlorine monoxide gives the same Lewis structure you drew.

Actually, it seems the highest occupied molecular orbital (where the unpaired electron is) is mostly localized on the oxygen atom according to the MO Diagram for the hypochlorite ion taken from University of Rhode Island — Chemistry 401 (Intermediate Inorganic Chemistry):

MO diagram for ClO-

That is to say, your structure is more correct than the one in the answer key. However, the justification involves molecular orbital arguments outside the scope of a general chemistry course. I too like your argument about formal charges and would accept it as a justification that the unpaired electron belongs to oxygen rather than to chlorine.

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  • $\begingroup$ You can come to the same generalization using electronegativity. O is more EN than Cl; O preferentially takes the remaining electron. $\endgroup$
    – Dissenter
    Commented Jul 27, 2014 at 21:50

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