You want to remember that in resonance structures, the atoms stay in exactly the same positions and only the electrons are arranged differently. Resonance structures really represent the same molecule, but with the crudeness of dot structures, this is the best we can do to represent molecules that have more complicated bonding. (Later you'll use "hybrid" atomic orbitals to make "sigma" and "pi" bonds in molecules like this one, which will give a more accurate but still incomplete picture.)
I believe that the order of bonding is H-C-N-O (see here for example). We will fill in the rest of the electrons (we have 16 valence electrons total) in different ways, trying to maintain octets on atoms if possible.
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One structure is H-C=N=O. There is one lone electron pair on the carbon and two lone pairs on the oxygen. The carbon has a formal -1 charge and the nitrogen has a formal +1 charge. Note that these formal charges are not in agreement with the relative electronegativities of carbon and nitrogen, making this a less favorable structure.
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The triple bond structure will be H-C=N-O, with three lone pairs on the oxygen atom. The nitrogen has a +1 formal charge and the oxygen has a -1 formal charge.
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The last structure is H-C=N-O, with one lone pair on the nitrogen and three lone pairs on the oxygen, with a formal charge of +1 on carbon and -1 on oxygen. Note that this structure has less than an octet on the carbon, making this structure less favorable, but that is allowable.
It is possible to draw more structures, but they will have unreasonable formal charges and are therefore excluded.
Hope this helps!
Steve
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