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For the incomplete Lewis structure drawn, assign the charges to the C, N and O atoms of an isocyanate (overall the molecule has a single negative charge).

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Correct Answer: C. aO0, bC0 and cN-1
The normal valency of carbon is 4, oxygen is 2 and nitrogen 3. Adding electrons to make a full octet means adding two lone pairs to both oxygen and nitrogen atoms. For a -1 charge this must therefore be situated on N as it only has two bonding pairs of electrons instead of three.
If you ever get stuck during the exam, just take a moment to consider what you know for sure: (1) when oxygen is bonded twice, it has no formal charge (i.e. water); (2) when carbon is bonded 4 times, it has no formal charge (i.e. methane); (3) when nitrogen is bonded 4 times, it has a positive charge (i.e. ammonium), 3 times and it’s neutral (i.e. ammonia) and so just twice, it must be negatively charged.
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