Formal Charges and How to Calculate Them

This knowledge comes ultimately from experiments. If you have forgotten the structures 

Problem 1.5), this may be a good time for a review of the relevant portions of your gen- Check your progress by doing - -, each Review Problem as you come 

Assume that the atoms are connected in the same way they are written in the formula, and write a Lewis structure for the toxic gas hydrogen cyanide (HCN). 

We find the total number of valence electrons on all of the atoms  

We use one pair of electrons to form a single bond between the hydrogen atom and the carbon atom (see below), and we use three pairs to form a triple bond between the carbon atom and the nitrogen atom. This leaves two electrons. We use these as an unshared pair on the nitrogen atom. Now each atom has the electronic structure of a noble gas. The carbon atom has two electrons (like helium) and the carbon and nitrogen atoms each have eight electrons (like neon). 

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