Formal Charge

Remember how “electrons are the currency of chemistry?”. And every chemical reaction is a transaction of electrons between atoms?

One of the key survival skills for getting through organic chemistry is learning electron accounting. Where do the electrons come from, where do they go? What bonds are broken and formed?
Here’s one of the two major accounting tools you have to master: formal charge(see diagram). This is going to be one of the building blocks of all of the future topics you’ll cover in organic chemistry.

Here’s the short cut: Find the # of valence electrons for the neutral atom (e.g. 4 for carbon). Subtract from this number the # of bonds AND the non-bonding electrons. That’s your formal charge.

It’s important to master because you’ll often be given a structure with just a negative charge or positive on an atom – and you’ll be expected to be able to figure out how many electrons are on that atom, just by being given the formal charge. It’s a skill that will bide you well all through Org 1 and Org 2.

Thanks for reading! James

P.S.  Why do we call it “formal” charge and not just “charge”? Because if a molecule bears a charge, for accounting purposes we need to assign a charge to one of the atoms. This is the method for it. A word of warning:  formal charge doesn’t always represent the “true” electron density around an atom. More on that soon, but here’s a preview: