Formal charge

Formal charge is not currently taught in the current course but will be taught from this August as part of the new syllabus (that you may have heard your teacher moaning about!)

It is a really good idea / concept that is similar to that of oxidation numbers but which applies to covalent compounds.

But what is it?

 

Well, it is certainly not the type of charge intimated in the above image!

It is actually given as:

FC = V – (N + B/2)

You did ask! A key may make this rather complicated equation seem simpler.

FC = Formal charge

V = Number of valence electrons of the atom.

N = Number of non bonding valence electrons on the atom

B = Total number of electrons shared in covalent bonds with other atoms in the molecule

Still confused?

OK, let us look at a worked example.

What is the formal charge of carbon in methane?

FC = 4 – (0 + 8/2)

FC = 0

What is the formal charge of nitrogen in ammonia (NH₃)?

FC = 5 – (2 + 6/2)

FC = 5 – 5

FC = 0

However, it is not always zero!

What is the formal charge of nitrogen in ammonium (NH₄⁺)?

FC = 5 – (0 – 8/2)

FC = 5 – 4

FC = 1

But what is the point of it? What does it allow us to do?

Formal charge discards electronegativity. If you like, all atoms are thought of as having equal electronegativity.

Ultimately, it allows us to determine the efficiency of electron distribution in a molecule and lets us determine if a molecule has gained or lost electrons.

A positive number (as in the case of N in NH₄⁺) tells us that the N contains the bulk of the positive charge in the molecule. Conversely, a formal charge of say, -1 would tell us an atom contains the bulk of the negative charge.

When formal charges are added to Lewis diagrams it is possible to see which part of a molecule are electron deficient or rich and this allows us to predict / deduce mechanisms.

So all in all, it is a pretty important concept, linking together different aspects of chemistry (for example, bonding and organic chemistry).

Why don’t you try these for size?

Please post your answers to the IB Chemistry questions below and I will get back to you!

• What is the formal charge of O in methanol?
• What is the formal charge of O in methanl?
• What is the FC of N in nitromethane?
• What is the formal charge of S in sulphuric acid?