- \n
- evaluating the number of valence electrons (VE<\/span>) <\/strong>the neutral<\/strong> atom has (e.g. 3 for boron, 4 for carbon, 5 for nitrogen, and so on).\u00a0 (note: this is also equivalent to the effective nuclear charge Zeff<\/sub> , the number of protons that an electron in the valence orbital “sees” due to screening by inner-shell electrons.)<\/em><\/span><\/li>\n
- counting the number of\u00a0non-bonded valence electrons (NBE<\/span>)<\/strong> on the atom. Each lone pair counts as\u00a02<\/strong>, and each unpaired electron counts as 1.<\/strong><\/li>\n
- counting the number of\u00a0bonds (B<\/span>)<\/strong> to the atom, or alternatively, counting the number of bonding electrons <\/strong>and dividing this by 2<\/strong>.<\/li>\n<\/ul>\n
The formal charge\u00a0FC<\/strong> is then calculated by subtracting NBE<\/span><\/strong>\u00a0and B<\/span><\/strong>\u00a0from VE<\/span><\/strong>.<\/p>\n
FC <\/b>= VE<\/span><\/strong> \u2013 (NBE<\/span> + B<\/span>)\u00a0<\/strong><\/p>\n
which is equivalent to<\/p>\n
FC = VE<\/span> \u2013 NBE<\/span> \u2013 B<\/span><\/strong><\/p>\n
The calculation is pretty straightforward if<\/strong> all the information is given to you. However, for brevity’s sake, there are many times when lone pairs<\/strong> and C-H bonds<\/strong> are not explicitly drawn out<\/strong>.<\/p>\n
So part of the trick for you will be to calculate the formal charge in situations where you have to take account of implicit<\/strong>\u00a0lone pairs and C-H bonds.<\/p>\n
In the article below, we’ll address many of these situations. We’ll also warn you of the situations where the calculated formal charge of an atom is not necessarily a good clue as to its reactivity<\/strong>, which is extremely important going forward.<\/p>\n
![\"Formal](\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2010\/09\/0-Formal-charge-formula-how-to-calculate-the-formal-charge-of-an-atom-master-organic-chemistry-1.gif\")
<\/a><\/p>\nTable of Contents<\/strong><\/p>\n
- \n
- Formal Charge<\/a><\/li>\n
- Simple Examples For First-Row Elements<\/a><\/li>\n
- Formal Charge Calculations When You Aren’t Given All The Details<\/a><\/li>\n
- Some Classic Formal Charge Problems<\/a><\/li>\n
- Formal Charges and Curved Arrows<\/a><\/li>\n
- Halogens<\/a><\/li>\n
- Conclusion<\/a><\/li>\n
- Notes<\/a><\/li>\n
- Quiz Yourself!<\/a><\/li>\n
- (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
\n<\/a>1. Formal Charge<\/h2>\n
Formal charge is a book-keeping<\/strong> formalism<\/strong> for assigning a charge to a specific atom.<\/p>\n
To obtain the formal charge of an atom, we start by counting the number of valence electrons<\/strong> [Note 1<\/span><\/a>] for the neutral atom<\/strong>, and then subtract from it the number of electrons that it “owns<\/strong>” (i.e. electrons in lone pairs, or singly-occupied orbitals<\/em><\/span>) and half<\/strong> of the electrons that it shares<\/strong> (half the number of bonding electrons, which is equivalent to the number of bonds<\/em><\/span>)<\/p>\n
The simplest way to write the formula for formal charge\u00a0 (FC) <\/strong>\u00a0is:<\/p>\n
FC<\/strong> = VE<\/strong><\/span> – NBE<\/strong><\/span> – B<\/strong><\/span><\/p>\n
where<\/p>\n
- \n
- VE<\/strong> corresponds to the number of electrons around the neutral atom (3 for boron, 4 for carbon, 5 for nitrogen, 6 for oxygen, 7 for fluorine)<\/em><\/span><\/li>\n
- NBE<\/strong> corresponds to the number of non-bonded electrons around the atom (2 for a lone pair, 1 for a singly-occupied orbital, 0 for an empty orbital)<\/em><\/span><\/li>\n
- B<\/strong> is the number of\u00a0bonds<\/strong> around the atom (equivalent to half the number of bonding electrons)<\/em><\/span><\/li>\n<\/ul>\n
It’s called “formal<\/strong>” charge because it assumes that all bonding electrons are shared equally<\/strong>. It doesn’t account for electronegativity differences (i.e. dipoles).<\/p>\n
For that reason formal charge isn’t always a good guide to where the electrons actually are<\/strong> in a molecule and can be an unreliable guide to reactivity. We’ll have more to say on that below<\/a>.<\/p>\n
<\/a>2. Simple Examples For First-Row Elements<\/h2>\n
When all the lone pairs are drawn out for you, calculating formal charge is fairly straightforward.<\/p>\n
Let’s work through the first example in the quiz below.<\/p>\n
- \n
- In the hydronium ion (H3<\/sub>O) the central atom is oxygen<\/strong>, which has 6<\/strong> valence electrons in the neutral atom<\/li>\n
- The central atom has 2<\/strong> unpaired electrons and 3<\/strong> bonds<\/li>\n
- The formal charge on oxygen is [6 – 2 – 3 = +1 ]<\/strong> giving us H3<\/sub>O+<\/sup><\/strong><\/li>\n<\/ul>\n
See if you can fill in the rest for the examples below.<\/p>\n
- The central atom has 2<\/strong> unpaired electrons and 3<\/strong> bonds<\/li>\n
- NBE<\/strong> corresponds to the number of non-bonded electrons around the atom (2 for a lone pair, 1 for a singly-occupied orbital, 0 for an empty orbital)<\/em><\/span><\/li>\n
- Simple Examples For First-Row Elements<\/a><\/li>\n
- counting the number of\u00a0non-bonded valence electrons (NBE<\/span>)<\/strong> on the atom. Each lone pair counts as\u00a02<\/strong>, and each unpaired electron counts as 1.<\/strong><\/li>\n