Calculating Formal Charges Simple but significant

Calculating Formal ChargesSimple but significant

What is the Formal Charge of an Atom?

• The Formal Charge is a mathematical summation of the number of actual electrons associated with an atom in a molecule. “Electronic Bookkeeping”

• The Formal Charge is all determined relative to the number of valence electrons an atom would have in the ground state.

• Remember, valence electrons are those found in the outermost unfilled shell.


• Anytime you see a charge in a molecule, it tells you that the atom with the charge has more or less electrons than it normally would.

• A -1 charge equals one extra electron on an atom.

• A +1 charge equals one less electron on an atom.


• Charges can be found on all sorts of atoms common to organic chemistry, including carbons, oxygens, nitrogens and halides.

• Remember that carbon typically has four covalent bonds. If it has only three, then it will be a charged atom, depending on whether it has a lone pair or not.


• Oxygen atoms, when neutral, have two covalent bonds and two lone pairs, like in water. Often though oxygen atoms may have negative or positive charges.

• Nitrogen atoms, when neutral, have three covalent bonds and one lone pair. These can also have negative or positive charges, depending on what is bonded.


• The presence of more or less electron density in a molecule dictates the flow of electrons in a process so this is really helpful information.

• So – how do you calculate the Formal Charge on an atom?


• The equation to determine the Formal Charge on an atom:FC = (The number of valence electrons on the atom in the ground state) – ½ (total number of electrons in covalent bonds attached to the atom, with each covalent bond having two electrons) – (all electrons in lone pairs on an atom, with each lone pair equal to two electrons).

• So:FC = #Valence – ½ (bonding) – (all lone pair electrons)


• Keep in mind that Formal Charge is for a specific atom so you have to know exactly which atom you are doing the calculation for.

• Determine the Formal Charge on the nitrogen atom in the following:

Calculating some Formal Charges

• Nitrogen is in Group V and has 5 valence electrons in the ground state.

• FC = #Valence – ½ (bonding) – (all lone pair electrons)

• FC(N) = 5 – ½ (6) – (2) = 5 – 3 – 2 = 0• Nitrogen has no charge – its neutral.


• Calculate the formal charge on oxygen in the hydronium ion shown:

• Oxygen is in Group VI and has 6 valence electrons in the ground state.




• FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1


• Every time you see that positive charge on an atom, it technically means the atom is missing one entire electron.

• In this case, the oxygen only has five electrons, not six.


• Calculate the formal charge on oxygen in the molecule shown:

• Again, oxygen is in Group VI and has 6 valence electrons in the ground state.


• FC(O) = 6 – ½ (2) – (6) = 6 – 1 – 6 = -1• As in this sodium salt:



• FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1




• FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1


• Calculate the formal charge on nitogen in the molecule shown:


• FC(O) = 5 – ½ (8) – (0) = 5 – 4 – 0 = +1


• Calculate the formal charge on the carbon (highlighted in red) in the molecule shown:


• Carbon is in Group IV.• FC(O) = 4 – ½ (6) – (2) = 4 – 3 – 2 = -1


• Calculate the formal charge on the sulfur (highlighted in red) in the molecule shown:


• Sulfur is in Group IV.• FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1


• Final Problem: Calculate the formal charge on the oxygen (highlighted in red) in the molecule shown:


• Oxygen is in Group IV.• FC(O) = 6 – ½ (2) – (6) = 6 – 1 – 6 = -1

Formal Charges

• Just remember: Valence Electrons minus half the bonding electrons (two per bond) minus all of the lone pair electrons (two per pair)…

• Thanks for reading…

Documents

Calculating Formal Charges Simple but significant