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1/30/16Calculating FormalChargesSimple but significantWhat is the Formal Charge ofan Atom? The Formal Charge is a mathematical summation ofthe number of actual electrons associated with anatom in a molecule. “Electronic Bookkeeping” The Formal Charge is all determined relative to thenumber of valence electrons an atom would have inthe ground state. Remember, valence electrons are those found in theoutermost unfilled shell.What is the Formal Charge ofan Atom?What is the Formal Charge ofan Atom? Anytime you see a charge in a molecule, it tells youthat the atom with the charge has more or lesselectrons than it normally would. Charges can be found on all sorts of atoms commonto organic chemistry, including carbons, oxygens,nitrogens and halides. A -1 charge equals one extra electron on an atom. Remember that carbon typically has four covalentbonds. If it has only three, then it will be a chargedatom, depending on whether it has a lone pair orOnot. A 1 charge equals one less electron on an atom.HCCHWhat is the Formal Charge ofan Atom?What is the Formal Charge ofan Atom? Oxygen atoms, when neutral, have two covalentbonds and two lone pairs, like in water. Oftenthough oxygen atoms may have negative or positivecharges. The presence of more or less electron density in amolecule dictates the flow of electrons in a processso this is really helpful information. Nitrogen atoms, when neutral, have three covalentbonds and one lone pair. These can also havenegative or positive charges, depending on what isbonded. So – how do you calculate the Formal Charge on anatom?1

1/30/16What is the Formal Charge ofan Atom?What is the Formal Charge ofan Atom? The equation to determine the Formal Charge on anatom: Keep in mind that Formal Charge is for a specificatom so you have to know exactly which atom youare doing the calculation for.FC (The number of valence electrons on the atom inthe ground state) – ½ (total number of electrons incovalent bonds attached to the atom, with each covalentbond having two electrons) – (all electrons in lone pairson an atom, with each lone pair equal to two electrons). Determine the Formal Charge on the nitrogen atomin the following:HHN So:HFC #Valence – ½ (bonding) – (all lone pair electrons)Calculating some FormalChargesCalculating some FormalCharges Nitrogen is in Group V and has 5 valence electronsHHin the ground state. Calculate the formal charge on oxygen in thehydronium ion shown: HHNOH FC #Valence – ½ (bonding) – (all lone pairelectrons) FC(N) 5 – ½ (6) – (2) 5 – 3 – 2 0 Nitrogen has no charge – its neutral.Calculating some FormalCharges FC #Valence – ½ (bonding) – (all lone pairelectrons) FC(O) 6 – ½ (6) – (2) 6 – 3 – 2 1HHH Oxygen is in Group VI and has 6 valence electronsin the ground state. FC #Valence – ½ (bonding) – (all lone pairelectrons)Calculating some FormalCharges Every time you see that positive charge on an atom,it technically means the atom is missing one entireelectron.HHOHOH In this case, the oxygen only has five electrons, notsix.2

1/30/16Calculating some FormalChargesCalculating some FormalCharges Calculate the formal charge on oxygen in themolecule shown:O FC(O) 6 – ½ (2) – (6) 6 – 1 – 6 -1 As in this sodium salt:O Na Again, oxygen is in Group VI and has 6 valenceelectrons in the ground state.Calculating some FormalChargesCalculating some FormalCharges Calculate the formal charge on oxygen in themolecule shown:H Calculate the formal charge on oxygen in themolecule shown:HOO FC(O) 6 – ½ (6) – (2) 6 – 3 – 2 1HOCalculating some FormalCharges FC(O) 6 – ½ (6) – (2) 6 – 3 – 2 1HOCalculating some FormalCharges Calculate the formal charge on nitogen in themolecule shown:HN3

1/30/16Calculating some FormalCharges FC(O) 5 – ½ (8) – (0) 5 – 4 – 0 1HCalculating some FormalCharges Calculate the formal charge on the carbon(highlighted in red) in the molecule shown:ONHSH3CCHCalculating some FormalCharges Carbon is in Group IV. FC(O) 4 – ½ (6) – (2) 4 – 3 – 2 -1Calculating some FormalCharges Calculate the formal charge on the sulfur(highlighted in red) in the molecule shown:OOHSH3CSCH3CHCalculating some FormalCharges Sulfur is in Group IV. FC(O) 6 – ½ (6) – (2) 6 – 3 – 2 1CH3Calculating some FormalCharges Final Problem: Calculate the formal charge on theoxygen (highlighted in red) in the molecule shown:OOSH3CSH3CCH3CH34

1/30/16Calculating some FormalCharges Oxygen is in Group IV. FC(O) 6 – ½ (2) – (6) 6 – 1 – 6 -1Formal Charges Just remember: Valence Electrons minus half thebonding electrons (two per bond) minus all of thelone pair electrons (two per pair) O Thanks for reading SH3CCH35

atom, depending on whether it has a lone pair or not. C H O H What is the Formal Charge of an Atom? 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.