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Calculating Formal Charges

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Calculating Formal Charges 1/30/16 What is the Formal Charge of an Atom? Calculating Formal • The Formal Charge is a mathematical summation of Charges the number of actual electrons associated with an atom in a molecule. “Electronic Bookkeeping” Simple but significant • 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. What is the Formal Charge of What is the Formal Charge of an Atom? an Atom? • Anytime you see a charge in a molecule, it tells you • Charges can be found on all sorts of atoms common that the atom with the charge has more or less to organic chemistry, including carbons, oxygens, electrons than it normally would. nitrogens and halides. • A -1 charge equals one extra electron on an atom. • Remember that carbon typically has four covalent bonds. If it has only three, then it will be a charged • A +1 charge equals one less electron on an atom. atom, depending on whether it has a lone pair or not. O H C C H What is the Formal Charge of What is the Formal Charge of an Atom? an Atom? • Oxygen atoms, when neutral, have two covalent • The presence of more or less electron density in a bonds and two lone pairs, like in water. Often molecule dictates the flow of electrons in a process though oxygen atoms may have negative or positive so this is really helpful information. charges. • Nitrogen atoms, when neutral, have three covalent bonds and one lone pair. These can also have • So – how do you calculate the Formal Charge on an negative or positive charges, depending on what is atom? bonded. 1 1/30/16 What is the Formal Charge of What is the Formal Charge of an Atom? an Atom? • The equation to determine the Formal Charge on an • Keep in mind that Formal Charge is for a specific atom: atom so you have to know exactly which atom you FC = (The number of valence electrons on the atom in are doing the calculation for. the ground state) – ½ (total number of electrons in covalent bonds attached to the atom, with each covalent • Determine the Formal Charge on the nitrogen atom bond having two electrons) – (all electrons in lone pairs in the following: on an atom, with each lone pair equal to two electrons). H H N • So: H FC = #Valence – ½ (bonding) – (all lone pair electrons) Calculating some Formal Calculating some Formal Charges Charges • Nitrogen is in Group V and has 5 valence electrons • Calculate the formal charge on oxygen in the in the ground state. H H hydronium ion shown: N H H O H H • FC = #Valence – ½ (bonding) – (all lone pair electrons) • Oxygen is in Group VI and has 6 valence electrons in the ground state. • FC(N) = 5 – ½ (6) – (2) = 5 – 3 – 2 = 0 • FC = #Valence – ½ (bonding) – (all lone pair • Nitrogen has no charge – its neutral. electrons) Calculating some Formal Calculating some Formal Charges Charges • FC = #Valence – ½ (bonding) – (all lone pair • Every time you see that positive charge on an atom, electrons) it technically means the atom is missing one entire electron. H H • FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1 O H H H O H • In this case, the oxygen only has five electrons, not six. 2 1/30/16 Calculating some Formal Calculating some Formal Charges Charges • Calculate the formal charge on oxygen in the • FC(O) = 6 – ½ (2) – (6) = 6 – 1 – 6 = -1 molecule shown: O • As in this sodium salt: O Na • Again, oxygen is in Group VI and has 6 valence electrons in the ground state. Calculating some Formal Calculating some Formal Charges Charges • Calculate the formal charge on oxygen in the • Calculate the formal charge on oxygen in the molecule shown: H molecule shown: H O O H • FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1 O Calculating some Formal Calculating some Formal Charges Charges • FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1 • Calculate the formal charge on nitogen in the molecule shown: H H O N 3 1/30/16 Calculating some Formal Calculating some Formal Charges Charges • FC(O) = 5 – ½ (8) – (0) = 5 – 4 – 0 = +1 • Calculate the formal charge on the carbon (highlighted in red) in the molecule shown: H N O S H H3C C H Calculating some Formal Calculating some Formal Charges Charges • Carbon is in Group IV. • Calculate the formal charge on the sulfur (highlighted in red) in the molecule shown: • FC(O) = 4 – ½ (6) – (2) = 4 – 3 – 2 = -1 O O S H S H3C C H3C CH3 H Calculating some Formal Calculating some Formal Charges Charges • Sulfur is in Group IV. • Final Problem: Calculate the formal charge on the oxygen (highlighted in red) in the molecule shown: • FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1 O O S S H3C CH3 H3C CH3 4 1/30/16 Calculating some Formal Formal Charges Charges • Oxygen is in Group IV. • Just remember: Valence Electrons minus half the bonding electrons (two per bond) minus all of the • FC(O) = 6 – ½ (2) – (6) = 6 – 1 – 6 = -1 lone pair electrons (two per pair)… O S • Thanks for reading… H3C CH3 5 .
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