calculating formal charges simple but significant
DESCRIPTION
What is the Formal Charge of an Atom? 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.TRANSCRIPT
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Calculating Formal ChargesSimple but significant
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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.
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What is the Formal Charge of an Atom?
• 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.
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What is the Formal Charge of 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.
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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.
• 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.
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What is the Formal Charge of an Atom?
• 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?
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What is the Formal Charge of 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)
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What is the Formal Charge of an Atom?
• 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:
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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.
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Calculating some Formal Charges
• 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 = #Valence – ½ (bonding) – (all lone pair electrons)
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Calculating some Formal Charges
• FC = #Valence – ½ (bonding) – (all lone pair electrons)
• FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1
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Calculating some Formal Charges
• 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.
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Calculating some Formal Charges
• 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.
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Calculating some Formal Charges
• FC(O) = 6 – ½ (2) – (6) = 6 – 1 – 6 = -1• As in this sodium salt:
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Calculating some Formal Charges
• Calculate the formal charge on oxygen in the molecule shown:
• FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1
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Calculating some Formal Charges
• Calculate the formal charge on oxygen in the molecule shown:
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Calculating some Formal Charges
• FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1
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Calculating some Formal Charges
• Calculate the formal charge on nitogen in the molecule shown:
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Calculating some Formal Charges
• FC(O) = 5 – ½ (8) – (0) = 5 – 4 – 0 = +1
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Calculating some Formal Charges
• Calculate the formal charge on the carbon (highlighted in red) in the molecule shown:
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Calculating some Formal Charges
• Carbon is in Group IV.• FC(O) = 4 – ½ (6) – (2) = 4 – 3 – 2 = -1
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Calculating some Formal Charges
• Calculate the formal charge on the sulfur (highlighted in red) in the molecule shown:
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Calculating some Formal Charges
• Sulfur is in Group IV.• FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1
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Calculating some Formal Charges
• Final Problem: Calculate the formal charge on the oxygen (highlighted in red) in the molecule shown:
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Calculating some Formal Charges
• Oxygen is in Group IV.• FC(O) = 6 – ½ (2) – (6) = 6 – 1 – 6 = -1
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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…