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Chapter 6 Molecules and Covalent Compounds
6.3 Multiple Covalent Bonds and Resonance Copyright © by Pearson Education, Inc. Publishing as Benjamin Cummings
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Multiple Bonds In a single bond, One pair of electrons is shared.
In a double bond, Two pairs of electrons are shared. In a triple bond, Three pairs of electrons are shared.
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Electron-Dot Formula of`CS2
Write the electron-dot formula for CS2. STEP 1 Determine the atom arrangement. The C atom is the central atom. S C S STEP 2 Add the valence electrons for 1C and 2S. 1C(4e-) + 2S(6e-) = 16e-
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Electron-Dot Formula CS2
STEP 3 Attach each S atom to C with one electron pair. S : C : S Calculate the remaining electrons. 16e e- = 12e- left
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Electron-Dot Formula CS2
STEP 4 Attach 12 remaining electrons as 6 lone pairs to complete octets. : S : C : S : STEP 5 To complete octets, move two lone pairs between C and S atoms to give two double bonds. : S : : C : : S : or : S = C = S : `
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Multiple Bonds in N2 In nitrogen N2,
Octets are achieved by sharing three pairs of electrons, which is a triple bond.
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Some Electron-Dot Formulas
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Resonance Structures Resonance structures are:
Two or more electron-dot formulas for the same arrangement of atoms Related by a double-headed arrow Written by changing location of a double bond from the central atom to a different attached atom Sometimes written as a hybrid resonance structure
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Resonance Structures :O : :O: :O: :O : :O: :O:
Resonance structures for NO3− are: − − − :O: :O: :O: ║ │ │ N N N :O : :O: :O: :O : :O: :O:
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Learning Check Carbonate has three resonance structures. If the following is one, what are the other two? 2- :O: C :O: :O:
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Solution :O : :O: :O: :O : :O: :O:
Carbonate has three resonance structures. If the following is one, what are the other two? 2− 2− − :O: :O: :O: │ │ C C C :O : :O: :O: :O : :O: :O:
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Writing Resonance Structures
Cyanate ion NCO− has three resonance structures. STEP 1 Write the arrangement of atoms. N C O − STEP 2 Count the valence electrons. 1N(5e−) + 1C(4e−) + 1 O(6e−) + charge (1e−) = 16e− STEP 3 Connect bonded atoms by single electron pairs. N: C :O − 4e− used Determine the remaining electrons. 16e− - 4e− = 12e−
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Writing Resonance Structures
STEP 4 Add 12 remaining electrons as 6 lone pairs. − : N─C─O : STEP 5 Form double or triple bonds to make octets. − − − :N=C=O: :N─CO: :NC─O:
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Learning Check Write two resonance structures for nitrite NO2−.
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Solution :O=N─O: :O─N=O: NO2− = 18 e− Electron-dot formula
− : O─N─O : Using a double bond to complete octets gives two resonance structures. − − :O=N─O: :O─N=O:
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