8.9 – 8.12

 

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The localized electron Model

  • A molecule is composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms
  • Localized on the atom = Unbonded electrons = lone pairs
  • Found in the space between atoms = Shared electrons = bonding pairs
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8.10 Lewis structures

  • Shows how the valence electrons are arranged among the atoms in a molecule
  • Duet rule for H, octet rule for other non-metals
  • Arrange the remaining electrons to satisfy the duet/octet rules 
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8.11 exceptions to the octet rule

  • The second row elements, C, N, O and F should always be assumed to obey the octet rule
  • The second row elements B and Be often have fewer than eight electrons around them in their compounds. These electron deficient compounds are very reactive
  • The second row elements never exceed the octet rule, since their valence orbitals (2s and 2p) can accommodate only eight electrons
  • The third row and heavier elements often satisfy the octet rule but can exceed the octet rule by using their empty valence d orbitals
  • When writing Lewis structures for a molecule, satisfy the octet rule for the atoms first. If electrons remain after the octet rule has been satisfied, then place them on the elements having available d orbitals ( elements in period 3 or beyond)
  • When it is necessary to exceed the octet rule for one of several third row or higher elements, assume that the extra electrons should be placed on the central atom
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8.12 Resonance

 

  • Sometimes more than one valid Lewis structure is possible for a given molecule
  • The correct description for NO3- is not given by any one of the three Lewis Structures but is given only by the superposition of all three

 

  • Resonance occurs when more than one valid Lewis structure can be written for a particular molecule
  • The actual structure is an average of the three resonance structures Electrons are delocalized in nature
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Formal charge

 

  • Formal charge is the difference between the number of valence electrons on the free atoms and the number of valence electrons assigned to the atom in a molecule.
  • Formal charge = (number of valence electrons on free atom) – (number of valence electrons assigned to the atom in a molecule)
  • Lone pair electrons belong entirely to the atom in question
  • Shared electrons are divided equally between the two sharing atoms
  • Valence electronsassigned = number of lone pair electrons + ½ number of shared electrons
  • Atoms in molecules try to achieve formal charges as close to zero as possible
  • Any negative formal charges are expected to reside on most electronegative atoms
  • The sum of the formal charges of all atoms in a given molecule or ion must equal the overall charge on that species.
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