Lewis Structures

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Chapter 13

• Lewis Structures

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Lewis Structures

• Lewis structures are diagrams that show the
  bonding between atoms of a molecule, and the lone
  pairs of valence electrons in the molecule.

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Steps for Writing Lewis Structures

• Count the total number of outer level (valence)
  electrons. To do this use the Roman numeral
  from the group number above each element in the
  periodic table.
• PO4
• PO43-
• PO43

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Steps for Writing Lewis Structures

• Determine the layout of the molecule. The
  formula of the molecule will often give you a
  hint as to its layout. For example in the
  molecule H3CCH3 there are two carbon atoms in the
  center with three hydrogen atoms bonded to each
  carbon.
• Carbon is always central
• Hydrogen is never central
• The element with the lowest electronegativity is
  central
• The element which you have the least of is
  usually central
• Group 17 elements are usually not central (unless
  you have no other choice) because they can only
  form single bonds.
• After determining the layout of the molecule.
  Arrange the elements symmetrically around your
  central atom(s).

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Steps for Writing Lewis Structures

• 3. The valence electrons from step 1 are now
  used to stabilize the atoms. This is done by
  using shared pairs (bonds) (see Table 1) to
  attach the atoms to the central atom. Use single
  bonds for all atoms first.

Note the strengths and lengths of the different
types of bonds.
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Steps for Writing Lewis Structures

• 3. After adding single bonds you may find that
  the atoms still need more valence electrons to
  achieve their octets. If more valence electrons
  are needed use unshared pairs (lone pairs) (see
  Table 1) around each atom to give the atom an
  octet (8 valence electrons). There are some
  exceptions to the octet rule. The most common
  exception is hydrogen which only requires 2
  electrons to fill its outer level and become
  stable.

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Steps for Writing Lewis Structures

• 3. The valence electrons from step 1 are now
  used to stabilize the atoms. This is done by
  using shared pairs (bonds) (see Table 1) to
  attach the atoms to the central atom. Use single
  bonds for all atoms first.
• After adding single bonds you may find that the
  atoms still need more valence electrons to
  achieve their octets. If more valence electrons
  are needed use unshared pairs (lone pairs) (see
  Table 1) around each atom to give the atom an
  octet (8 valence electrons). There are some
  exceptions to the octet rule. The most common
  exception is hydrogen which only requires 2
  electrons to fill its outer level and become
  stable.
• If you still do not have a stable structure you
  may try double and triple bonds if C, N, or O is
  involved in the bond.

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Steps for Writing Lewis Structures

• If you cannot write a stable structure for the
  molecule using rules 1 3 add or remove unshared
  pairs to/from the central atom until you arrive
  at the desired number of valence electrons
  determined in step 1. This may give you a
  structure that appears to be unstable however
  some molecules can form which do not have stable
  octets.
• Common exceptions to the octet rule H is stable
  with 2 valence electrons B is stable with 6
  valence electrons.

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Writing Lewis Structures

• CO2

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Writing Lewis Structures

• PO43-

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Writing Lewis Structures

• H2CO

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Writing Lewis Structures

• BrNO

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Writing Lewis Structures

• BrNO

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Steps for Writing Lewis Structures

• Determine the layout of the molecule. The
  formula of the molecule will often give you a
  hint as to its layout. For example in the
  molecule H3CCH3 there are two carbon atoms in the
  center with three hydrogen atoms bonded to each
  carbon.
• Carbon is always central
• Hydrogen is never central
• The element with the lowest electronegativity is
  central
• The element which you have the least of is
  usually central
• Group 17 elements are usually not central (unless
  you have no other choice) because they can only
  form single bonds.
• After determining the layout of the molecule.
  Arrange the elements symmetrically around your
  central atom(s).

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Writing Lewis Structures

• HCCH

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Steps for Writing Lewis Structures

• Determine the layout of the molecule. The
  formula of the molecule will often give you a
  hint as to its layout. For example in the
  molecule H3CCH3 there are two carbon atoms in the
  center with three hydrogen atoms bonded to each
  carbon.
• Carbon is always central
• Hydrogen is never central
• The element with the lowest electronegativity is
  central
• The element which you have the least of is
  usually central
• Group 17 elements are usually not central (unless
  you have no other choice) because they can only
  form single bonds.
• After determining the layout of the molecule.
  Arrange the elements symmetrically around your
  central atom(s).

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Writing Lewis Structures

• HCCH

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Writing Lewis Structures

• HCCH

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Writing Lewis Structures

• BeCl2

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Steps for Writing Lewis Structures

• If you cannot write a stable structure for the
  molecule using rules 1 3 add or remove unshared
  pairs to/from the central atom until you arrive
  at the desired number of valence electrons
  determined in step 1. This may give you a
  structure that appears to be unstable however
  some molecules can form which do not have stable
  octets.

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Homework

• Lewis Structures Worksheet

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Homework

• Lewis Structures Worksheet