Section 8.3 Molecular Structures

1
Section 8.3 Molecular Structures

• List the basic steps used to draw Lewis
  structures.

• Explain why resonance occurs, and identify
  resonance structures.
• Identify three exceptions to the octet rule, and
  name molecules in which these exceptions occur.

ionic bond the electrostatic force that holds
oppositely charged particles together in an ionic
compound
2
Section 8.3 Molecular Structures (cont.)
structural formula resonance coordinate
covalent bond
Structural formulas show the relative positions
of atoms within a molecule.
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Section 8-1
Why do atoms bond?

• Atoms gain stability when they share electrons
  and form covalent bonds.

• Lower energy states make an atom more stable.
• Gaining or losing electrons makes atoms more
  stable by forming ions with noble-gas electron
  configurations. (octet)
• Sharing valence electrons with other atoms also
  results in noble-gas electron configurations.

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Section 8-1
Why do atoms bond? (cont.)

• Atoms in non-ionic compounds share electrons.
• Water is formed when Hydrogen and Oxygen atoms
  share electrons.

• The chemical bond that results from sharing
  electrons is a covalent bond.
• A molecule is formed when two or more atoms bond.

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• Shared electrons are considered to be part of the
  outer energy level of all the atoms that share
  it.
• This type of bonding usually occurs between
  elements near each other on the periodic table.
• Most of this type of bonding is done with
  nonmentals.

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• The chemical bond that results from sharing
  electrons is a covalent bond
• 3 types of Covalent bonds
• Single Covalent bond sharing 2 electrons
• Double Covalent bond Sharing 4 eletrons
• Triple Covalent Bond Sharing 6 electrons

• Atoms do not share more than 6 electrons between
  them (quadruple bond does not exist)

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Section 8-1
Why do atoms bond? (cont.)

• The most stable arrangement of atoms exists at
  the point of maximum net attraction, where the
  atoms bond covalently and form a molecule.
• F 1s22s2 2p5
• Shared are covalently bondedloan pairs are
  unbonded.

8
Structural Formulas

• In a Lewis structure dots or a line are used to
  symbolize a single covalent bond.
• A structural formula uses letter symbols and
  bonds to show relative positions of atoms.

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Structural Formulas

• Electron Dot Diagrams
• show valence e- as dots
• distribute dots like arrows in an orbital
  diagram
• 4 sides 1 s-orbital, 3 p-orbitals
• EX oxygen

O
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Drawing Lewis Diagrams

• Arrange atoms least electronegative atom
  usually in the middle. (never hydrogen)
• Calculate the total number of valence e-.
• Place one bonding pair (line) between the central
  atom and the terminal atoms
• Place the remaining pairs of electrons around the
  terminal atoms and any remaining pairs go on the
  central atom
• Check to be sure the central atom has four pairs
  of electrons, if not convert a pair of electrons
  from a terminal atom to make a double or triple
  bond

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Drawing Lewis Diagrams

• CF4

1 C 4e- 4e- 4 F 7e- 28e- 32e-
F F C F F
- 8e- 24e-
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Drawing Lewis Diagrams

• CO2

1 C 4e- 4e- 2 O 6e- 12e- 16e-
O C O
- 4e- 12e-
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Polyatomic Ions

• To find total of valence e-
• Add 1e- for each negative charge.
• Subtract 1e- for each positive charge.
• Place brackets around the ion and label the
  charge.

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Polyatomic Ions

• ClO4-

1 Cl 7e- 7e- 4 O 6e- 24e- 31e-
O O Cl O O
1e- 32e-
- 8e- 24e-
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Polyatomic Ions

• NH4

1 N 5e- 5e- 4 H 1e- 4e- 9e-
H H N H H
- 1e- 8e-
- 8e- 0e-
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Resonance Structures

• Resonance is a condition that occurs when more
  than one valid Lewis structure can be written for
  a molecule or ion.

• This figure shows three correct ways to draw the
  structure for Nitrate.

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Resonance Structures (cont.)

• Two or more correct Lewis structures that
  represent a single ion or molecule are resonance
  structures.

• The molecule behaves as though it has only one
  structure.
• The bond lengths are identical to each other and
  intermediate between single and double covalent
  bonds.

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Resonance Structures

• SO3

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Exceptions to the Octet Rule

• Some molecules do not obey the octet rule.

• A small group of molecules might have an odd
  number of valence electrons.
• NO2 has five valence electrons from nitrogen and
  12 from oxygen and cannot form an exact number of
  electron pairs.

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Section 8-3
Exceptions to the Octet Rule (cont.)

• A few compounds form stable configurations with
  less than 8 electrons around the atoma suboctet.

• A coordinate covalent bond forms when one atom
  donates both of the electrons to be shared with
  an atom or ion that needs two electrons.

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Section 8-3
Exceptions to the Octet Rule (cont.)

• A third group of compounds has central atoms with
  more than eight valence electrons, called an
  expanded octet.

• Elements in period 3 or higher have a d-orbital
  and can form more than four covalent bonds.

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Octet Rule

• Exceptions

• Hydrogen ? 2 valence e-

• Groups 1,2,3 get 2,4,6 valence e-

• Expanded octet ? more than 8 valence e-, central
  atom only and must be in the third row or lower

• Radicals ? odd of valence e-

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Section 8-3
Section 8.3 Assessment
What is it called when one or more correct Lewis
structures can be drawn for a molecule?
A. suboctet B. expanded octet C. expanded
structure D. resonance

1. A
2. B
3. C
4. D

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Section 8-3
Section 8.3 Assessment
Where do atoms with expanded octets occur?
A. transition metals B. noble gases
C. elements in period 3 or higher D. elements
in group 3 or higher

1. A
2. B
3. C
4. D