Chapter 10: structure and shape

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Chapter 10 structure and shape

• Theme shape is determined by electron repulsion

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Two-minute problem

• Select the group that is arranged in order of
  increasing ionic character.
• KF, MgO, AlN
• KF, AlN, MgO
• MgO, KF, AlN
• MgO, AlN, KF
• AlN, KF, MgO
• AlN, MgO, KF

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Drawing Lewis Structures

• 1. Count the valence electrons
• Anions add one electron for each negative charge
• Cations subtract one electron for each positive
  charge
• 2. Write the symbols for the atoms
• least electronegative in center
• connect with single bonds
• 3. Complete octets of outer atoms
• Place left-over e- on the central atom
• If central atom is short of electrons
• Multiple bonds

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Lewis Structures

• CH4
• CO2
• NH3

• NO3-
• NH4

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Exceptions to Octet Rule

• Expanded valence shell
• PCl5
• XeF4

Reduced valence shell

• BeF2
• BCl3

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Exceptions to Octet Rule

• Radicals
• NO2
• OH

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Formal Charge

• Formal charge bookkeeping of valence electrons
• the charge an atom would have if all atoms had
  the same electronegativity
• F.C. (valence e-)
• 1/2(bonding e-) (lone pair e-)

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Formal Charge

• CH4
• OH-

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Two Minute Problem

• Calculate the formal charge on the C atom in
  CH3
• -1
• 0
• 1
• 2
• I dont know

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Formal Charge

• If several Lewis structures are possible, the
  most stable is
• the smallest formal charges
• no adjacent opposite charges
• negative formal charges on the more
  electronegative atom

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Formal Charge Problem

• Use the concept of formal charge to determine the
  correct Lewis structure for SO4-2.

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Formal Charge Problem

• Use the concept of formal charge to determine the
  correct Lewis structure for NCS-.

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Resonance Structures

• All possible Lewis structures for a molecule
  having delocalized bonding
• bonding in which a bonding pair of electrons is
  spread over more than two of atoms
• Example Describe the electron structure of O3
  (ozone).

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Two minute problem

• Draw the Lewis structure for N2
• Each nitrogen atom has ___ lone pairs.
• 0
• 1
• 2
• 3

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Resonance Structures

• CO3-2
• SO4-2

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Resonance Stabilization

• Benzene

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Bond Order

• The number of bonds between two atoms
• As bond order increases, bond length decreases
• Why?

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Two-minute problem

• Which resonance structure is more important?

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Valence-Shell Electron-Pair Repulsion (VSEPR)
Model

• Write down Lewis structure
• Count the number of groups of electrons around
  the central atom
• Assign electron pair geometry
• Assign molecular geometry (shape of molecule)

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Electron geometry

• Two groups of electrons linear

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Electron geometry

• Three groups of electrons trigonal planar

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Electron geometry

• Four groups of electrons tetrahedral

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Electron geometry

• Five groups of electrons trigonal bipyramidal

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Electron geometry

• Six groups of electrons octahedral

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Two-minute problem

• What is the electron geometry for the oxygen in
  hypochlorous acid?
• Linear
• Trigonal planar
• Tetrahedral
• Trigonal bipyramidal
• I dont know

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Molecules with Linear Electron geometry

• HCN
• BeF2
• All have linear molecular geometry

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Molecules with Trigonal Planar Electron geometry

• BF3
• SO3
• Either bent or trigonal planar molecular geometry

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Molecules with Trigonal Planar Electron geometry

• SO2
• Either bent or trigonal planar molecular geometry

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Molecules With Tetrahedral Electron Geometry

• CH4
• Tetrahedral, trigonal pyramidal, or bent
  molecular geometry

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Molecules With Tetrahedral Electron Geometry

• NH3
• Tetrahedral, trigonal pyramidal, or bent
  molecular geometry

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Molecules With Tetrahedral Electron Geometry

• H2O
• Tetrahedral, trigonal pyramidal, or bent
  molecular geometry

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Molecules With Trigonal Bipyramidal Electron
Geometry

• PF5
• Trigonal Bipyramidal, see-saw, T-shaped, or
  linear molecular geometry

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Molecules With Trigonal Bipyramidal Electron
Geometry

• SF4
• Trigonal Bipyramidal, see-saw, T-shaped, or
  linear molecular geometry

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Molecules With Trigonal Bipyramidal Electron
Geometry

• ClF3
• Trigonal Bipyramidal, see-saw, T-shaped, or
  linear molecular geometry

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Molecules With Trigonal Bipyramidal Electron
Geometry

• XeF2
• Trigonal Bipyramidal, see-saw, T-shaped, or
  linear molecular geometry

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Molecules With Octahedral Electron Geometry

• SF6
• Octahedral, square pyramidal, or square planar
  molecular geometry

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Molecules With Octahedral Electron Geometry

• BrF5
• Octahedral, square pyramidal, or square planar
  molecular geometry

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Molecules With Octahedral Electron Geometry

• XeF4
• Octahedral, square pyramidal, or square planar
  molecular geometry

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Two-minute problem

• What is the shape of ICl4-?
• Square planar
• Tetrahedral
• See-saw
• I dont know

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Molecules with Octahedral Electron geometry

• VSEPR model to predict the electron pair and
  molecular geometries of the following molecules.
  Include the bond angles.
• AsF3
• ICl4 -

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QOD

• What is the shape of KrF4?
• Square planar
• Tetrahedral
• See-saw
• I dont know

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Dipole Moment

• definition
• a quantitative measure of the degree of charge
  separation in a molecule
• related to the polarity of a molecule

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Polarity Molecular Geometry

• The geometry of a molecule can be used to
  describe the polarity of a molecule
• dipole moments are VECTOR quantities
• they have both magnitude and direction
• Consider the molecules CO2 and H2O

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CO2 H2O
Vectors cancel (symmetric) dipole moment 0
Vectors do not cancel dipole moment 0
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Practice Problem

• Which of the following molecules would be
  expected to have a zero dipole moment on the
  basis of their geometry?
• GeF4
• SF2
• XeF2
• AsF3