Valence shell electron pair repulsion theory (VSEPR)

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Valence shell electron pair repulsion theory
(VSEPR)

1. 1. How many electron groups total are on the
   central atom?
2. 2. How many of the electron groups are bonding
   groups?
3. 3. What is the molecular geometry?

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VSEPR theory steps

• Draw valid electron dot structure
• Count the number of electron groups around
  central atom(multiple bonds count as one)
• Decide on overall electron arrangement including
  lone pairs

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VSEPR Valence Shell Electron Pair
Repulsion
X E Overall Structure Forms
2 Linear AX2
3 Trigonal Planar AX3, AX2E
4 Tetrahedral AX4, AX3E, AX2E2
5 Trigonal bipyramidal AX5, AX4E, AX3E2, AX2E3
6 Octahedral AX6, AX5E, AX4E2
A central atom
X atoms bonded to A
E nonbonding electron pairs on A
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VSEPR Linear
AX2
CO2
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VSEPR Trigonal Planar
AX3
BF3
AX2E
SnCl2
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VSEPR Tetrahedral
AX4
CCl4
AX3E
PCl3
AX2E2
Cl2O
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VSEPR Octahedral
AX6
SF6
AX5E
BrF5
AX4E2
ICl4-
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A Lone Pear
A Lone Pair
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..
O
H
..
104.5o
H
CH4, methane
NH3, ammonia
H2O, water
..
lone pair electrons
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To determine the angle

• Each lone pair on an atom compresses the
  remaining bond angles about that atom by 2o

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Molecular Shapes
AB2 Linear
AB3 Trigonal planar
AB2E Angular or Bent
AB4 Tetrahedral
AB3E Trigonal pyramidal
AB2E2 Angular or Bent
AB2E3 Linear
AB5 Trigonal bipyramidal
AB4E Irregular tetrahedral (see saw)
AB3E2 T-shaped
AB6E Square pyramidal
AB5E2 Square planar
AB6 Octahedral
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Predicting the Geometry of Molecules

• Lewis electron-pair approach predicts number and
  types of bonds between the atoms in a substance
  and indicates which atoms have lone pairs of
  electrons but gives no information about the
  actual arrangement of atoms in space
• Valence-shell electron-pair repulsion (VSEPR)
  model predicts the shapes of many molecules and
  polyatomic ions but provides no information about
  bond lengths or the presence of multiple bonds

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Polarity of Molecules

• Intermolecular attractive forces large net
  attractive forces among molecules
• Composition and shape of molecules helps to
  determine strength of force

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Polar and nonpolar

• Polar(dipolar) molecules have two electrical
  poles
• Nonpolar molecules equal sharing of electrons
• If electronegativity difference is 0.4 or more
  between central atom and attached atom polar
  molecule

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Polar and nonpolar

• Arrows are used to indicate polarity
• Arrowhead points to the negative end and it is
  crossed at the positive end, the longer the arrow
  the greater the dipole moment (more polar
  molecule is)

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Process to determine polarity

• 1.Determine the total number of valence electrons
• 2.Connect atoms using single bonds
• 3.Put in remaining dots as lone pairs to satisfy
  octets
• 4.If octets are not satisfied send lone pairs
• 5.Apply VSEPR to central atom
• 6.Draw all bond dipole moments(consider
  electronegativities)
• 7.Determine the net molecular dipole moment