vsepr

• VSEPR Theory

  Predicts the molecular shape of a bonded molecule by arranging electrons around the central atom as far apart from each other as possible
• Molecules with both paired and unshared (lone) pairs of electrons around the central atom

  • Bent
  • Trigonal Pyramidal
• Unshared pairs of electrons (lone pairs)

  Repel the most
• Trigonal Planar


  • 3 atoms attached to center atom
  • 0 lone pairs
  • Bond angle = 120o
  • Type: AB3
  • Ex. : AlF3
• Linear

  • 2 atoms attached to center atom
  • 0 unshared pairs (lone pairs)
  • Bond angle = 180o
  • Type: AB2
  • Ex. : BeF2
• Trigonal Pyramidal


  • 3 atoms attached to center atom
  • 1 lone pair
  • Bond angle = 107o
  • Type: AB3E
  • Ex. : NH3
• Molecules with only paired electrons around the central atom

  • Linear
  • Trigonal Planar
  • Tetrahedral
  • Trigonal Bipyramidal
  • Octahedral
• Trigonal Bipyramidal

  • 5 atoms attached to center atom
  • 0 lone pairs
  • Bond angle = equatorial -> 120o, axial -> 90o
  • Type: AB5
  • Ex. : PF5
• Only look at what is connected to the central atom
• Tetrahedral

  • 4 atoms attached to center atom
  • 0 lone pairs
  • Bond angle = 109.5o
  • Type: AB4
  • Ex. : CH4
• Octahedral

  • 6 atoms attached to center atom
  • 0 lone pairs
  • Bond angle = 90o
  • Type: AB6
  • Ex. : SF6
• Bent


  • 2 atoms attached to center atom
  • 2 lone pairs
  • Bond angle = 104.5o
  • Type: AB2E2
  • Ex. : H2O
• Vespa theory stands for valence shell electron pair repulsion and predicts the shape of a molecule based on the fact that electrons repel each other
• Trigonal pyramidal structure


  Atom at the center with a lone pair, surrounded by three other atoms, forming a pyramid-like shape
• Linear molecular geometry

  Molecules where you have an atom at the center and two other atoms on the sides, forming a straight line with a bond angle of 180 degrees
• Trigonal planar structure


  Atom at the center surrounded by three things, with a bond angle of approximately 120 degrees
• Tetrahedral molecular structure


  Atom surrounded by four other atoms, with a bond angle of about 109.5 degrees in a three-dimensional arrangement
• Linear molecular geometry

  • BeCl2
  • Carbon dioxide
  • Triiodide ion
• Trigonal pyramidal structure

  • NH3
  • PH3
• Tetrahedral molecular structure

  • Methane
  • Silicon tetrafluoride
• Trigonal planar structure

  • BH3
  • COCl2
• Elements with trigonal planar structure tend to be in group 3a and those with trigonal pyramidal structure tend to be in group 5a
• Bond angle for ammonia


  Approximately 107 degrees
• Elements with trigonal planar structure

  Tend to be in group five like NH3, PH3, AsH3 when hydrogen is the only other atom attached
• Bent molecular geometry


  Shape where lone pairs cause the atoms to be bent with respect to each other
• Tetrahedral structure


  Shape where there are four things or four atoms attached to the center atom
• Bond angle for water

  104.5 degrees
• Comparison of bond angles

  Water is somewhat similar to a tetrahedral structure with a bond angle of 104.5 degrees
• For trigonal pyramidal structures, if there are no double bonds involved, the elements will be in group 3a and 5a
• Bond angle for ammonia is a common test question
• Elements with trigonal planar structure

  Tend to be in group three like BH3, AlCl3
• Trigonal pyramidal structure

  • Ammonia (NH3)
  • PH3
  • AsH3
• Bond angle for sulfur dioxide


  Less than 120 degrees
• Bond angle for tetrahedral structure is 109.5 degrees
• Bent molecular geometry


  • Water (H2O)
  • Sulfur dioxide (SO2)
• Comparison of bond angles

  SO2 has a bond angle close to 120 degrees
• Comparison of bond angles

  BH3 has a bond angle of 120 degrees
• Comparison of bond angles

  Ammonia is somewhat similar to a tetrahedral structure with a bond angle of 107 degrees