ValenceShell ElectronPair Repulsion Theory

1
Valence-Shell Electron-Pair Repulsion Theory
2
VSEPR Theory

• Gillespie and Nyholm developed model in 1957
• Predicts shape of molecules
• Fundamental principle the bonding pairs and lone
  pairs of electrons in the valance level of an
  atom repel one another due to their negative
  charges.

3
VSEPR Continued

• Pairs of electrons around the central atom try to
  be as far away from one another as possible while
  remaining in the molecule
• Lone pair electrons will spread out more than
  bonding pair electrons
• This means the repulsion is greatest between lone
  pairs
• Bonding pairs are more localized between nuclei
  so they spread out less than lone pairs
• The repulsion between lone pairs and bonding
  pairs is in between repulsions for lone to lone
  pairs, and bonding to bonding pairs
• Can be expressed in decreasing order of
  repulsion
• LP-LPgtLP-BPgtBP-BP

4
VSEPER Continued

• Double bonds(two bonding pairs), triple
  bonds(three bonding pairs) act as single electron
  group when repelling other groups of electrons
• You can determine the shape of the molecule
  around the atom by analyzing the types and number
  of electron pairs around the central atom
• We will be looking at five different shapes that
  atoms can form but there are many more that we
  will not discuss.

5
Linear Shape

• Two electron groupings around central atom
• Can be a combination of two double bonds or a
  single and triple bond
• Bond angle, which is the angle formed by two
  bonds is 180 in a linear shape
• Two dimensional
• Examples CO2

6
Trigonal Planar

• Three electron groupings around central atom
• Can be a combination of two single bonds and one
  double bond or three single bonds
• Bond angle is 120
• Two dimensional
• Examples CH2O

7
Tetrahedral

• Four electron groupings around central atom
• All bonding pairs has four single bonds
• Completely symmetrical
• Bond angles are each 109.5
• Three dimensional
• Example CH4

8
Pyramidal

• Four electron groupings around the central atom
• One lone pair of electrons and three single bonds
• Bond angle less than 109 (because the lone pair
  exerts a stronger force on the bonding pairs than
  they do on eachother
• Example NH3

9
Bent

• Four electron groupings around central atom
• Two lone pairs, two single bonds
• Bond angle is 104.5 (two lone pairs reduce two
  bonding pairs even more than pyramidal shape)
• Example H2O