Shapes of molecules and ions

2.7.1 Valence shell electron pair repulsion (VSEPR) theory

VSEPR

Valence Shell Electron Pair Repulsion Theory is a way how to draw the shape of hybridised molecules. The number of bonds and lone pairs are counted and the shapes depend on the total number of electron pairs in the molecule.

The VSEPR predicted shapes of molecules can be found in a systematic way by using the number of electron pairs to determine the shape of the molecules.

To predict the shape of the molecules, first, draw out the Lewis structure of the molecule.

In order to be able to draw the correct shape always do the following steps, in order.

  • Identify the central atom.
  • Note the number of valence electrons.
  • Note the number of bonding electrons.
  • Note the charge of the central atom.
  • Find the number of lone pairs:
    • \frac{{\color{Blue} valence\: electrons} - {\color{Purple} bonding \: electrons} - {\color{Green} charge}}{2}
  • Electron Pairs: number of bonded atoms + lone pairs

Example

CH3–                                                          CH3+                                                               SO42-                                 

Central Atom: C                            Central Atom: C                                 Central Atom: S

Number of atoms: 3                    Number of atoms: 3                          Number of atoms: 4

Number of bonded e-: 3              Number of bonded e-: 3                   Number of bonded e-: 6

Charge on central atom: -1        Charge on central atom: +1          Charge on central atom: 0

Valence electrons: 4                    Valence electrons: 4                          Valence electrons: 6

Lone Pairs = (4–3–{-1})/2              Lone Pairs = (4–3–1)/2                    Lone Pairs = (6–6)/2

Steric Number: 4                          Steric Number: 3                              Steric Number: 4

Pyramidal                                      Trigonal planar                                Tetrahedral

VSEPR