Using VSEPR

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 θ bonds.
• Note the number of Π bonds.
• Note the charge of the central atom.
• Note the valence electrons.
• Find the number of lone pairs:
  • valence electrons – θ bonds – Π bonds – charge                                                                                                     2
• Electron Pairs: θ bonds + lone pairs

Example

CH₃^–                                                          CH₃⁺                                                               SO₄^2-                                 

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

Number of θ bonds: 3                 Number of θ bonds: 3                      Number of θ bonds: 4

Number of Π bonds: 0                Number of Π bonds: 0                      Number of Π bonds: 2

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 = (6–4–2)/2

Electron Pairs: 4                          Electron Pairs: 3                                  Electron Pairs: 4

Pyramidal                                     Trigonal planar                                  Tetrahedral