Determining the Shapes of molecules: 

The shape of the molecule can be found using the VSEPR model:

e.g. CH3–

  1. Determine the central atom

C

  1. Count the number of valence electrons

C = +4

  1. Add one electron for each bonding atom

+4 + 3 (hydrogens) = 7

  1. Add or subtract electrons for charge (see Top Tip)

Negative ( – ) charge, therefore -1

7 -1 = 6

  1. Divide the total of these by 2 to find the total

number of electron pairs

6 /2 = 3

As CH3— has 3 bonding groups according to VESPR, and has no lone pairs, it is a Trigonal Planar.