Determining the Shapes of molecules:
The shape of the molecule can be found using the VSEPR model:
e.g. CH3–
- Determine the central atom
C
- Count the number of valence electrons
C = +4
- Add one electron for each bonding atom
+4 + 3 (hydrogens) = 7
- Add or subtract electrons for charge (see Top Tip)
Negative ( – ) charge, therefore -1
7 -1 = 6
- 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.