The VSEPR Approach.

Lewis structures can lead very quickly to approximate molecular shapes by applying the VSEPR approach of distributing the electron groups evenly round an atom. Here an electron group is either all the electrons bonding two atoms (2, 4, or 6), or a lone pair.

Molecular Shapes with No Lone Pairs.

For simple organic molecules there are only three electronic shapes to worry about:

With four (4) electron groups around an atom, the electronic shape is tetrahedral. methane, CH4, is an example:
With three (3) electron groups around an atom, the electronic shape is trigonal planar ethene, CH2=CH2, is an example
with two (2) electron groups around an atom, the electronic shape is linear. ethyne, C2H2, is an example

Molecular Shapes with Lone Pairs Present.

The actual shape of the molecule will depend on the presence or absence of lone pairs of electrons with the lone pair taking the place of one of the bonded atoms or groups of atoms.

1. Four groups around the atom; called either a tetrahedral centre or an sp3 centre.
With four bonds, no lone pairs, the molecule centre is tetrahedral. Example: methane, CH4
With three bonds, one lone pair, the molecule is pyramidal. Example: ammonia, NH3
With two bonds, two lone pairs, the molecule is angular (or bent). Example: water, H2O
2. Three groups around an atom; called either a trigonal planar center or an sp2 centre.
With three bonds, no lone pairs, the molecule is trigonal planar. Example: ethene, C2H4
With two bonds, one lone pair, the molecule is bent. Example: the nitrogen atom in acetone oxime, C3H7NO There are also 2 tetrahedral C atoms, 1 trigonal planar C atom, and a bent (sp3) O atom.
3. Two groups around an atom; called either a linear centre or an sp centre.
With two bonds, no lone pairs, the molecule is linear. Example: ethyne, C2H2

For further examples, here is an excellent site showing molecular models with their VSEPR geometry.

Self-study.

Draw a Kekulé structure for the each of following molecules, then predict their shape using VSEPR. Finally look at the model.(Use the "render2D" option on the chime right-button menu to see Kekulé structure of the molecule.)

H₂O₂, hydrogen peroxide

NH₄⁺, ammonium ion

NH₂^-, amide anion

NH₂NH₂, hydrazine

CH₃⁺, methyl cation

CH₃^-, methyl anion

CH₃CH₃, ethane: note how shape is assigned to each centre individually.

HCOOH, methanoic acid

HNO₃, nitric acid

H₂CO, methanal

CO₃^2-, carbonate anion

HNO₂, nitrous acid

HCN, hydrogen cyanide

CH₃CH₂CH₃, propane

CH₃NH₂, methanamine