Beginning Organic Chemistry (BOC)
1. Basic Knowledge
d. Molecular Shapes from Kekulé Structures.

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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
CO32-, carbonate anion
HNO2, nitrous acid
HCN, hydrogen cyanide
CH3CH2CH3, propane
CH3NH2, methanamine

Date created: 2005 06 08.