Shapes of Molecules

vsepr_geometries1

The shape of a molecule depends on the number of pairs of electrons in the outer most shell surrounding a central atom. Once you find out how many bonding pairs and non-bonding pairs there are the rest is easy

Quick note about bonding pairs!
Ignore double bonds, they count as one bonding pair!

i.e. CO2 for this purpose has just two bonding pairs (although we know that in reality it has 4, 2 pairs between each oxygen and the hydrogen)

i.e. CO2 for this purpose has just two bonding pairs (although we know that in reality it has 4, 2 pairs between each oxygen and the hydrogen)

The shapes molecules form is all to do with Valence shell electron pair repulsion theory (VSEPR). Which sounds complicated but the general rule is that all the pairs will repel each other so that all electron pairs will spread out in all three dimensions so as to get as far away as possible. The shapes that are formed by this spreading out are very important and can influence the chemistry of the molecule a huge amount.

Shapes of molecules using VSEPR
No pairs
of e
No pairs of bonding e No non- bonding pairs of e Name of shape Arrangement in space Bond Angles Examples
2 2 0 Linear Linear 180o CO2
3 3 0 Trigonal Planar Trigonal 120o BF3
3 2 1 Angular, V-shaped or Bent Bent 120o SO2
4 4 0 Tetrahedral Tetrahedral 109.5o CH4
4 3 1 Trigonal Pyramidal Pyramidal 107o NH3
4 2 2 Angular, V-shaped or Bent Bent 104.5o H2O
6 6 0 Octahedral  Octahedral 90o SF6