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!
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.
|No pairs of bonding e–||No non- bonding pairs of e–||Name of shape||Arrangement in space||Bond Angles||Examples|
|3||2||1||Angular, V-shaped or Bent||120o||SO2|
|4||2||2||Angular, V-shaped or Bent||104.5o||H2O|