How can I determine molecular geometry from the Lewis structure?

Key: Single bonds are bonds of 1 pair of electrons Double bonds are bonds of 2 pairs of electrons

The Lewis structure molecules have five geometries: - Tetrahedral: The central atom forms 4 single covalent bonds with 4 atoms (such as ##CH_4##). - Pyramidal: The central atom forms 3 single covalent bonds with 3 atoms and 1 lone pair of electrons (such as ##NH_3##). - Bent: The central atom forms 2 double covalent bonds with 2 atoms and 2 lone pairs of electrons (such as ##H_2O##). - Linear: The central atom forms 2 double covalent bonds with 2 atoms (such as ##CS_2##). - Triagonal Planer: The central atom forms 3 single covalent bonds with 3 atoms.

Now to answer your question: each Lewis structure has a specific angle between it's bonds. That is: The bonds in the tetrahedral form an angle of 109.5 degrees. The bonds in the pyramidal form an angle of 107.5 degrees. The bonds in the Bent form an angle of 104.5 degrees. CAREFUL: Sometimes, there are exceptions where bent structures have only 1 pair. In that case, they form an angle less that 120 degrees (not a constant angle for all molecules). The bonds in the linear form an angle of 180 degrees. The bonds in the triagonal planer form an angle of 120 degrees.

Here's a diagram for more clarification: