Every molecule with two electron domains around its central atom is categorized into one group. Formally, this grouping is called its domain geometry and given a name: linear. This is helpful because finding the domain geometry of a molecule narrows down its possible shapes. In the case of linear domain geometry, all molecules are shaped like a straight line. Therefore, they all have linear molecular geometry.
Every molecule with three electron domains around its central atom has trigonal planar domain geometry. There are two possible shapes: trigonal planar and bent. To find out which it is, determine how many of the electron domains you counted around its central atom are made up of bonds and which--if any--of unbonded electron pairs.
If all three are made up of bonds, the molecular geometry is trigonal planar.
If two are made up of bonds and one is made up of an unbonded electron pair, the molecular geometry is bent.
The difference occurs because unbonded electron pairs exert a greater force of repulsion than bonded ones, and this strong force shifts the shape of the molecule.
Every molecule with four electron domains around its central atom has tetrahedral domain geometry and three possible shapes: tetrahedral, trigonal pyramidal, or bent.
If all four electron domains are made up of bonds, the molecular geometry is tetrahedral.
If three are made up of bonds and one of a free electron pair, the molecular geometry is trigonal pyramidal.
If two are made up of bonds and two of free electron pairs, the molecular geometry is bent.
Every molecule with five electron domains around its central atom has trigonal bipyramidal domain geometry and four possible shapes: trigonal bipyramidal, seesaw, T-shaped, or linear.
If all five electron domains are made up of bonds, the molecular geometry is trigonal bipyramidal.
If four are made up of bonds and one of a free electron pair, the molecular geometry is seesaw.
If three are made up of bonds and two of free electron pairs, the molecular geometry is T-shaped.
If two are made up of bonds and three of free electron pairs, the molecular geometry is linear.
Every molecule with six electron domains around its central atom has octahedral domain geometry and three possible shapes: octahedral, square pyramidal, or square planar.
If all six electron domains are made up of bonds, the molecular geometry is octahedral.
If five are made up of bonds and one of a free electron pair, the molecular geometry is square pyramidal.
If four are made up of bonds and two of free electron pairs, the molecular geometry is square planar.