The molecular geometry of a molecule can be determined using the
VSEPR theory.
VSEPR (Valence Shell Electron Pair Repulsion) Theory: The basic
premise of this simple theory is that electron pairs (bonding and
nonbonding) repel one another; so the electron pairs will adopt a
geometry about an atom that minimizes these repulsions. Use the
method below to determine the molecular geometry about an atom.
Write the Lewis dot structure for the molecule.
Count the number of things (atoms, groups of atoms, and lone
pairs of electrons) that are directly attached to the central atom
(the atom of interest) to determine the overall (electronic)
geometry of the molecule.
Now ignore the lone pairs of electrons to get the molecular
geometry of the molecule. The molecular geometry describes the
arrangement of the atoms only and not the lone pairs of electrons.
If there are no lone pairs in the molecule, then the overall
geometry and the molecular geometry are the same. If the overall
geometry is tetrahedral, then there are three possibilities for the
molecular geometry; if it is trigonal planar, there are two
possibilities; and if it is linear, the molecular geometry must
also be linear. The diagram below illustrates the relationship
between overall (electronic) and molecular geometries. To view the
geometry in greater detail, simply click on that geometry in the
graphic below. Although there are many, many different geometries
that molecules adopt, we are only concerned with the five shown
below.