Molecular Geometry

For more than a century, scientists have intensely studied the geometry of compounds. Swiss chemist Alfred Werner (1866–1919) won the Nobel Prize in 1913 for his pioneering work predicting the shapes of molecules. Since those early studies, scientists have developed rules and guidelines based upon physical laws that predict molecular shapes.

An understanding of molecular geometry can be key in determining chemical reactivity or in determining the functional relationships of biological molecules. Basic molcular forms are shown above. Identifying the presence of unshared pairs of electrons—and the space-occupying, bond-bending stronger concentration of negative electrical charge associated with those unshared pairs—is key to predicting the overall molecular structure. For example, water is a bent molecule because the oxygen atom has two unshared pairs of valence elections.The bent shape is important in determining the bonding relationships between water molecues that are critical in determing the physical characteristics (e.g., melting point, boiling point, surface tensions) that make water the universal solvent and make life on Earth possible. Illustration by K. Lee Lerner and Argosy. The Gale Group.

The rules and principles of molecular geometry accurately predict the shapes of simple molecules such as methane (CH₄), water (H₂0), or ammonia (NH₃). As molecules become increasingly complex, however, it becomes very difficult, but not impossible, to predict and describe complex geometric arrangements of atoms. The number of bonds between atoms, the types of bonds, and the presence of lone electron pairs on the central atom in the molecule critically influence the arrangement of atoms in a molecule. In addition, use of valance shell electron pair repulsion theory (VSEPR) allows chemists to predict the shape of a molecule.