Molecular Geometry Definition in Chemistry

In chemistry, molecular geometry describes the three-dimensional shape of a molecule and the relative position of the atomic nuclei of a molecule. Understanding the molecular geometry of a molecule is important because the spatial relationship between atom determines its reactivity, color, biological activity, state of matter, polarity, and other properties.

Molecule Shapes

Molecular geometry may be described according to the bond angles formed between two adjacent bonds. Common shapes of simple molecules include:

Linear: Linear molecules have the shape of a straight line. The bond angles in the molecule are 180°. Carbon dioxide (CO₂) and nitric oxide (NO) are linear.

Angular: Angular, bent, or v-shaped molecules contain bond angles less than 180°. A good example is water (H₂O).

Trigonal Planar: Trigonal planar molecules form a roughly triangular shape in one plane. The bond angles are 120°. An example is boron trifluoride (BF₃).

Tetrahedral: A tetrahedral shape is a four-faced solid shape. This shape occurs when one central atoms has four bonds. The bond angles are 109.47°. An example of a molecule with a tetrahedral shape is methane (CH₄).

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Molecular Geometry Introduction

By Anne Marie Helmenstine, Ph.D.

Octahedral: An octahedral shape has eight faces and bond angles of 90°. An example of an octahedral molecule is sulfur hexafluoride (SF₆).

Trigonal Pyramidal: This molecule shape resembles a pyramid with a triangular base. While linear and trigonal shapes are planar, the trigonal pyramidal shape is three-dimensional. An example molecule is ammonia (NH₃).

Methods of Representing Molecular Geometry

It's usually not practical to form three-dimensional models of molecules, particularly if they are large and complex. Most of the time, the geometry of molecules is represented in two dimensions, as on a drawing on a sheet of paper or a rotating model on a computer screen.

Some common representations include:

Line or stick model: In this type of model, only sticks or lines to represent chemical bonds are depicted. The colors of the ends of the sticks indicate the identity of the atoms, but individual atomic nuclei are not shown.

Ball and stick model: This is common type of model in which atoms are shown as balls or spheres and chemical bonds are sticks or lines that connect the atoms. Often, the atoms are colored to indicate their identity.

Electron density plot: Here, neither the atoms nor the bonds are indicated directly. The plot is a map of the probability of finding an electron. This type of representation outlines the shape of a molecule.

Cartoon: Cartoons are used for large, complex molecules that may have multiple subunits, like proteins. These drawings show the location of alpha helices, beta sheets, and loops. Individual atoms and chemical bonds are not indicated. The backbone of the molecule is depicted as a ribbon.

Isomers

Two molecules may have the same chemical formula, but display different geometries. These molecules are isomers. Isomers may share common properties, but it's common for them to have different melting and boiling points, different biological activities, and even different colors or odors.

How Is Molecular Geometry Determined?

The three-dimensional shape of a molecule may be predicted based on the types of chemical bonds it forms with neighboring atoms. Predictions are largely based on electronegativity differences between atoms and their oxidation states.

Empirical verification of predictions comes from diffraction and spectroscopy. X-ray crystallography, electron diffraction, and neutron diffraction may be used to assess the electron density within a molecule and the distances between atomic nuclei. Raman, IR, and microwave spectroscopy offer data about the vibrational and rotational absorbance of chemical bonds.

The molecular geometry of a molecule may change depending on its phase of matter because this affects the relationship between atoms in molecules and their relationship to other molecules. Similarly, the molecular geometry of a molecule in solution may be different from its shape as a gas or solid. Ideally, molecular geometry is assessed when a molecule is at a low temperature.

Sources

• Chremos, Alexandros; Douglas, Jack F. (2015). "When does a branched polymer become a particle?". J. Chem. Phys. 143: 111104. doi:10.1063/1.4931483
• Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th ed.). New York: Wiley-Interscience. ISBN 0-471-19957-5.
• McMurry, John E. (1992). Organic Chemistry (3rd ed.). Belmont: Wadsworth. ISBN 0-534-16218-5.