Molecular Geometry Introduction

Three-Dimensional Arrangement of Atoms in a Molecule

Most molecular model sets include the proper bond angles for atoms so you can see the molecular geometry of molecules when you make them.
Most molecular model sets include the proper bond angles for atoms so you can see the molecular geometry of molecules when you make them. Grzegorz Tomasiuk / EyeEm / Getty Images

Molecular geometry or molecular structure is the three-dimensional arrangement of atoms within a molecule. It is important to be able to predict and understand the molecular structure of a molecule because many of the properties of a substance are determined by its geometry. Examples of these properties include polarity, magnetism, phase, color, and chemical reactivity. Molecular geometry may also be used to predict biological activity, to design drugs or decipher the function of a molecule.

The Valence Shell, Bonding Pairs, and VSEPR Model

The three-dimensional structure of a molecule is determined by its valence electrons, not its nucleus or the other electrons in the atoms. The outermost electrons of an atom are its valence electrons. The valence electrons are the electrons that are most often involved in forming bonds and making molecules.

Pairs of electrons are shared between atoms in a molecule and hold the atoms together. These pairs are called "bonding pairs".

One way to predict the way electrons within atoms will repel each other is to apply the VSEPR (valence-shell electron-pair repulsion) model. VSEPR can be used to determine a molecule's general geometry.

Predicting Molecular Geometry

Here is a chart that describes the usual geometry for molecules based on their bonding behavior. To use this key, first draw out the Lewis structure for a molecule. Count how many electron pairs are present, including both bonding pairs and lone pairs.

Treat both double and triple bonds as if they were single electron pairs. A is used to represent the central atom. B indicates atoms surrounding A. E indicates the number of lone electron pairs. Bond angles are predicted in the following order:

lone pair versus lone pair repulsion > lone pair versus bonding pair repulsion > bonding pair versus bonding pair repulsion

Molecular Geometry Example

There are two electron pairs around the central atom in a molecule with linear molecular geometry, 2 bonding electron pairs and 0 lone pairs. The ideal bond angle is 180°.

GeometryType# of Electron PairsIdeal Bond AngleExamples
linearAB22180°BeCl2
trigonal planarAB33120°BF3
tetrahedralAB44109.5°CH4
trigonal bipyramidalAB5590°, 120°PCl5
octohedralAB6690°SF6
bentAB2E3120° (119°)SO2
trigonal pyramidalAB3E4109.5° (107.5°)NH3
bentAB2E24109.5° (104.5°)H2O
seesawAB4E5180°,120° (173.1°,101.6°)SF4
T-shapeAB3E2590°,180° (87.5°,<180°)ClF3
linearAB2E35180°XeF2
square pyramidalAB5E690° (84.8°)BrF5
square planarAB4E2690°XeF4

 

Isomers in Molecular Geometry

Molecules with the same chemical formula may have atoms arranged differently. The molecules are called isomers. Isomers may have very different properties from each other. There are different types of isomers:

  • Constitutional or structural isomers have the same formulas, but the atoms are not connected to each other the same water.
  • Stereoisomers have the same formulas, with the atoms bonded in the same order, but groups of atoms rotate around a bond differently to yield chirality or handedness. Stereoisomers polarize light differently from each other. In biochemistry, they tend to display different biological activity.

    Experimental Determination of Molecular Geometry

    You can use Lewis structures to predict molecular geometry, but it's best to verify these predictions experimentally. Several analytical methods can be used to image molecules and learn about their vibrational and rotational absorbance. Examples include x-ray crystallography, neutron diffraction, infrared (IR) spectroscopy, Raman spectroscopy, electron diffraction, and microwave spectroscopy. The best determination of a structure is made at low temperature because increasing the temperature gives the molecules more energy, which can lead to conformation changes. The molecular geometry of a substance may be different depending on whether the sample is a solid, liquid, gas, or part of a solution.

    Molecular Geometry Key Takeaways

    • Molecular geometry describes the three-dimensional arrangement of atoms in a molecule.
    • Data that may be obtained from a molecule's geometry includes the relative position of each atom, bond lengths, bond angles, and torsional angles.
    • Predicting a molecule's geometry makes it possible to predict its reactivity, color, phase of matter, polarity, biological activity, and magnetism.
    • Molecular geometry may be predicted using VSEPR and Lewis structures and verified using spectroscopy and diffraction.

    References

    • 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.
    • Miessler G.L. and Tarr D.A. Inorganic Chemistry (2nd ed., Prentice-Hall 1999), pp. 57-58.