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Chemistry LibreTexts

Molecular Geometry Overview

  • Page ID
    55230
  • The specific three dimensional arrangement of atoms in molecules is referred to as molecular geometry. We also define molecular geometry as the positions of the atomic nuclei in a molecule. There are various instrumental techniques such as X-Ray crystallography and other experimental techniques which can be used to tell us where the atoms are located in a molecule. Using advanced techniques, very complicated structures for proteins, enzymes, DNA, and RNA have been determined. Molecular geometry is associated with the chemistry of vision, smell and odors, taste, drug reactions and enzyme controlled reactions to name a few.

    Note: The AXE Method

    It is common practice to represent bonding patterns by "generic" formulas such as \(AX_4\), \(AX_2E_2\), etc., in which "X" stands for bonding pairs and "E" denotes lone pairs. This convention is known as the "AXE Method."

    Molecular geometry is associated with the specific orientation of bonding atoms. A careful analysis of electron distributions in orbitals will usually result in correct molecular geometry determinations. In addition, the simple writing of Lewis diagrams can also provide important clues for the determination of molecular geometry. Click on a picture to link to a page with the GIF file and a short discussion of the molecule.

    Steric Number (# bonded atoms + # electron pairs)
    6 5 4 3 2
    AX6
    ax6.gif
    octahedral
    AX5
    ax5.gif
    trigonal bipyramidal
    AX4
    ax4.gif
    tetrahedral
    AX3
    ax3.gif
    trigonal planar
    AX2
    nopic.gif
    linear
    1 lone pair of electrons
    AX5E
    ax5e.gif
    square pyramidal
    AX4E
    ax4e.gif
    distorted tetrahedron
    AX3E
    ax3e.gif
    pyramidal
    AX2E
    ax2e.gif
    nonlinear
    AXE
    nopic.gif
    linear
    2 lone pairs of electrons
    AX4E2
    ax4e2.gif
    square planar
    AX3E2
    ax3e2.gif
    T-shaped
    AX2E2
    ax2e2.gif
    bent
    AXE2
    nopic.gif
    linear
    3 lone pairs of electrons
    AX3E3
    nopic.gif
    T-shaped
    AX2E3
    ax2e3.gif
    linear
    AXE3
    nopic.gif
    linear
    4 lone pairs
    AX2E4
    nopic.gif
    linear
    AXE4
    nopic.gif
    linear
    5 lone pairs
    AXE5
    nopic.gif
    linear

    Valence Shell Electron Pair Repulsion (VSEPR) theory

    Electron pairs around a central atom arrange themselves so that they can be as far apart as possible from each other. The valence shell is the outermost electron-occupied shell of an atom that holds the electrons involved in bonding. In a covalent bond, a pair of electrons is shared between two atoms. In a polyatomic molecule, several atoms are bonded to a central atom using two or more electron pairs. The repulsion between negatively charged electron pairs in bonds or as lone pairs causes them to spread apart as much as possible.

    The idea of "electron pair repulsion can be demonstrated by tying several inflated balloons together at their necks. Each balloon represents an electron pair. The balloons will try to minimize the crowding and will spread as far apart as possible. According to VSEPR theory, molecular geometry can be predicted by starting with the electron pair geometry about the central atom and adding atoms to some or all of the electron pairs. This model produces good agreement with experimental determinations for simple molecules. With this model in mind, the molecular geometry can be determined in a systematic way.

    • Lewis diagrams provide information about what atoms are bonded to each other and the total electron pairs involved.
    • Electron pair geometry is determined from the total electron pairs.

    Molecules can then be divided into two groups:

    • Group 1: Molecules with NO lone electron pairs. In this case the molecular geometry is identical to the electron pair geometry.
    • Group 2: Molecules with one or more lone electron pairs. In this case an extra step is needed to to translate from electron pair geometry to the final molecular geometry, since only the positions of bonded atoms are considered in molecular geometry.

    Outside Links

    • Pfennig, Brian W.; Frock, Richard L. "The Use of Molecular Modeling and VSEPR Theory in the Undergraduate Curriculum to Predict the Three-Dimensional Structure of Molecules." J. Chem. Educ.1999 76 1018.
    • Covalent Radii - Wikipedia: http://en.wikipedia.org/wiki/Covalent_radius

    Contributors

    • Charles Ophardt, Professor Emeritus, Elmhurst College; Virtual Chembook
    • Robyn Rindge (Class of '98) who now works for PDI Dreamworks (look for his name in the credits of Shrek2.). Robyn drew these rotating molecules using Infini-D (MetaCreations).
    • Paul Groves, chemistry teacher at South Pasadena High School and Chemmy Bear