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Chemistry LibreTexts

9.1: Molecular Shapes

  • Page ID
    21751
  • The Lewis electron-pair approach described previously can be used to predict the number and types of bonds between the atoms in a substance, and it indicates which atoms have lone pairs of electrons. This approach gives no information about the actual arrangement of atoms in space, however.

    Molecular Geometry

    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, taste, drug reactions, and enzyme controlled reactions to name a few.

    Example \(\PageIndex{1}\): Carbon Tetrachloride

    The Lewis structure of carbon tetrachloride provides information about connectivities, provides information about valence orbitals, and provides information about bond character.

    imageedit_2_4813302658.gif

    However, the Lewis structure provides no information about the shape of the molecule, which is defined by the bond angles and the bond lengths. For carbon tetrachloride, each C-Cl bond length is 1.78Å and each Cl-C-Cl bond angle is 109.5°. Hence, carbon tetrachloride is tetrahedral in structure:

    imageedit_5_4545398637.png

    Molecular Geometries of \(AB_n\) molecules

    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. Molecular shapes, or geometries, are critical to molecular recognition and function. Table \(\PageIndex{1}\) shows some examples of geometries where a central atom \(A\) is bonded to two or more \(X\) atoms. As indicated in several of the geometries below, non-bonding electrons \(E\) can strongly influence the molecular geometry of the molecule; this is discussed in more details in Section 9.2.

    Table \(\PageIndex{1}\):
    6 5 4 3 2
    AX6
    ax6.gif
    octahedral
    AX5
    ax5.gif
    trigonal bipyramidal
    AX4
    ax4.gif 
    tetrahedral
    AX3
      ax3.gif
    trigonal planar
    AX2
    linear_label.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
    linear_label.gif
    linear
    2 lone pairs of electrons
    AX4E2
    ax4e2.gif
    square planar
    AX3E2
    ax3e2.gif
    T-shaped
    AX2E2
    ax2e2.gif
    bent
    AXE2
    linear_label.gif
    linear
     

    These structures can generally be predicted, when A is a nonmetal, using the "valence-shell electron-pair repulsion model (VSEPR) discussed in the next section.

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