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6.3: Molecular shapes –Valence shell electron pair repulsion (VSEPR) theory

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    The Lewis structure tells the connection between atoms and any lone pair present, but it does not tell the exact angles of bonds around the central atom or the actual shape of the molecule. The conventional way of presenting a Lewis structure of a molecule shows it as a planer, e.g., CH4 as clipboard_ee2b18a861c8fdde551b1bfc7bf554b8b.png which implies the molecule to be planar and H-C-H bond angles to be either 90o or 180o. The actual CH4 molecule is nonplanar as Tetrahedral structure of CH4. with all H-C-H bond angles 109.5o. the following theory helps explain the actual shapes of molecules.

    Valence shell electron pair repulsion (VSERP) theory

    The valence shell electron pair repulsion theory predicts the shape of the molecule and bond angles based on the fact that valence shell electrons around the central atom in a molecule are grouped. The electron groups repel each other and go as far apart from each other as possible.

    Electron groups

    A lone pair, a single bond, a double bond, and a triple bond, each of these is one electron group. This is because two elections of single bond, four electrons of a double bond, and six electrons of a triple bond are located in the region along the axis of the bond, i.e., they are grouped together. Similarly, a lone pair is located in a defined space around the atom. For example, carbon in methane (clipboard_ee2b18a861c8fdde551b1bfc7bf554b8b.png) has four electron groups that are the four single bonds (C-H bonds) around the carbon. Carbon in carbon dioxide (O=C=O) has two electron groups that are the two double bonds around the carbon, and in H-C≡N has two electron groups that are a single bond (C-H bond) and a triple bond (C≡N bond).

    Molecular shapes and bond angles based on VSEPR theory

    One electron group

    One electron group between two atoms is always a linear molecule. For example, H-H ,O=O, N≡N, and H-Cl clipboard_e2345797732145448c37be656ebac3eea.png, are linear molecules, where hydrogen is white and chlorine is green in the H-Cl model.

    Two-electron groups

    Two-electron groups are farthest apart in a linear geometry with the central atom in the middle of the line and the bond angles of 180o around the central atom. The examples include CO2 clipboard_e587d5d66e4d325417072928a0406c912.png and HCN clipboard_e7b39dd5a7dd5975dde145c02662ccb9a.png, where the central carbon atom is gray, hydrogen is white, nitrogen is blue, and oxygen is red.

    Three-electron groups

    Three-electron groups are farthest apart when they are at the corners of a triangle in a planar trigonal geometry with the central atom in the middle of the triangle and the bond angles of 120o around the central atom. Examples include BF3 clipboard_efe9780eec0332f8b99c252f16dcc378d.png , and H2CO clipboard_ea2123bca03623bb44d97ee0220ab2e04.png, where boron is pink, F is green, carbon is gray, oxygen is red, and hydrogen is white.

    If one of the electron domains is a lone pair, the electron domain geometry remains the same, but the geometry of the atoms in the molecule, i.e., molecule geometry, is bent. For example, SO2 has three electron domains and trigonal planar electron domain geometry, but there is one lone pair. So, the molecule geometry is bent as clipboard_e43cbf1393c18f7ec92d23e3427795895.png, where sulfur is yellow, and oxygen is red (lone pair in not shown).

    Four-electron groups

    Four-electron groups are farthest apart when they are at the corners of a tetrahedron in a tetrahedral geometry with the central atom at the center of the tetrahedron and the bond angles of 109.5o around the central atom as: . An example is methane CH4 Tetrahedral structure of CH4., where carbon is gray, and hydrogens are white.

    If one of the electron domains is a lone pair, the electron domain geometry is still tetrahedral, but the molecule geometry is trigonal pyramidal asclipboard_e0bc10705b86d7dd0b7a9d4670ff79b45.png with three pereferal atoms at the corners of the triangel and the central atom raised to the top of the pyramid. An example is ammonia (:NH3) clipboard_e279a05d0785cb1c2a0b2c4c0be750b03.png, where nitrogen is blue, and hydrogens are white.

    If two electron domains are lone pairs, the electron domains geometry is still tetrahedral, but the molecule geometry is bent. An example is water (Lewis structure of H2O. Bent structure of H2O.), where oxygen is red, and hydrogens are white.

    Table 1 is the summary of the electron domain geometries and the corresponding molecular geometries.

    Table 1: Common molecular geometries around the central atom
    Electron domain Lone pair Electron domain geometry Molecule geometry Bond angles Examples
    1 0 Linear Linear - HCl clipboard_e2345797732145448c37be656ebac3eea.png
    2 0 Linear Linear 180o

    CO2 clipboard_e587d5d66e4d325417072928a0406c912.png

    HCN clipboard_e7b39dd5a7dd5975dde145c02662ccb9a.png

    3 0 Trigonal pyramidal Trigonal pyramidal 120o H2CO clipboard_ea2123bca03623bb44d97ee0220ab2e04.png
    3 2 Trigonal pyramidal Bent 120o clipboard_e3320bb339e861ee156edb300c9858db0.png clipboard_e43cbf1393c18f7ec92d23e3427795895.png
    4 0 Tetrahedral Tetrahedral 109.5o CH4 clipboard_e18105a9ef61a135729d8ec123a095273.png
    4 1 Tetrahedral Trigonal pyramidal 109.5o :NH3 clipboard_e279a05d0785cb1c2a0b2c4c0be750b03.png
    4 2 Tetrahedral Bent 109.5o clipboard_ebc0895f0b82d0e99a60b6dcb456adce4.png clipboard_e9fa6fd03d1694bd0b2fdafb424403d31.png

    This page titled 6.3: Molecular shapes –Valence shell electron pair repulsion (VSEPR) theory is shared under a Public Domain license and was authored, remixed, and/or curated by Muhammad Arif Malik.

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