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Chemistry LibreTexts

4.7: The Shapes of Molecules

  • Page ID
    258859
  • Learning Objectives

    • Predict the general shape of a simple covalent molecule.

    Molecular Shape: VSEPR Theory

    Unlike ionic compounds, with their extended crystal lattices, covalent molecules are discrete units with specific three-dimensional shapes. The shape of a molecule is determined by the fact that covalent bonds, which are composed of negatively charged electrons, tend to repel one another. This concept is called the valence shell electron pair repulsion (VSEPR) theory. For example, the two covalent bonds in \(\ce{BeCl2}\) stay as far from each other as possible, ending up 180° apart from each other. The result is a linear molecule:

    imageedit_64_9647175464.jpg

    The three covalent bonds in BF3 repel each other to form 120° angles in a plane, in a shape called trigonal planar:

    15171678226139609.jpg

    The molecules \(\ce{BeCl2}\) and \(\ce{BF3}\) actually violate the octet rule; however, such exceptions are rare and will not be discussed in this text.

    Try sticking three toothpicks into a marshmallow or a gumdrop and see if you can find different positions where your “bonds” are farther apart than the planar 120° orientation.

    The four covalent bonds in CCl4 arrange themselves three dimensionally, pointing toward the corner of a tetrahedron and making bond angles of 109.5°. CCl4 is said to have a tetrahedral shape:

    imageedit_70_6538282096.jpg clipboard_e5a77c4564ef717fb1d79813cc4e265e6.png
     
    Atoms Around Central Atom Geometry Example
    2 \(\ce{AB_2}\) Linear \(\ce{BeCl_2}\)
    3 \(\ce{AB_3}\) Trigonal Planar \(\ce{BF_3}\)
    4 \(\ce{AB_4}\) Tetrahedral \(\ce{CCl_4}\)

    In determining the shapes of molecules, it is useful to first determine the Lewis diagram for a molecule. The shapes of molecules with multiple bonds are determined by treating the multiple bonds as one bond. Thus, formaldehyde (CH2O) has a shape similar to that of BF3. It is trigonal planar.

    imageedit_79_7227788746.jpg clipboard_e896c0741278e39c337d38544b5f9550e.png

    Molecules With Lone Pairs Around Central Atom

    Molecules with lone electron pairs around the central atom have a shape based on the position of the atoms, not the electron pairs. For example, NH3 has one lone electron pair and three bonded electron pairs. These four electron pairs repel each other and adopt a tetrahedral arrangement. However, the shape of the molecule is described in terms of the positions of the atoms, not the lone electron pairs. Thus, NH3 is said to have a trigonal pyramidal shape, not a tetrahedral one.

    imageedit_73_4726698555.jpg clipboard_ed79bd66cb66ca157536990b08a8ee10b.png

    Similarly, H2O has two lone pairs of electrons around the central oxygen atom and two bonded electron pairs. Although the four electron pairs adopt a tetrahedral arrangement, the shape of the molecule is described by the positions of the atoms only. The shape of H2O is bent with an approximate 109.5° angle.

    imageedit_76_3022246848.jpg clipboard_ec3bf18d92b3cf4bc660c7c69d89dda64.png

    In summary, to determine the molecular geometry:

    Step 1: Draw the Lewis structure.

    Step 2: Count the number of bonds (a double/triple bond counts as one) and lone pairs around the central atom.

    Step 3: Use Table 4.5.1 to determine the molecular geometry.

    clipboard_e348cff530a5e90c14191dcd5d8d522e3.png

    Table 4.5.1: The molecular geometry depends on the number of bonds and lone pairs around the central atom, A.

    Example \(\PageIndex{1}\)

    What is the geometry of the ammonium ion, NH4+? Its Lewis structure is shown below. How is this different from ammonia, NH3?

    clipboard_e6c86d2942b2a10989bb188cd4c396536.png

    Solution

    In ammonium ion, the central atom N has 4 bonds and no lone pair. It is equivalent to the below in Table 4.5.1. Hence, this is tetrahedral.

    clipboard_e0ffd4bb7ef4ad01e8289f7b6360aaeca.png

    In ammonia (NH3), shown below, N has 3 bonds and one lone pair.

    clipboard_e6495e34a49afe42ffacd48ed9d275806.png

    It is equivalent to the below in Table 4.5.1. Hence, the shape of this molecule is trigonal pyramid.

    clipboard_e887d09558d4f27e7ce1aba8cca00f195.png

    Exercise \(\PageIndex{1}\)

    What is the molecular shape of nitrosyl chloride, a highly corrosive, reddish-orange gas? Its Lewis structure is shown below.

    clipboard_e7d2ade87d477bdd8794c20512012676e.png

    Answer

    Focus on the central atom, N. It has a double bond to O, count this as one bond. It also has a single bond to Cl. Thus, N has 2 bonds and one lone pair. These 3 electron pairs will spread out 120 degrees from each other. But, since the shape is defined by the arrangement of the atoms only, the shape is bent or angular. If you consult Table 4.5.1, this molecule is equivalent to the below. Hence, two bonds and one lone pair has a bent or angular shape.

    clipboard_e275fe6c17864e78d32abc3fcacbf7846.png

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