{"id":513,"date":"2018-03-20T14:43:42","date_gmt":"2018-03-20T14:43:42","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-orgbiochemistry\/?post_type=chapter&p=513"},"modified":"2018-04-05T16:40:12","modified_gmt":"2018-04-05T16:40:12","slug":"4-5-characteristics-of-molecules","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-orgbiochemistry\/chapter\/4-5-characteristics-of-molecules\/","title":{"raw":"4.5 Characteristics of Molecules","rendered":"4.5 Characteristics of Molecules"},"content":{"raw":"
\r\n
\r\n
\r\n
\r\n

Learning Objectives<\/h3>\r\n
    \r\n \t
  1. Determine the molecular mass of a molecule.<\/li>\r\n \t
  2. Predict the general shape of a simple covalent molecule.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

    Unlike the ions in ionic compounds, which are arranged in lattices called crystals, molecules of covalent compounds exist as discrete units with a characteristic mass and a certain three-dimensional shape.<\/p>\r\n\r\n

    \r\n

    Molecular Mass<\/h3>\r\n

    The mass of a molecule\u2014the molecular mass\u00a0<\/span><\/span>(sometimes called the molecular weight)\u2014is simply the sum of the masses of its atoms. As with formula masses, it is important that you keep track of the number of atoms of each element in the molecular formula to obtain the correct molecular mass. (For more information about formula masses, see Chapter 3 \"Ionic Bonding and Simple Ionic Compounds\"<\/a>, Section 3.5 \"Formula Mass\".)<\/p>\r\n\r\n

    \r\n

    Example 7<\/h3>\r\n

    What is the molecular mass of each covalent compound?<\/p>\r\n\r\n

      \r\n \t
    1. H2<\/sub>O<\/li>\r\n \t
    2. C6<\/sub>H6<\/sub><\/li>\r\n \t
    3. NO2<\/sub><\/li>\r\n \t
    4. N2<\/sub>O5<\/sub><\/li>\r\n<\/ol>\r\n

      Solution<\/p>\r\n

      [reveal-answer q=\"820491\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"820491\"]Use the masses of the atoms from the periodic table in Chapter 21 \"Appendix: Periodic Table of the Elements\".<\/p>\r\n\r\n

        \r\n \t
      1. \r\n

        The molecular formula H2<\/sub>O indicates that there are two hydrogen atoms and one oxygen atom in each molecule. Summing the masses of these atoms,<\/p>\r\n\r\n

        \r\n\r\n\r\n\r\n\r\n\r\n
        2 H:<\/td>\r\n2 \u00d7 1.01 =<\/td>\r\n2.02 u<\/td>\r\n<\/tr>\r\n
        1 O:<\/td>\r\n<\/td>\r\n+ 16.00 u<\/span><\/td>\r\n<\/tr>\r\n
        Total:<\/td>\r\n<\/td>\r\n18.02 u<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

        The molecular mass of H2<\/sub>O is 18.02 u.<\/p>\r\n<\/li>\r\n \t

      2. \r\n
        \r\n\r\n\r\n\r\n\r\n\r\n
        6 C:<\/td>\r\n6 \u00d7 12.01 =<\/td>\r\n72.06 u<\/td>\r\n<\/tr>\r\n
        6 H:<\/td>\r\n6 \u00d7 1.01 =<\/td>\r\n+ 6.06 u<\/span><\/td>\r\n<\/tr>\r\n
        Total:<\/td>\r\n<\/td>\r\n78.12 u<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

        The molecular mass of C6<\/sub>H6<\/sub> is 78.12 u.<\/p>\r\n<\/li>\r\n \t

      3. \r\n
        \r\n\r\n\r\n\r\n\r\n\r\n
        1 N:<\/td>\r\n<\/td>\r\n14.01 u<\/td>\r\n<\/tr>\r\n
        2 O:<\/td>\r\n2 \u00d7 16.00 =<\/td>\r\n+ 32.00 u<\/span><\/td>\r\n<\/tr>\r\n
        Total:<\/td>\r\n<\/td>\r\n46.01 u<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

        The molecular mass of NO2<\/sub> is 46.01 u.<\/p>\r\n<\/li>\r\n \t

      4. \r\n
        \r\n\r\n\r\n\r\n\r\n\r\n
        2 N:<\/td>\r\n2 \u00d7 14.01 =<\/td>\r\n28.02 u<\/td>\r\n<\/tr>\r\n
        5 O:<\/td>\r\n5 \u00d7 16.00 =<\/td>\r\n+ 80.00 u<\/span><\/td>\r\n<\/tr>\r\n
        Total:<\/td>\r\n<\/td>\r\n108.02 u<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

        The molecular mass of N2<\/sub>O5<\/sub> is 108.02 u. (Note that the two different nitrogen and oxygen compounds in these examples have different molecular masses.)<\/p>\r\n

        [\/hidden-answer]<\/p>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n

        \r\n
        \r\n

        Skill-Building Exercise<\/h3>\r\n
        \r\n\r\nWhat is the molecular mass of each covalent compound?\r\n
          \r\n \t
        1. C2<\/sub>H2<\/sub><\/li>\r\n \t
        2. CO<\/li>\r\n \t
        3. CO2<\/sub><\/li>\r\n \t
        4. BF3<\/sub><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
          \r\n

          Molecular Shape: VSEPR Theory<\/h2>\r\n

          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)<\/span><\/span>\u00a0theory. For example, the two covalent bonds in BeCl2<\/sub> stay as far from each other as possible, ending up 180\u00b0 apart from each other. The result is a linear molecule:<\/p>\r\n\r\n

          \"image\"<\/div>\r\n

          The three covalent bonds in BF3<\/sub> repel each other to form 120\u00b0 angles in a plane, in a shape called trigonal planar<\/em>:<\/p>\r\n\r\n

          \"image\"<\/div>\r\n

          The molecules BeCl2<\/sub> and BF3<\/sub> actually violate the octet rule; however, such exceptions are rare and will not be discussed in this text.<\/p>\r\n\r\n

          \r\n
          \r\n

          Note<\/h3>\r\n

          Try sticking three toothpicks into a marshmallow or a gumdrop and see if you can find different positions where your \u201cbonds\u201d are farther apart than the planar 120\u00b0 orientation.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n

          The four covalent bonds in CCl4<\/sub> arrange themselves three dimensionally, pointing toward the corner of a tetrahedron and making bond angles of 109.5\u00b0:<\/p>\r\n\r\n

          \"image\"<\/div>\r\n

          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<\/sub> has one lone electron pair and three bonded electron pairs. These four electron pairs repel each other and adopt a tetrahedral arrangement:<\/p>\r\n\r\n

          \"image\"<\/div>\r\n

          However, the shape of the molecule is described in terms of the positions of the atoms, not the lone electron pairs. Thus, NH3<\/sub> is said to have a pyramidal shape, not a tetrahedral one. Similarly, H2<\/sub>O has two lone pairs of electrons around the central oxygen atom, in addition to the two bonded pairs:<\/p>\r\n\r\n

          \"image\"<\/div>\r\n

          Although the four electron pairs adopt a tetrahedral arrangement due to repulsion, the shape of the molecule is described by the positions of the atoms only. The shape of H2<\/sub>O is bent.<\/p>\r\n

          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, CH2<\/sub>O has a shape similar to that of BF3<\/sub>:<\/p>\r\n\r\n

          \"image\"<\/div>\r\n
          \r\n

          Example 8<\/h3>\r\n

          Describe the shape of each molecule.<\/p>\r\n\r\n

            \r\n \t
          1. PCl3<\/sub><\/li>\r\n \t
          2. CO2<\/sub><\/li>\r\n<\/ol>\r\n

            Solution<\/p>\r\n

            [reveal-answer q=\"24231\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"24231\"]<\/p>\r\n1. The Lewis diagram for PCl3 is as follows:\r\n\r\n\"image\"\r\n\r\nThe four electron pairs arrange themselves tetrahedrally, but the lone electron pair is not considered in describing the molecular shape. Like NH3<\/sub>, this molecule is pyramidal.\r\n\r\n2. The Lewis diagram for CO2<\/sub> is as follows:\r\n\r\n\"image\"\r\n

            The multiple bonds are treated as one group. Thus, CO2<\/sub> has only two groups of electrons that repel each other. They will direct themselves 180\u00b0 apart from each other, so CO2<\/sub> molecules are linear.\u00a0[\/hidden-answer]<\/p>\r\n\r\n<\/div>\r\n

            \r\n
            \r\n

            Skill-Building Exercise<\/h3>\r\nDescribe the shape of each molecule.\r\n
              \r\n \t
            1. \r\n
              \r\n

              CBr4<\/sub><\/p>\r\n\r\n<\/div><\/li>\r\n \t

            2. \r\n
              \r\n

              BCl3<\/sub><\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

              \r\n
              \r\n
              \r\n

              Concept Review Exercises<\/h3>\r\n
                \r\n \t
              1. \r\n
                \r\n

                How do you determine the molecular mass of a covalent compound?<\/p>\r\n\r\n<\/div><\/li>\r\n \t

              2. \r\n
                \r\n

                How do you determine the shape of a molecule?<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n

                \r\n
                \r\n

                [reveal-answer q=\"966683\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"966683\"]<\/p>\r\n\r\n

                  \r\n \t
                1. The molecular mass is the sum of the masses of the atoms in the formula.<\/li>\r\n \t
                2. The shape of a molecule is determined by the position of the atoms, which in turn is determined by the repulsion of the bonded and lone electron pairs around the central atom.[\/hidden-answer]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
                  \r\n
                  \r\n

                  Key Takeaways<\/h3>\r\n<\/div>\r\n
                  \r\n