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4.5: Characteristics of Molecular Compounds

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  • Learning Objectives

    • Compare the properties of ionic and molecular compounds.

    Physical Properties of Molecular Compounds

    The physical state and properties of a particular compound depend in large part on the type of chemical bonding it displays. Molecular compounds, sometimes called covalent compounds, display a wide range of physical properties due to the different types of intermolecular attractions such as different kinds of polar interactions. The melting and boiling points of molecular compounds are generally quite low compared to those of ionic compounds. This is because the energy required to disrupt the intermolecular forces (discussed further in Chapter 8) between molecules is far less than the energy required to break the ionic bonds in a crystalline ionic compound. Since molecular compounds are composed of neutral molecules, their electrical conductivity is generally quite poor, whether in the solid or liquid state. Ionic compounds do not conduct electricity in the solid state because of their rigid structure, but conduct well when either molten or dissolved into a solution. The water solubility of molecular compounds is variable and depends primarily on the type of intermolecular forces involved. Substances that exhibit hydrogen bonding or dipole-dipole forces are generally water soluble, whereas those that exhibit only London dispersion forces are generally insoluble. Most, but not all, ionic compounds are quite soluble in water. The table below summarizes some of the differences between ionic and molecular compounds.

    Property Ionic Compounds Molecular Compounds
    Type of elements Metal and nonmetal Nonmetals only
    Bonding Ionic - transfer of electron(s) between atoms Covalent - sharing of pair(s) of electrons between atoms
    Representative unit Formula unit Molecule
    Physical state at room temperature Solid Gas, liquid, or solid
    Water solubility Usually high Variable
    Melting and boiling temperatures Generally high Generally low
    Electrical conductivity Good when molten or in solution Poor

    Table 4.5.2: Comparison of Ionic and Molecular Compounds

    In summary, covalent compounds are softer, have lower boiling and melting points, are more flammable, are less soluble in water and do not conduct electricity compared to ionic compounds. The individual melting and boiling points, solubility and other physical properties of molecular compounds depend on molecular polarity.

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