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7.1: Comparing Ionic and Molecular Substances

  • Page ID
    221371
  • Learning Objectives

    • Know the physical properties of ionic and molecular substances.

    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 between molecules is far less than the energy required to break the ionic bonds in a crystalline ionic compound (Figure \(\PageIndex{1}\)) . Ionic solids typically melt at high temperatures and boil at even higher temperatures. For example, sodium chloride melts at 801 °C and boils at 1413 °C. (As a comparison, the molecular compound water melts at 0 °C and boils at 100 °C.). The water solubility of molecular compounds is variable and depends primarily on the type of intermolecular forces involved.

    Figure \(\PageIndex{1}\) Interactions in Ionic and Covalent Solids.

    (a) The positively and negatively charged ions in an ionic solid such as

    sodium chloride (NaCl) are held together by strong electrostatic interactions.

    (b) In this representation of the packing of methane (CH4) molecules in solid methane,

    a prototypical molecular solid, the methane molecules are held together

    in the solid only by relatively weak intermolecular forces, even though

    the atoms within each methane molecule are held together by strong covalent bonds.

    Since molecular compounds are composed of neutral molecules, their electrical conductivity is generally quite poor, whether in the solid or liquid state. In solid form, an ionic compound is not electrically conductive because its ions are unable to flow (“electricity” is the flow of charged particles). When molten, however, it can conduct electricity because its ions are able to move freely through the liquid (Figure \(\PageIndex{2}\); Video \(\PageIndex{1}\)).

    This figure shows three photos connected by right-facing arrows. The first shows a light bulb as part of a complex lab equipment setup. The light bulb is not lit. The second photo shows a substances being heated or set on fire. The third shows the light bulb again which is lit.
    Figure \(\PageIndex{2}\) Sodium chloride melts at 801 °C and conducts electricity when molten. (credit: modification of work by Mark Blaser and Matt Evans)

    Conductivity of Molten Salt

    Video \(\PageIndex{1}\) Watch this video to see a mixture of salts melt and conduct electricity.

    The table below summarizes some of the differences between ionic and molecular compounds.

    Table \(\PageIndex{1}\) Comparison of 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

    One type of molecular compound behaves quite differently than that described so far. A covalent network solid is a compound in which all of the atoms are connected to one another by covalent bonds. Diamond is composed entirely of carbon atoms, each bonded to four other carbon atoms in a tetrahedral geometry. Melting a covalent network solid is not accomplished by overcoming the relatively weak intermolecular forces. Rather, all of the covalent bonds must be broken, a process that requires extremely high temperatures. Diamond, in fact, does not melt at all. Instead, it vaporizes to a gas at temperatures above \(3500^\text{o} \text{C}\).

    Summary

    • The physical properties of a material are affected by the intermolecular forces holding the molecules together.
    • Ionic compounds usually form hard crystalline solids with high melting points. Covalent molecular compounds, in contrast, consist of discrete molecules held together by weak intermolecular forces and can be gases, liquids, or solids at room temperature and pressure.
    • Ionic compounds in molten form or in solution can conduct electricity while molecular compounds do not..

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