Physical Properties of Covalent Molecules



 

Learning Objective

  • Discuss the qualitative predictions of covalent bond theory on the boiling and melting points, bond length and strength, and conductivity of molecules

Key Points

    • The Lewis theory of covalent bonding says that the bond strength of double bonds is twice that of single bonds, which is not true.
    • General physical properties that can be explained by the covalent bonding model include boiling and melting points, electrical conductivity, bond strength, and bond length.

Terms

  • intermolecular forcesAttractive forces or interactions between different molecules in a sample of a substance. The strength of these interactions is an important factor that determines the substance’s physical properties.
  • bond lengthThe distance between the nuclei of two bonded atoms. It can be experimentally determined.
  • bond strengthDirectly related to the amount of energy required to break the bond between two atoms. The more energy required, the stronger the bond is said to be.
  • octet ruleAtoms lose, gain, or share electrons in order to have a full valence shell of eight electrons. Hydrogen is an exception because it can hold a maximum of two electrons in its valence level.

First described by Gilbert Lewis, a covalent bond occurs when electrons of different atoms are shared between the two atoms. These cases of electron sharing can be predicted by the octet rule. The octet rule is a chemical rule that generalizes that atoms of low atomic number (< 20) will combine in a way that results in their having 8 electrons in their valence shells. Having 8 valence electrons is favorable for stability and is similar to the electron configuration of the inert noble gases. In a covalent bond, the shared electrons contribute to each atom’s octet and thus enhance the stability of the compound.

The Lewis bonding theory can explain many properties of compounds. For example, the theory predicts the existence of diatomic molecules such as hydrogen, H2, and the halogens (F2, Cl2, Br2, I2). A H atom needs one additional electron to fill its valence level, and the halogens need one more electron to fill the octet in their valence levels. Lewis bonding theory states that these atoms will share their valence electrons, effectively allowing each atom to create its own octet.

Several physical properties of molecules/compounds are related to the presence of covalent bonds:

  • Covalent bonds between atoms are quite strong, but attractions between molecules/compounds, or intermolecular forces, can be relatively weak. Covalent compounds generally have low boiling and melting points, and are found in all three physical states at room temperature.
  • Covalent compounds do not conduct electricity; this is because covalent compounds do not have charged particles capable of transporting electrons.
  • Lewis theory also accounts for bond length; the stronger the bond and the more electrons shared, the shorter the bond length is.

However, the Lewis theory of covalent bonding does not account for some observations of compounds in nature. The theory predicts that with more shared electrons, the bond between the two atoms should be stronger. According to the theory, triple bonds are stronger than double bonds, and double bonds are stronger than single bonds. This is true. However, the theory implies that the bond strength of double bonds is twice that of single bonds, which is not true. Therefore, while the covalent bonding model accounts for many physical observations, it does have its limitations.