Properties of Covalent Substances (SQA National 5 Chemistry): Revision Note

Covalent molecular substances

• Covalent substances are made of non-metal atoms

• They can exist in one of two structures:

  • Covalent molecular

  • Covalent network

• A covalent molecular structure is a small, distinct group of atoms joined by strong covalent bonds

  • Examples include:

  • Water, H₂O

  • Carbon dioxide, CO₂

  • Ammonia, NH₃

• It is crucial to understand the two different types of forces in a covalent molecular substance:

  • Strong covalent bonds exist within each molecule

    • These hold the atoms together

  • Weak forces of attraction exist between the separate molecules:

    • These are called intermolecular forces
      

Properties of covalent molecular substances

Low melting and boiling points

• At room temperature, covalent molecular substances are typically:

  • Gases

  • Liquids

  • Soft solids

• This indicates they have low melting and boiling points

• When a covalent molecular substance melts or boils, energy is used to overcome the weak forces of attraction between the molecules

  • The strong covalent bonds inside the molecules do not break

• Since little energy is needed to overcome these weak forces, the melting and boiling points are low

Electrical conductivity

• Covalent molecular substances do not conduct electricity in any state

  • For a substance to conduct electricity, it must contain charged particles that are free to move

• Covalent molecules are neutral

  • Their electrons are held in fixed bonds

  • So, there are no free-moving charged particles available to carry a current

Covalent substances as insulators

Solubility

• Most covalent molecular substances do not dissolve in water

• They may, however, dissolve in other covalent solvents (such as hexane)

Covalent network structures

• Some covalent substances do not form small, separate molecules

• Instead, they form a covalent network structure

  • This is a giant three-dimensional (3D) lattice where all the atoms are held together by a vast network of strong covalent bonds

• There are no weak intermolecular forces in these structures; only strong covalent bonds exist throughout.

• The main examples you need to know are

  • Graphite

  • Diamond

  • Silicon dioxide

Graphite, diamond & silicon dioxide

Properties of covalent network structures

• The properties of covalent network substances are a direct result of their very strong and rigid structure

High melting and boiling points

• Covalent network structures are very hard, strong solids at room temperature.

• They have extremely high melting and boiling points

• To melt or boil a covalent network, a very large amount of energy is needed to break the strong covalent bonds throughout the giant structure

• This is very different from molecular substances, where only weak forces are overcome

Electrical conductivity

• In general, covalent network substances do not conduct electricity

• In most covalent network structures, like diamond and silicon dioxide, all the outer electrons are held tightly in fixed bonds between atoms

  • So, there are no free-moving charged particles available to carry a current

Solubility

• Covalent network structures are insoluble

  • They do not dissolve in any solvents

• The strong covalent bonds holding the atoms together in the network are too strong to be broken by the solvent particles