Aids to chemistry: Structure and bonding II
Francine Taylor Campbell, Contributor

Students from Wolmer's Boys and Wolmer's Girls. - Carlington Wilmot Photo

POINTS TO NOTE

* Metals are able to conduct electricity and heat due to the presence of mobile electrons.

* Solids can be divided into the following groups: ionic crystals, molecular (simple and giant) and metallic crystals. The differences in the properties of these compounds can be explained by the type of bonds they possess.

* Ionic compounds are crystalline solids able to conduct electricity when molten due the movement of ions, which can carry an electric current. These compounds also have high melting and boiling temperatures due to the strength of the attractive forces between the ions.

* Simple molecular crystals have low melting and boiling temperatures due to weak forces of attraction between molecules.

* Giant structures of atoms and molecules have very high melting and boiling points due to strong covalent bonds throughout their three-dimensional network.

The arrangement of atoms and ions in a crystal helps to determine the physical properties of thermal and electrical conductivity, melting and boiling points, physical state at room temperature and solubility in solvents.

In metallic crystals, the outer electrons of each atom are mobile or delocalised (that is they do not belong to any particular cation) and come together to form a 'band or sea' of electrons. These electrons will bind to the cations formed from the electron loss forming a strong bond. In this way metals are able to conduct heat and electricity since the mobile electrons can move throughout the metal. The strong bonds between the cations and electrons mean that they are hard to break thus metals have high melting and boiling points. Metals are also solids (except mercury) and are malleable and ductile. The bonding in metals can be represented by the diagram below.

+ e + e + e + e The mobile electrons form a cloud or band surrounding the cations. The difference in charges holds them together into a strong bond.

In an ionic crystal the attraction between cations and anions holds the crystal together into a regular three-dimensional framework. Each cation is surrounded by anions and vice versa. These crystals are solids at room temperature and are unable to conduct electricity in this state.

However, imagine what happens when these ionic crystals are heated; the ions gain more energy to move but, because they are oppositely charged, it requires vast amounts of energy to break this force of attraction and so these crystals have high melting and boiling points.

Note: Ionic solids can only conduct electricity when molten, as only then are the ions free enough to move. Examples of ionic solids are sodium chloride, magnesium oxide and potassium iodide.

In giant molecular crystals such as graphite, diamond and silicon dioxide, strong covalent bonds exist between the atoms, which make them difficult to melt or boil. On the other hand, simple molecular crystals have covalent bonds within molecules but weak bonds between molecules. Hence the molecules separate easily at fairly low temperatures.

Attempt to explain the following:

1. Most ionic crystals are solids while simple covalent molecules are not.

2. Sodium chloride does not conduct electricity in its solid state.

3. Metals can conduct heat and electricity.

* Francine Taylor-Campbell is an independent contributor.