Bonding & Structure

KEY POINTS:

• The types of bonding in a substance can be easily deduced by looking at the chemical formulae of the substance. There are four major types of structure to be looked at:
  • Compounds made up of a metal and a non-metal, are ionic compounds. Sodium chloride (NaCl) is the most common example of this. The particles involved are positive and negative ions arranged in a giant three-dimensional lattice. Physical properties include high melting and boiling points, conductivity in the molten state or when dissolved in water. But remember – not all ionic compounds dissolve in water.
  • If the substance is composed entirely of non-metal atoms, the structure is referred to as simple molecular. There are covalent bonds within the molecules. This includes simple molecules, e.g. pure helium (He), oxygen (O₂), carbon dioxide (CO₂). More complex molecules include polymers and DNA. Physical properties include relatively low melting and boiling points, no conduction in any state of matter, and a tendency to be insoluble in water.
  • Giant covalent structures are rare but to be noted. Diamond, graphite, and silicon dioxide are all composed entirely of non-metal atoms, but have incredibly high melting points and are insoluble in water. These substances truly reflect the strength of covalent bonds.
  • A pure metal is described as having a giant metallic structure. The physical properties are obvious – high melting point, conductivity in the solid state, etc. Remember there is no such thing as a compound made of two metals. (Metals can of course be mixed – alloys!)