Syllabus ref: 1.2 The nature, or properties, of a pure substance is determined by the type of particles that it contains. These may be atoms, molecules or ions. Ionic solids contain a giant lattice (network) of ions, simple covalent solids contain discrete molecules held together by loose forces and giant covalent structures have a lattice (network) of atoms all joined together by strong covalent bonds. Metals may be considered to be a regular arrangement of atoms for the purpose of mass calculations. Terminology The mole

Terminology

The terminology used depends on the type of structure under consideration. There are three ways to describe the relative mass of solids:

• Relative atomic mass (abbreviated A^r)
• Relative molecular mass (abbreviated M^r)
• Relative formula mass (abbreviated M^r)

Where each of these applies is considered below.

1. Relative atomic mass

Relative atomic mass is used to define the relative mass of one atom of a metallic element, or non-metallic element, which may be considered to consist of unique atoms for the purposes of calculations.

Note: Although sulfur and phosphorus contain discrete molecules (S₈ and P₄, respectively), they are usually treated in calculations as monatomic.

2. Relative molecular mass

Relative molecular mass is used to define the relative mass of one molecule of a covalent substance, whether element or compound.

Note The halogens bromine and iodine are liquid and solid respectively, at room temperature and pressure.

3. Relative formula mass

Relative formula mass is used to define the mass of the simplest ratio of particles in ionic and giant covalent compounds.

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The Mole

The amount of substance that contains an Avogadro number of particles (6.02 x 10²³) is called 1 mole of that substance. Where the word amount is used it always refers to the number of moles.

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