Each chemical element has a chemical symbol and a combination of chemical symbols gives a chemical formula.

The chemical formula shows what elements are present in a substance. If a pure element is present the chemical formula is merely the chemical symbol for the element. This situation is encountered most often with the noble gases. In some cases, representing an element by its chemical symbol is technically inaccurate but the symbol is used as a shorthand convention. For example, sodium is normally represented by the symbol Na, although in natural situations sodium is encountered as a giant metallic lattice with many sodium molecules bonded together. Each sodium molecule is held in place adjacent to the next molecule by weak intermolecular forces. This produces a large sheet of identical repeating units of sodium. Many layers are joined in a similar manner. Some chemicals are generally encountered as...