A chemical symbol is an abbreviation that chemists use to represent atoms of an element. Symbols of some elements are derived from the common names of the element while others are derived from their names in other languages. Most symbols have one or two letters. The recently synthesised (prepared) elements use three- letter symbols. In order to avoid confusion with other notations, only the first letter of a symbol is capitalised, e.g. Co is the symbol for the element cobalt while CO is a notation for the compound, carbon monoxide containing one atom each of carbon (C) and oxygen (O) elements.
Names of chemical compounds are represented in abbreviations called formulae. Carbon monoxide, for example, is represented by the formula, CO. Formula also represents the composition of the compound. Thus the expression, MgCL2, identifies atoms of the element magnesium (Mg) and chlorine (Cl) as the constituents of magnesium chloride. When more than one atom of any given element is present in the formula, subscripts are used to denote the relative numbers of atoms of each type of the elements in the compound. In the above formula, therefore, each unit of magnesium chloride contains one atom of magnesium and two atoms of chlorine.
Parenthesis or brackets in a formula indicate a group of atoms that behaves or functions as a unit. For instance, the formula Al2(SO4)3 indicates that aluminium tetraoxosulphate (Vl) has two atoms of aluminium (Al) for every three groups of tetraoxosulphate(VI) (SO4).