A chemical formula represents the atomic composition of one molecule of a substance. It is a combination of symbols that denote the atoms which compose the molecule together with subscript numbers that indicate how many atoms of each element are present in it. The subscript one is never written. For example, the formula for oxygen molecule is O2 which shows that a molecule of oxygen contains two atoms of oxygen. This means it is a diatomic molecule. A molecule of a compound consists of two parts , one metallic and the other nonmetallic. The metallic part includes metal or hydrogen and the nonmetallic part includes nonmetallic radicals. The formula for water is H2O which indicates that a molecule of water consists of two atoms of hydrogen and one atom of oxygen. To write the formula of a compound, knowledge of valency of the metal and that of the radical ( group of atoms ) is essential. Magnesium is divalent and the nitrate is monovalent. Hence the formula of magnesium nitrate is Mg(NO3)2. Similarly, the formula of aluminium sulphate is Al2 (SO4)3, as aluminium is trivalent and sulphate is divalent. Thus metal atoms and radicals combine together in the inverse ratio of their valencies.
A formula is a symbolic representation of a molecule of an element or a compound .
The formula indicates one molecule of that substance. It also stands for the molecular mass of that substance. It also indicates the fixed proportion by weight in which the atoms combine. For example, the formula of ammonia is NH3 while that of carbon dioxide is CO2. The formula of calcium carbonate is CaCO3 while that of magnesium chloride is MgCl2.