The types of atoms and the number of each type in each unit (molecule) of a given compound are conveniently expressed by a chemical formula. In a chemical formula the atoms are indicated by the element symbols, and the number of each type of atom is indicated by a subscript, a number that appears to the right of and below the symbol for the element. The formula for water is written H2O, indicating that each molecule of water contains two atoms of hydrogen and one atom of oxygen (the subscript 1 is always understood and not written). Following are some general rules for writing formulas:Rules for Writing Formulas1. Each atom present is represented by its element symbol.2. The number of each type of atom is indicated by a subscript written to the right of the element symbol.3. When only one atom of a given type is present, the subscript 1 is not written