A chemical compound contains atoms that are chemically bonded in a specific whole number ratio, and is described by a chemical formula. Some examples are:
1. Water, H2O, contains two hydrogen atoms and one oxygen atom per molecule.
2. Glucose, C6H12O6, contains six carbon atoms, twelve hydrogen atoms and six oxygen atoms per molecule.
3. Ethanol, CH3OH, contains one carbon, four hydrogen and one oxygen atom per molecule.
These three compounds are molecular, meaning that they exist as individual molecules. Ionic compounds exist as a collection of ions. Sodium chloride, for example, is an ionic compound that has the formula NaCl. This means that the lowest whole number ratio of sodium ions to chloride ions is one to one. This is called the formula unit. Sodium chloride exists as a crystal lattice containing made of many ions in this ratio, not as individual molecules of one sodium and one chloride.