An ionic compound is a compound that is composed of ions. An ion is an atom or molecule that has lost or gained one or more electrons, giving it a net positive or negative charge. The formula for an ionic crystal is AB, where A is a positively charged ion and B is a negatively charged ion. The subscripts x and y indicate the relative numbers of A and B ions in the compound. For example, NaCl is an ionic crystal with the formula AB. The A ion in NaCl is Na+ and the B ion is Cl-.