By Carter McBride — Updated August 30, 2022
When two elemental compounds combine, the resulting ionic substance has a unique chemical composition. The cross‑over method provides a systematic, evidence‑based approach to deduce the empirical formula of such compounds. By consulting a valency chart, you can determine each element’s charge and calculate the correct stoichiometric ratio. For example, reacting sodium (Na) with chlorine (Cl) yields sodium chloride (NaCl), the familiar table salt.
Consult a reliable periodic table to confirm the symbols of the reactants. For instance, sodium and oxygen are represented as Na and O, respectively.
List each element’s chemical symbol followed by its common oxidation state. A valency chart displays these charges. For sodium and oxygen, you would write Na +1 and O −2, indicating sodium’s +1 charge and oxygen’s −2 charge.
Swap the numerical values of the charges between the two elements, discarding their signs. This “cross‑over” step gives the subscript ratio. Using the example, the 2 from oxygen moves to sodium and the 1 from sodium moves to oxygen, resulting in Na 2 and O 1.
Eliminate any common factors or units of one from the subscripts. In the example, the 1 next to oxygen can be omitted, yielding the empirical formula Na₂O, known as sodium oxide.
By following these steps, you can confidently determine the formula of any ionic compound formed by combining two elements.
