In chemistry, a net ionic equation is an equation that shows only the species that are involved in the chemical reaction, excluding spectator ions. Spectator ions are ions that do not participate in the reaction and do not change during the reaction.

To write a net ionic equation, first write the balanced molecular equation. Then, identify the spectator ions and remove them from the equation. Finally, write the coefficients for the reactants and products so that the equation is balanced.

For example, consider the following reaction:

$$BaCl_2(aq) + Na_2SO_4(aq) → BaSO_4(s) + 2NaCl(aq)$$

The spectator ions in this reaction are sodium and chloride. They do not participate in the reaction and do not change during the reaction. The net ionic equation for this reaction is:

$$Ba^{2+}(aq) + SO_4^{2-}(aq) → BaSO_4(s)$$

The net ionic change for this reaction is the formation of barium sulfate from barium and sulfate ions.