A net ionic equation is a chemical equation that only shows the species that are involved in the actual chemical reaction. Spectator ions, which are ions that do not participate in the reaction, are excluded from the net ionic equation. To write a net ionic equation, first, write the overall balanced chemical equation. Then, remove the spectator ions from both sides of the equation. The remaining equation is the net ionic equation.

For example, consider the following reaction:

$$\text{BaCl}_2(aq) + \text{Na}_2\text{SO}_4(aq) \rightarrow \text{BaSO}_4(s) + \text{NaCl}(aq)$$

The spectator ions in this reaction are $\text{Na}^+$ and $\text{Cl}^-$. To write the net ionic equation, we remove these ions from both sides of the equation:

$$\text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s)$$

This is the net ionic equation for the reaction.