A net ionic equation keeps only the species that actually undergo chemical or physical change in an aqueous reaction. It is obtained from a complete ionic equation by cancelling spectator ions that appear unchanged on both sides.

Start with the complete ionic equation

$$\mathrm{Ag^+(aq)+NO_3^-(aq)+Na^+(aq)+Cl^-(aq)}$$ $$\mathrm{\rightarrow AgCl(s)+Na^+(aq)+NO_3^-(aq)}.$$

The ions $\mathrm{Na^+}$ and $\mathrm{NO_3^-}$ occur in the same form on both sides, so they are spectators and can be cancelled. The remaining equation is

$$\boxed{\mathrm{Ag^+(aq)+Cl^-(aq)\rightarrow AgCl(s)}}.$$

This is the net ionic equation. It identifies the essential transformation: dissolved silver and chloride ions form solid silver chloride.

A valid net ionic equation must conserve both atoms and total electric charge. Here the left side has one Ag atom, one Cl atom and net charge zero; the solid product has the same atoms and net charge zero.

Cancellation is allowed only when a species is genuinely identical on both sides, including its chemical form and physical state. For example, $\mathrm{H_2O(l)}$ cannot be cancelled against $\mathrm{H_2O(g)}$, and an ion incorporated into a precipitate is not a spectator.

Different molecular equations can therefore reduce to the same net ionic equation. Mixing $\mathrm{AgNO_3}$ with $\mathrm{NaCl}$ or with another soluble chloride can produce the same essential reaction if the accompanying ions remain spectators.

Net ionic equations are valuable because they factor away chemically irrelevant counterions and expose the reusable reaction pattern underneath a particular choice of soluble reagents.