To write the equation for a reaction in ionic form, we indicate the ionic species that actually exist in solution.
For the reaction between zinc and hydrogen chloride, the equation in the ionic equation form is:
Zn + 2H+ + 2Cl- --> H2 + Zn2+ + 2Cl-
The net ionic equation is arrived at by looking at ions that do not undergo a change. These are known as spectator ions and can be eliminated from both the sides.
In the reaction between zinc and hydrogen chloride, we see that the Cl- ions are common on both the sides of the equation. They are spectator ions and can be eliminated.
The resultant net ionic equation is:
Zn + 2H+ --> H2 + Zn2+