The chemical symbol 2Cl− means that you have 2 moles of Cl− ion. For an ion, the oxidation state is the charge. Since one chloride ion has a charge of -1, then that would be its oxidation state.
The overall oxidation state of the 2 moles Cl− ion is, therefore, -2 (as 2 moles multiplied by the charge -1).
Now for Cl2, the symbol means that there are two Cl atoms sharing electrons to form a substance, which is the chlorine gas. Thus we can calculate the formal charge as:
Cl atom: 7 valence electrons; therefore, total valence electrons is 14.
)
The sharing of electrons brings a formal charge of +1 for one of the Cl atoms and a charge of -1 to the other Cl atom, bringing the overall charge/oxidation state to zero [(+1) + (-1)].
