$\Delta H_f^{\circ}\left(\mathrm{Cl}_2, g\right)=0$
$\Delta H_f^{\circ}$ of elements in their standard state is taken to be zero. $\mathrm{Cl}_2$ is gas, $\mathrm{Br}_2$ is liquid and $\mathrm{I}_2$ is solid at room temperature.
Energetics
Conceptual, implication of rules/laws III