The reaction $\text{Cl}_2(g) \rightarrow 2\text{Cl}(g)$ is endothermic (requires energy to break bond). Therefore, the reverse reaction $2\text{Cl}(g) \rightarrow \text{Cl}_2(g)$ is exothermic ($\Delta_r\text{H} < 0$). Since two gaseous atoms combine to form one gaseous molecule, randomness decreases ($\Delta_r\text{S} < 0$).