To form more than one binary compound with chlorine, the element must exhibit variable oxidation states.

• Zinc ($\text{Zn}$) exhibits only a $+2$ oxidation state and forms only $\text{ZnCl}_2$.
• Potassium ($\text{K}$) is an alkali metal, exhibits only a $+1$ oxidation state, and forms $\text{KCl}$.
• Calcium ($\text{Ca}$) is an alkaline earth metal, exhibits only a $+2$ oxidation state, and forms $\text{CaCl}_2$.
• Iron ($\text{Fe}$) is a transition element that exhibits variable oxidation states (commonly $+2$ and $+3$). Therefore, it can combine with chlorine to form more than one binary compound, namely Iron(II) chloride ($\text{FeCl}_2$) and Iron(III) chloride ($\text{FeCl}_3$).