Final answer:
The compound written incorrectly is 'ClNa'; it should be written as 'NaCl' with the cation (sodium) before the anion (chloride).
Step-by-step explanation:
The chemical compound written incorrectly among the options provided is ClNa. In chemical formulas, the cation (positively charged ion) is written before the anion (negatively charged ion). Thus, instead of ClNa, the correct chemical formula should be NaCl, which is sodium chloride.
All other compounds are written correctly, with the cations coming before the anions, as shown in the provided examples such as CaF₂ (calcium fluoride), KCl (potassium chloride), and MgSO₄ (magnesium sulfate).