Which of the following is the least stable oxide of chlorine?

Correct answers: (a) Cl₂O or (b) ClO₂

Explanation

The oxides of chlorine are all unstable, and some explode on heating or shock. Their stability generally increases with the oxidation state of chlorine:

• Cl₂O (chlorine +1): a yellowish-brown gas that explodes easily; by this trend, the least stable;
• ClO₂ (+4) and Cl₂O₆, which the option writes as ClO₃ (+6): unstable and strongly oxidising;
• Cl₂O₇ (+7): the anhydride of perchloric acid and the most stable of them.

The book's key is (a), from the rule that stability rises with chlorine's oxidation state. ClO₂ is also very explosive and some keys give (b), so (b) is also accepted.