Here's why:

* Electronegativity: Chlorine is highly electronegative, meaning it strongly attracts electrons. This leads to the formation of polar covalent bonds with oxygen in its oxide (Cl₂O₇).

* Polarity: The high electronegativity difference between chlorine and oxygen creates a highly polar bond, resulting in a highly polar Cl₂O₇ molecule.

* Acidity: The highly polar nature of Cl₂O₇ makes it a strong Lewis acid. When Cl₂O₇ reacts with water, it readily donates a proton (H+) to form perchloric acid (HClO₄), which is a very strong acid.

Other elements that form acidic oxides include:

* Sulfur (S)

* Bromine (Br)

* Iodine (I)

However, the acidity of their oxides generally decreases down the group.