* Chlorine is a highly electronegative element. This means it strongly attracts electrons.

* Oxygen is also electronegative, but less so than chlorine.

* In ClO, the chlorine atom pulls electron density away from the oxygen atom. This creates a partial positive charge on the oxygen atom and a partial negative charge on the chlorine atom.

* The oxygen atom, with its partial positive charge, can readily accept a pair of electrons from a base. This is the defining characteristic of an acid, according to the Lewis definition.

Therefore, chlorine oxide acts as an acid in chemical reactions.