(a) In oxy-acids the central elements always exhibits positive oxidation states. Fluorine , being the most electronegative , never shows positive oxidation states, hence `F` does not form oxy-acids. Other halogens have the tendency to show positive oxidation states and hence form oxy-acids.
(b) In `NO` the size of nitrogen atom is small and the odd electrons is attracted by only oxygen atom while in `ClO_(2)` the size of chlorine atom is comparatively large and an odd electron is attracted by two oxygen atoms.As a result, the odd electron on `N` in `NO` is localised while the odd electron on chlorine in `ClO_(2)` is delocalised. Thus, `NO` has a tendency to dimerise but `ClO_(2)` does not.
