`NO_2` forms dimer but `ClO_2` does not why?

Fluorine does not form oxyacid, but other halogens do. Why? (b) Both NO and ClO_(2) are odd electron species. NO dimeries but ClO_(2) does not . Why?

ClO_2 is an/a

NO_(2) readily dimenise, whereas ClO_(2) does not. Why ?

AlCl_(3) forms dimer in vapour phase but BCl_(3) does not because