The type of hybrid orbitals used by the chlorine atom in ClO2−  is

H=12(V+M−C+A) where H=No. of orbitalsinvolved in hybridisation (viz. 2, 3, 4, 5, 6) and hence nature of hybridisation  (viz. sp2⋅sp3,sp3d,sp3d2 can be ascertained. V = No. of electrons in valence shell of the central atom: M = No. of monovalent atoms, C = charge on cation. A = Charge on anion, ⇒H=12(7+1)=4 or sp3 hybridisation as 4 orbitals are involved.