luorine does not form any oxoacid since it is the strongest oxidizing agent. Chlorine, bromine and iodine mainly form four series of oxoacids namely hypohalous acid (HXO), halous acid (HXO₂) halic acid (HXO₃) and perhalic acid (HXO₄) as given below :

Oxidation state	Chlorine	Bromine	Iodine	Thermal stability and acid strength	Oxidising power
+1	HClO	HBrO	HlO
+3	HClO2	—	—
+5	HClO3	HBrO3	HIO3
+7	HClO4	HBrO4	HIO4
	Acidity decreases ——

(i) Hybridized ion : In all these oxoacids, the halogen atom is sp³ -hybridized.

(ii) Acidic character : All these acids are monobasic containing an—OH group. The acidic character of the oxoacids increases with increase in oxidation number, i.e., HClO < HClO₂ < HClO₃ < HClO₄ and the strength of the conjugate bases of these acids follows the order,

ClO > ClO > ClO > ClO₄^–

(iii) Oxidising power and thermal stability : The oxidizing power of these acids decreases as the oxidation number increases, i.e., HClO < HClO₂ < HClO₃ < HClO₄. Stability of oxoacids of chlorine in the increasing order is, HClO < HClO₂ < HClO₃ < HClO₄ and the increasing stability order of anions of oxoacids of chlorine is, CIO^– < CIO₂^– < CIO₃^– < CIO₄^–.

As the number of oxygen atoms in an ion increases there will be a greater dispersal of negative charge and thus greater will be the stability of ion formed. For different halogen having the name oxidation number, the thermal stability decreases with increase in atomic number i.e., it is in the order HClO > HBrO > HIO and ClO^– > BrO^– > IO^– However, in HXO₃ is most stable. The stability order being HClO₃ < HBrO₃ < HIO₃.

(iv) Perhalates are strong oxidizing agents, the oxidizing power is in the order, BrO₄^–, IO₄^– > CIO₄^–.

Thus BrO₄ is the strongest oxidizing agent (though its reaction is quite slow) and CIO₄^– is the weakest.

(v) The acidity of oxoacids of different halogens having the same oxidation number decreases with increase in the atomic size of the halogen i.e. HCIO₄ > HBrO₄ > HIO₄.