In my book it 's stated that the bond order of the oxy acids increases with increasing value of $n$ in $\ce{HClO_n}$ , and also the energy required to break $\ce{Cl-O}$ bond decreases with increasing n leading to the greater strength in acidity . But with increasing bond order bond length should decrease making it tougher for it to break the $\ce{Cl-O}$ bond, so how's that possible?

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    $\begingroup$ For starters, your book is wrong; the $\ce{Cl-O}$ bond order for all chlorine oxyacids is very close to 1. Any bond order differences are so minor that they do not play any role. (Draw out the molecular structures observing the octet rule to confirm that.) $\endgroup$ – Jan Nov 7 '17 at 11:25