dic strength of the oxoacids of chlorine.** \(\mathrm{HOCl} < \mathrm{HClO_2} < \mathrm{HClO_3} < \mathrm{HClO_4}\) The acid strength of a halogen oxoacid **increases with the oxidation state of the halogen**, here \(+1, +3, +5, +7\). Each added oxygen pulls electron density away from the \(\mathrm{O{-}H}\) bond and spreads the negative charge of the anion left behind over more oxygens, so the proton is released more readily. Thus \(\mathrm{HOCl}\) is a weak acid, \(K_a = 3.5 \times 10^{-8}\), while \(\mathrm{HClO_4}\) is very strong, \(K_a \gg 1\). **(ii) Ionic character of the halides.** \(\mathrm{MI} < \mathrm{MBr} < \mathrm{MCl} < \mathrm{MF}\) The ionic character of an \(\mathrm{M{-}X}\) bond **increases with the electronegativity difference** between the metal and the halogen. Electronegativity falls down group 17 — \(\mathrm{F}\ 4.0\), \(\mathrm{Cl}\ 3.2\), \(\mathrm{Br}\ 3.0\), \(\mathrm{I}\ 2.7\) — so for the same metal the difference is greatest with fluorine and least with iodine. The large, easily polarised iodide ion also gives the \(\mathrm{M{-}I}\) bond the most covalent character. **(iii) Thermal stability of the hydrogen halides.** \(\mathrm{HI} < \mathrm{HBr} < \mathrm{HCl} < \mathrm{HF}\) Thermal stability follows the **\(\mathrm{H{-}X}\) bond dissociation enthalpy**, which falls down the group as the halogen grows and the bond lengthens: \(\mathrm{HF}\ 574\), \(\mathrm{HCl}\ 432\), \(\mathrm{HBr}\ 363\), \(\mathrm{HI}\ 295\ \mathrm{kJ\ mol^{-1}}\), for bond lengths of 91.7, 127.4, 141.4 and 160.9 pm. The weakest bond breaks first, so \(\mathrm{HI}\) is the least stable and \(\mathrm{HF}\) the most. Note that acidic strength runs the **opposite** way to thermal stability, \(\mathrm{HF} < \mathrm{HCl} < \mathrm{HBr} < \mathrm{HI}\), and for the same reason: the weaker the \(\mathrm{H{-}X}\) bond, the more easily the proton is given up.23:T55c,Oxygen, the first member of group 16, differs from the rest of the group in the following respects. **i. Atomicity.** Oxygen is **diatomic**,

Elements of Groups 16, 17 and 18

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