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Explain the anomalous nature of fluorine?

(1) Fluorine is most reactive of all the halogens due to lower value of F — F bond dissociation energy (F₂ = 158, Cl₂ = 243, bromine = 192 and iodine = 151 kJ mol^–1) .

(2) Being the most electronegative element, it shows only an oxidation state of –1 and does not show positive oxidation states due to absence of d-orbitals in its valence shell. Other halogens show positive oxidation states of +1, +3, +5 and +7.

(3) Due to small atomic size and high electronegativity of H, HF undergoes strong H-bonding while other halogen acids do not. As a result,

(i) HF is a liquid (boiling point 292.5K), while other halogen acids are gases at room temperature (boiling point of HCl = 189 K, HBr = 206 K, HI = 238 K).

(ii) HF is weakest of all the halogen acids due to high strength of H — F bond.

(iii) Due to H-bonding, HF can form acid salts of the type KHF₂, i.e., K⁺[H – F ... F^–] while HCl, HBr and HI do not form such salts (i.e., no KHCl, KHBr₂ and KHl₂ are known).

(4) Fluorides have the maximum ionic character. For example AlF₃ is ionic while other halides of are covalent.

(5) Of all the halogens, fluorine has the highest positive electrode potential (F₂ = 2.87, Cl₂ = 1.36, Br₂ = 1.09 and 1₂ = 0.53 volt) i.e., it is most easily reduced and hence acts as the strongest oxidising agent. It brings about the highest oxidation of other elements with which it combines. For example with S, it gives SF₆, with I₂ it gives IF₁. Other halogens do not always bring about the highest oxidation state. For example, with sulphur gives Cl₂ gives SCl₄, Br₂ while SBr₂ does I₂ not react at all. F₂ is so powerful oxidising agent that it can even oxidise inert-gases.

(6) HF cannot be stored in glass bottles since it reacts with silicates to form fluorosilicates.

                 Na₂SiO₃ + 6 HF ———> Na₂SiF₆ + 3 H₂O

While other halogen acids (HCl, HBr and HI) do not react with silicates and hence can be stored in glass bottles.

(7) AgF is soluble in H₂O while all other silver halides i.e., AgCl, AgBr and Agl are insoluble in water. In constant, CaF₂ is insoluble while other calcium halides i.e., CaCl₂, CaBr₂, Cal₂ are soluble inH₂O.

(8) Due to absence of -orbitals, fluorine, does not form polyhalide ions while other halogens form polyhalides of the type I₃^–, Br₃^–, I₅^– etc.