Discuss the anamolous behaviour of fluorine among the halogens. Give reason also.

(A) Fluorine shows abnormal behaviour because of the following facts:
(i)    Smaller atomic size.
(ii)    Higher electronegativity.
(iii)    Non-availability of empty d-orbitals in its valency shell.
The main points of difference are:

(i)    Fluorine exhibits on oxidation state-1 only whereas remaining halogens may exhibit oxidation states –1, + 1, + 3, – 5 and + 7. The higher oxidation states arise due to the presence of vacant d-orbitals in their valency shells.

(ii)    On account of high. electronegativity fluorine enters into hydrogen bond formation in its compounds with hydrogen. Thus hydrogen fluoride is an associated molecule due to hydrogen bonding.

 

The phenomenon of hydrogen bonding is not shown by other hydracids.

(iii) Reactivity: Fluorine is most reactive among the halogens. This is due to high electronegativity, small size of its atom, extremely high oxidizing power and its low F—F bond energy (38.5 k cal/ mole). This is indicated by the following properties

(a)    Combination with hydrogen: Fluorine reacts with hydrogen in dark at a low temperature. The other halogens do not react with hydrogen in dark.

(b)    Action with metals : Fluorine reacts with metals like gold and platinum. The other halogens do not react with these metals.

(c) Action with non-metals: Fluorine combines directly with the non-metals like carbon, silicon, nitrogen etc. to give their fluorides.
$$\begin{gathered} \mathrm{C}+2{\mathrm{F}}_2 \to {\mathrm{CF}}_4 \\ {\mathrm{N}}_2+3{\mathrm{F}}_2 \to 2{\mathrm{NF}}_3 \end{gathered}$$
The other halogens do not combine directly with these elements.

(iv) Action with water : Fluorine reacts with water forming HF, O₂, and O₃.
$2{\mathrm{F}}_2+2{\mathrm{H}}_2\mathrm{O} \to 4\mathrm{HF}+{\mathrm{O}}_2\uparrow$
$3{\mathrm{F}}_2+3{\mathrm{H}}_2\mathrm{O} \to 6\mathrm{HF}+{\mathrm{O}}_3\uparrow$

The other halogens do not give ozone with water.

(v) Action with alkalies: Fluorine reacts with caustic alkalies to form oxygen difluoride
$2\mathrm{NaOH}+2{\mathrm{F}}_2 \to 2\mathrm{NaF}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{OF}}_2$
The other halogens react with cold and dilute alkalies to form hypohalites and with hot and conc. alkalies to form higher oxy salts, halates, with cold and dilute alkali:
$2\mathrm{NaOH}+{\mathrm{Cl}}_2 \to \mathrm{NaCl} + \underset{\mathrm{Sodium} \mathrm{hpochlorite}}{\mathrm{NaClO}}+{\mathrm{H}}_2\mathrm{O}$
with hot and conc. alkali $\to$
$6\mathrm{NaOH}+3{\mathrm{Cl}}_2 \to \underset{\mathrm{Sodium} \mathrm{Chlorate}}{5\mathrm{NaCl}} + {\mathrm{NaClO}}_3+3{\mathrm{H}}_2\mathrm{O}$

vi)    Formation of oxy acids: Fluorine does not form any oxyacid because it is the strongest oxidising agent while the remaining halogens form four types of oxyacids.
HXO — Hypohalous acid
HXO₂ — halous acid
HXO₃ — Halic acid
HXO₄, — Per-halic acid.

(vii)    Behaviour of hydracids:

(a) HF is a liquid whereas other hydracids HCI, HBr HI are gases at ordinary temperature.

(b)    HF is a weak acid, while the other hydr– acids are strong acids.

(c)    HF is most stable of all the hydracids of remaining halogens.

(d)    HF forms acid salts such as NaHF₂ while the other halogens acids do not form such salts.

(e)    HF can form complex acids such as HBF₄, H₂SiF₆, while the other halogen acids do not form such acids.

(f) HF is the only acid which reacts with silica, silicates and hence attacks glass.

$$\begin{gathered} \underset{\mathrm{Silica}}{{\mathrm{SiO}}_2} + 4\mathrm{HF} \to {\mathrm{SiF}}_4 + 2{\mathrm{H}}_2\mathrm{O} \\ \underset{\mathrm{Sodium} \mathrm{silicate}}{{\mathrm{Na}}_2{\mathrm{SiO}}_3}+ 6\mathrm{HF} \to \underset{\mathrm{Sodium} \mathrm{fluosilicate}}{{\mathrm{Na}}_2{\mathrm{SiF}}_6}+3{\mathrm{H}}_2\mathrm{O} \\ \underset{\mathrm{Calcium} \mathrm{silicate}}{{\mathrm{CaSiO}}_3}+6\mathrm{HF} \to \underset{\mathrm{Calcium} \mathrm{fluosilicate}}{{\mathrm{CaSiF}}_6} + 3{\mathrm{H}}_2\mathrm{O} \end{gathered}$$

(viii) Behaviour of salts:
(a) AgF is soluble in water, whereas other silver halides (AgCl, AgBr, Agl) are insoluble.

(b) The fluorides of calcium, strontium and barium are insoluble in water, whereas corresponding salts of other halogens are soluble.

(B) (i) Electron affinity: Cl > F > Br > I.
(ii) Oxidizing power: F₂ > Cl₂, > Br₂ > I₂.