p - Block Elements Part - 12 | Class 12th

ü  Anomalous Behaviour of Fluorine:

·       Due to the small size.

·       Highest Electronegativity.

·       Low F-F bond dissociation enthalpy.

·       Non availability of d orbitals in valence shell.

·       Most of the reactions of fluorine are exothermic.

·       F forms only one oxoacid while other halogens form a number of oxoacids.

·       Ionic and covalent radii, m.p. and b.p. and electron gain enthalpy are quite lower than expected.

·       Hydrogen fluoride is a liquid due to strong hydrogen bonding. While the hydrogen halides of other elements are gases.

Ø  Reactivity Towards Hydrogen: They all react with hydrogen to give hydrogen halides but affinity for hydrogen decreases from fluorine to iodine. They dissolve in water to form hydrohalic acids.

The acidic strength of these acids varies in the order: HF < HCl < HBr < HI. The stability of these halides decreases down the group this is due to decrease in bond (H–X) dissociation enthalpy in the order H – F > H – Cl > H –Br > H – I.

HF has strong intermolecular H bonding.

% Ionic character: HF > HCl > HBr > HI.

Dipole moment HF > HCl > HBr > HI.

Electronegativity decreases down the group.

Reducing power: HF < HCl < HBr < HI.

As the size increases van der Waals forces increases and hence boiling point increases.

Ø  Reactivity Towards Oxygen:

·       Halogens form many oxides with oxygen but most of them are unstable.

·       Fluorine forms two oxides OF₂ and O₂F₂. However, only OF₂ is the thermally stable at 298 K. These oxides are essentially oxygen fluorides because of the higher electronegativity of fluorine than oxygen.

·       Both are strong fluorinating agents. O₂F₂ oxidises plutonium to PuF₆ and the reactions is used in removing plutonium as PuF₆ from spent nuclear fuel.

·       Chlorine, bromine and iodine form oxides in which the oxidation states of these halogen range from + 1 to + 7. The higher oxides of halogens tend to be more stable than the lower ones.

·       Chlorine oxides, Cl₂O, ClO₂, Cl₂O₆ and Cl₂O₇ are highly reactive oxidising agents and tend to explode. ClO₂ is used as a bleaching agent for paper pulp and textiles and in water treatment.

·       The bromine oxides, Br₂O, BrO₂, BrO₃ are the least stable halogen oxides and exist only at low temperature. They are very powerful oxidising agents.

·       The iodine oxides, I₂O₄, I₂O₅, I₂O₇ are insoluble solids and decompose on heating. I₂O₅ is very good oxidising agent and is used in the estimation of carbon monoxide.

Ø  Reactivity towards metals: Halogen react with metals to form metal halides. For e.g., bromine reacts with magnesium to give magnesium bromide. Ionic character: MF > MCl > MBr > MI. Halides in higher oxidation state will be more covalent than the one in the lower oxidation state.

Ø Interhalogen Compounds: Reactivity of Halogens Towards other Halogens: Halogens combine amongst themselves to form a number of compounds known as interhalogen of the types X X’, X X’₃, X X’₅ and X X’₇. where X is a larger size halogen and X’ is smaller size halogen.