Elements of p-Block III (Group 17) Halogen Family


Elements of p-Block III (Group 17) Halogen Family – Class 12th ncert chemistry Elements of p-Block III (Group 17) Halogen Family notes…

Elements of p-Block III  (Group 17) Halogen Family

F, Cl, Br, I and At are the elements of group 17. These are collectively known as halogens. At (astatine) is a radioactive element. Their general valence shell configuration is ns2np5.


  • Fluorine and chlorine are most abundant gases among all halogens.
  • Fluorine is mainly present as insoluble as fluorides like CaF2, Na3AlF6 etc.
  • Bromine and iodine are present in very small quantity (mainly as salts in sea water).
  • Chlorine present on sea water as chlorides.

Electronegativity of Halogens

Among all the known elements fluorine has highest electonegative like 4. Electronegativity decreases by going down in a group.

Electron affinity of Halogens

Among all the known elements, chlorine has highest electron affinity. Fluorine has less electron affinity than chlorine because there is strong repulsion between electrons of 2p-orbitals due to comparatively smaller size of fluorine, so incoming electron feels less attraction. Electronegativity continuously decreases from chlorine to iodine.

Oxidising property of Halogens

Due to high electronegativity, halogen atom accept one electron and easily get reduced, because pf this reasons all the halogens act as strong oxidising agents. Order of oxidising property is as follows –

F2 > Cl2 > Br2 > I2

Therefore, fluorine is strongest and iodine is the weakest oxiding agents.

Hydrogen halides

Halogens firm HX type hydrogen halides with hydrogen. Thermal stability and bond dissociation enthalpy decreases down the group from HF to HI but acidic strength increases from HF to HI. Their boiling points also increases from HCl to HI. High boiling point of HF is due to the intermolecular H-bonding.

Oxygen Fluorides

OF2 and O2F2 both are strong fluorinating agents and are called oxygen fluorides. O2F2 is used in removing Pu as PuF6 from spent nuclear fuel. Cl, Br and I form oxides in which halogens exist +1 to +7 states.

Interhalogen Compound

Due to difference in electronegativity of halogen atoms, halogen combine mutually and form intrahalogen compound. Their general formula is ABn where A is larger size halogen and B is smaller size halogen. These are of different type like AB, AB3, AB5, and AB7. Interhalogens compounds are more reactive than halogen except fluorine).

Method of Preparation of Chlorine

(i) Cl2 is formed when strong oxidising agent like MnO2, KMnO4, K2Cr2O7 react with HCl.

{ MnO }_{ ( }2)+{ 4HClconc. }\overset { Heating }{ \longrightarrow } { MnCl }_{ 2 }+{ { 2H }_{ 2 }O }+\underset { Chlorine }{ { Cl }_{ 2 } } \uparrow

{2KMnO}_{4} + {16HCl} \overset { \Delta }{ \longrightarrow } {2KCl} + {2MnCl}_{2} + {{8H}_{2}O} + {5Cl}_{2} \uparrow

(ii) By reaction of Hcl or H2SO4 on bleaching powder

{CaOCL}_{2} + {{H}_{2}{SO}_{4}} \longrightarrow {CaSO}_{4} + {{H}_{2}O} + {Cl}_{2} \uparrow

(iii) Deacon’s process

{4HCl} + {O}_{2} \xrightarrow [ 450°C ]{ { CuCl }_{ 2 }(catalyst) } {{2H}_{2}O} + {2Cl}_{2}

Some important reactions-

Hydrochloric acid (HCl)            {NaCl} + {{H}_{2}{SO}_{4}} \overset{420  K}{\longrightarrow} {NaHSO}_{4} + {HCl}

{NaHSO}_{4} + {NaCl} \overset{823 K}{\longrightarrow} {{Na}_{2}{SO}_{4}} + {HCl} [latex]</p> <p style="text-align: center;">[latex] {{Na}_{2}{CO}_{3}} + {2HCl} \longrightarrow {2NaCl} + {{H}_{2}O} + {CO}_{2}

Interhalogen Compounds

{Cl}_{2} + {F}_{2} \overset{473 K}{\longrightarrow} {2ClF}

{Cl}_{2} + {3F}_{2}(excess) \overset{573 K}{\longrightarrow} {2ClF}_{3}

{I}_{2} + {Cl}_{2} \longrightarrow {2ICl}

{Br}_{2} + {3F}_{2} \longrightarrow {2BrF}_{3}

{Br}_{2} + {2I}^{-} \longrightarrow {2Br}^{-} + {I}_{2}

Test of HCl
  • Hydrochloric acid or gas with Ammonia gives white fumes of Ammonium Chloride.
  • Aqueous solution of HCl forms white precipitate of lead chloride with lead acetate or lead nitrate solution.
  • Aqueous solution of HCl forms white precipitate of AgCl with the solution of silver nitrate which is insoluble in nitric acid.
  • Dry heating concentrated HCl with MnO2 or KMnO2 it gives greenish yellow coloured chlorine gas.
Uses of HCl
  • It is used as a reagent in laboratory
  • HCl is employed in extraction of glue from bones and purification of bone black
  • It is used in manufacturing of NH4Cl, Cl2, glucose, etc.
  • Used in the form of aqua-regia for dissolving metals like gold, platinum.


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