P - Block Elements

Elements and Compounds of Group-17


  • VII A group is comprised of non-metallic elements, F, Cl, Br, I, At. These are P-block elements with general electronic configuration "ns2 np5".
  • Collectively these elements are known as halogens because their salts are present in sea. The last element astatine is radioactive having very short life.
  • Fluorine shows only −1 oxidation state due to high electronegativity and absence of d-orbitals in valency shell. Other elements show oxidation states +1, +3, +5 and +7 in addition to −1 state due to presence of vacant d-orbitals in their valency shell.
  • All these elements form hydro acids and order of strength of these acids is HF < HBr < HCl < HI
  • The order of reducing character is HF < HBr < HCl < HI
  • Acid strength of oxyacids with some halogens increase with increase in oxidation number of halogen atom HOCl < HClO2 < HClO3 < HClO4.
  • Among the oxyacids of different halogens acid strength decreases with increase in atomic number.
           ⇒ HClO > HBrO > HIO (hypohalous acids)
              HClO3 > HBrO3 > HIO3 (halic acids)
              HClO4 > HBrO4 > HIO4 (perhalic acids)
  • Fluorine forms only one oxyacid (HOF)
  • Increasing order of electron affinity is I < F < Br < Cl
  • Increasing order of electronegativity is I < Br < Cl < F
  • Chlorine is found only in the combined state.
  • Sea water and Laminaria sea grass are important source of iodine.
  • Cl2O is an anhydride of hypochlorous acid (HClO)
  • Bleaching powder which is mainly used as bleaching agent is manufactured by action of dry slaked lime over chlorine.
    Ca(OH)2 + Cl2 → CaOCl2 + H2O

Comparison of halogens

S.NO Property Chlorine Bromine Iodine
1. Physical State gas Liquid Solid
2. B.P & M.P(°C) −34, −102 58.5, −7.3 184, 114
3. Colour of vapour greenish yellow dark red violet
4. Combination with H2 explosive in light, slow in dark Only on heating on heating and in the presence of a catalyst
5. Reaction with non-metal Combines with all non-metals except N, O and C Combines with all non-metals except N, O and Si Combines only with halogens, P, As and H
6. Action of H2O decomposes slowly giving HCl and O2 decomposes slowly only in presence of light Little (or) no action
7. Oxidising action Strong oxidising agent good oxidising agent Weak oxidising agent
8. Bleaching action moist Cl2 is a good bleaching agent moist Br2 is a slow bleaching agent no bleaching action
9. Action of halides displaces Br2 and I2 from Br and I displaces I2 from I no action

 Comparison of Hydro acids HCl, HBr and HI

S.NO Property HCl HBr HI
1. Physical State gas gas gas
2. Stability Stable Unstable Least stable
3. Reducing power Least more than HCl Maximum
4. Strength of acid Weak Moderate Strong
5. Action of a mixture of MnO2 and H2SO4 Chlorine gas is evolved Br2 gas (reddish brown vapour) is evolved. Turns starch paper yellow. I2 gas (violet vapour) is evolved. Turns starch paper violet.
6. Action with AgNO3 solution White ppt insoluble in HNO3 but soluble in NH4OH Pale yellow ppt, insoluble in HNO3, sparingly soluble in NH4OH yellow ppt. insoluble both in HNO3 and NH4OH
7. Action of lead acetate solution White ppt. of PbCl2 solution in hot water White ppt. of PbBr2 soluble in hot water. Yellow ppt of PbI2 soluble in hot water giving colourless solution.
8. Action of mercurous nitrate solution White ppt. soluble in aqua regia _ _
9. Action of mercuric chloride solution. _ _ Scarlet ppt.
10. Action of CuSO4 solution Cl2 is liberated only by F2 Br2 is liberated by F2 and Cl2 I2 is liberated by F2, Cl2 and Br2
11. Action of X2 Cl2 is liberated only by F2 Br2 is liberated by F2 and Cl2 I2 is liberated by F2, Cl2 and Br2
12. Confirmatory test heating with K2Cr2O7 + H2SO4 gives chromyl chloride (CrO2Cl2) fumes which when passed into lead acetate solution gives a yellow ppt of lead chromate When treated with chlorine water in presence of CS2 orange colour is obtained in CS2 layer. When treated with chlorine water in presence of CS2 violet colour is obtained in CS2 layer

Inter halogen compound

Type Inter halogen compound Shape
AB ClF, BrF, BrCl Linear
AB3 ClF3, BrF3, IF3, ICl3 T shaped, distorted trigonal pyramidal due to
2 lone pair of electrons (sp3d)
AB5 ClF5, BrF5 distorted octahedral, square pyramidal due to
1 lone pair of electrons (sp3d2)
AB7 IF7 Pentagonal bi pyramidal (sp3d3)
  • With cold dilute alkali chlorine gives chloride & hypochlorite.
  • With hot conc. alkali chlorine gives chloride & chlorates.
  • With dry slaked lime chlorine gives bleaching powder.
  • Chlorine oxidises ferrous salts to ferric salts, hydrogen sulphide to sulphur, sulphites and thiosulphates to sulphates.
  • Chlorine used in preparation of poisonous gases like phosgene (COCl2), mustard gas, teargas and DDT (dichloro diphenyl trichloro ethane)
  • Fluorine has no known oxyacids.
  • Acidic nature and thermal stability of oxyacids increases with increasing oxidation state of chlorine :-
              HClO < HClO2 < HClO3 < HClO4.
  • Perchloric acid is strongest oxyacid and perchlorate is the weakest base known.
  • Order of bond length of Cl-O in oxyanions is, ClO_4^{^-} < ClO_3^{^-} < ClO_2^{^-} < ClO^{^-}
  • Order of bond energy of ClO in oxyanions of chlorine is, ClO^{^-} < ClO_2^{^-} < ClO_3^{^-} < ClO_4^{^-}

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