The 17 group of Periodic Table contains five elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At) combinedly known as halogens (salt forming elements). Astatine is artificially prepared radioactive element.
General Physical Properties of Group 17 Elements:
(i) Electronic configuration: Their valence shell electronic configuration is ns2 np5.
[He] 2s2 2p5
[Ne] 3s2 3p5
[Ar] 3d10, 4s2 Ap5
[Kr] 4d10, 5s25p5
[Xe] 4f14 5d10, 6s2 6p5
(ii) Physical state- Intermolecular forces in halogens are weak and increase down the group. Thus, F2 and Cl2 are gases, Br2 is volatile liquid and I2 is solid.
(iii) Atomicity- All are diatomic in nature.
(iv) Abundance- Being very reactive in nature, they are not found free in nature. Their presence in earth’s crust follows the order:
F2 > Cl2 > Br2 > I2
(v) Colour- They absorb light in the visible range forming excited states and are thus, coloured in nature.
(vi) Metallic character: All the elements are non-metals and metallic character increases down the group. Thus, 1 forms 1+.
(vii) Oxidation state:
-1 to +6
-1 to +6
-1, +1, +5
(viii) Bond energy and bond length: The bond length increases from fluorine to iodine and in the same order bond energy decreases. However, the bond dissociation energy of F2 is lesser due to its smaller size. The order of bond energy is:
(ix) Density: It increases down the group in a regular fashion and follows the order F > Cl > Br >I.
(x) Ionisation enthalpy: The ionisation enthalpy of halogens is very high and decreases down the group. The iodine also forms I+ and I3+ and forms compounds like LiI, lCN, IPO4. In molten state, the compounds conduct electricity showing ionic character.
(xi) Electron affinity- The halogens have the high values for electron affinity. The order of electron affinity is:
Cl2 > F2 > Br2 > I2
Due to small size of fluorine (hence, high electron density), the extra electron to be added feels more electron-electron repulsion. Therefore, fluorine has less value for electron affinity than chlorine.
(xii) Reduction potentials and oxidising nature: E°red of halogens are positive and decrease from F to I. Therefore, halogens act as strong oxidising agents and their oxidising power decreases from fluorine to iodine. Fluorine is the strongest oxidising agent and is most reactive. That’s why it is prepared by the electrolysis of a mixture of KHF2 and anhydrous HF using Monel metal as a catalyst.
(xii) Solubility Halogens are soluble in water which follows the order:
F2 > Cl2 > Br2 > I2
The solubility of iodine in water is enhanced in the presence of KI.
KI + I2 ⇔ KI3 ⇔ K+ + I–2
I2 forms blue colour complex with starch.
Chemical Properties of Group 17 Elements:
(i) Hydrides- HF is a low boiling liquid due to intermolecular hydrogen bonding, while HCI, HBr, HI are gases. The boiling point follows the trend-
HF > Hi > HBr > HCl
Some other properties show the following trend :
(ii) Oxides Fluorine forms two oxides, OF2 and O2F2, but only OF2 is thermally stable at 298K. O2F2 oxidises Plutonium to PuF6 and the reaction is used for removing plutonium as PuF6 from spent nuclear fuel.
Chlorine forms a number of oxides such as, CI2O, CI2O3, Cl2O5 , Cl2O7 , CIO2 and CIO2 is used as a bleaching agent for paper pulp, textiles and in water treatment.
Br2O, BrO2, BrO3 are the least stable bromine oxides and exist only at low temperatures. They are very powerful oxidising agents.
The iodine oxides, i.e., I2O4, I2O5, I2O7 are insoluble solids and decompose on heating. I2O5 is a very good oxidising agent and is used in the estimation of carbon monoxide.
(iii) Reaction with alkali-
Other halogens form hypohalite with dilute NaOH and halate with conc. NaOH4.
(iv) Oxoacids of halogens: Higher oxoacids of fluorine such as HFO2, HFO3 do not exist because fluorine is the most electronegative and has absence of d-orbitals.
+3 oxidation state of bromine and iodine are unstable due to inert pair effect. Therefore, HBrO2 and HIO2 do not exist.
Acidic character of oxoacids decreases as the electronegativity of halogen atom decreases. Thus, the order of acidic strength.
For the oxoacids of same halogens. acidic strength and thermal stability increase as the number of O atoms increases.
When two different halogens react with each other, interhalogen compounds are formed. These compounds are covalent and diamagnetic in nature. They are volatile solids or liquids except elf which is a gas at 298 K. Interhalogen compounds are more reactive than halogens (except fluorine).
The XY3 type compounds have bent ‘T’ shape, XY5 type compounds have square pyramidal shape and IF7 has pentagonal bipyramidal structure.
BrF3 has “T” shaped structure due to 3 bp and 2 lp.
ICI is more reactive than I2 due to weak bond. ClF3 and BrF3 are used for the production of UF6 in the enrichment of 235 U.
U(s) + 3CIF3(l) → UF6(g) + 3CIF(g)
Pseudohalogens and Pseudohalides:
The substances behaving like halogens are known as pseudohalides. Some examples are
Chlorine and its Compounds:
Common salt, NaCl is most important. Chlorine is also present in sea water and as rock salt.
Preparation of Chlorine:
(i) By oxidation of conc. HCl-
4NaCl + Mn02 + 4H2SO4 ----- > 4NaHSO4 + MnCl2 + 2H2O + Cl2
(ii) Weldon’s process
MnO2 + 4HCl ---- > MnCl2 + 2H2O + Cl2
(iii) Deacon’s process
In this process, HCl is oxidised by O2 in the presence of CuCl2 as catalyst at 400oC.
4HCl + O2 ---->2Cl2 + 2H2O
(iv) Electrolytic process: By the electrolysis of brine solution in Nelson cell.
2Na+ + 2e- ------> 2Na + H2O -----> 2NaOH + H2 (at cathode)
2Cl- -----> 2Cl + 2e- ------> Cl2 (at anode)
It is yellowish green gas, collected by upward displacement of air poisonous in nature, soluble in water. It’s aqueous solution is known as chlorine water.
(i) Action of water-
Coloured matter +  → colourless matter.
The bleaching action of chlorine is due to oxidation and is permanent.
(ii) Action of hydrogen-
(iii) Displacement reactions-
(iv) Action of NaOH (cold)-
Aqueous solution of NaOCl is called Javelle water,
(v) Action of H2S-
(vi) Action of dry SO2-
(vii) Action of CO-
(viii) Oxidising properties-
(ix) Reaction with amonia-
(x) Chromyl chloride test-
When a mixture of chloride and solid K2Cr2O7 is heated with concentrated H2SO4 in a dry test tube, deep red vapours of chromyl chloride are evolved.
When these vapours are passed through NaOH solution, the solution becomes yellow due to the formation of sodium chromate.
The yellow solution is neutralised with acetic acid and on addition of lead acetate gives a yellow precipitate of lead chromate.
It is used as a bleaching agent, disinfectant and in the manufacture of CHCl3, CCl4, DDT, anti-knocking compounds and bleaching powder.
Hydrochloric Acid (HCl)
It is a colourless and pungent smelling gas. It is extremely soluble in water and ionises as below-
[Noble metals like gold, platinum can dissolve in aqua-regia [three part conc. HCl and one part of conc. HNO3].
It is used in the manufacture of chlorides. Chlorine, in textile and dyeing industries, in medicine and in extraction of glue from animal tissues and bones.
It’s major source is deep sea weeds of laminaria variety. Their ashes which is called kelp contain 0.5% iodine as iodides.
Another source of I2 is caliche or crude chile saltpetre (NaNO3) which contains 0.2%, NaIO3. Iodine is purified by sublimation.
It shows no reaction with water. Tincture of iodine is a mixture of I2 and KI dissolved in rectified spirit.