Group 17 Elements: Halogens Family | Chemistry Class 12 - NEET PDF Download

What are Halogens?

The halogens are the elements that form group 17 of the periodic table. They are reactive nonmetals and include fluorine, chlorine, bromine, and iodine.

Halogens are highly reactive non-metals. These elements greatly resemble in property with each other. Group 17 elements are collectively called halogens (In Greek: halo means salt and genes mean producing, so collectively salt-producing) and it consists of fluorine, chlorine, bromine, iodine, and astatine.

Group 17 - Halogens

The similarity to this extent is not found in other groups of the periodic table. They have a regular gradation in the physical and chemical properties. Astatine is the only radioactive element in the group. They have seven electrons in their outermost shell (ns₂np₅) and are short of one electron from the configuration of the nearest noble gas. The chemical properties and reactivity of an element are determined by the oxidation state exhibited by them.

General Physical Properties of Group 17 Elements:

1. Electronic configuration: 

   Their valence shell electronic configuration is ns² np⁵.
   

2. 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.

3. Atomicity: 

   All are diatomic in nature.

4. Abundance: 

   Being very reactive in nature, they are not found free in nature. Their presence in the earth’s crust follows the order: F₂ > Cl₂ > Br₂ > I₂

5. Colour: 

   They absorb light in the visible range forming excited states and are thus, coloured in nature.
   F₂ ⇒ pale Yellow
   Cl₂ ⇒ yellowish green
   Br₂ ⇒ reddish brown
   I₂ ⇒ deep violet

6. Metallic character: 

   All the elements are non-metals and metallic character increases down the group. Thus, 1 forms 1+.

7. Oxidation state:

   

8. Bond energy & 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:
   

9. Density: 

   It increases down the group in a regular fashion and follows the order F > Cl > Br >I.

10. 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 a molten state, the compounds conduct electricity showing ionic character.

11. Electron affinity: 

    The halogens have high values for electron affinity. The order of electron affinity is: Cl₂ > F₂ > Br₂ > I₂
    Due to the 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.

12. 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.

13. 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 a blue colour complex with starch.

Chemical Properties of Group 17 Elements:

1. 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:
   

2. 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.

3. Reaction with alkali:

   Other halogens form hypohalite with dilute NaOH and hypohalite with conc. NaOH₄.

4. Oxoacids of halogens: 

   Higher oxoacids of fluorine such as HFO₂, HFO₃ do not exist because fluorine is the most electronegative and has the absence of d-orbitals.
   +3 oxidation state of bromine and iodine are unstable due to the inert pair effect. Therefore, HBrO₂ and HIO₂ do not exist.
   Acidic character of oxoacids decreases as the electronegativity of the halogen atom decreases. Thus, the order of acidic strength.For the oxoacids of the same halogens. acidic strength and thermal stability increase as the number of O atoms increases.

Interhalogen Compounds

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 for 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 a 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

Compounds of Group 17 Elements

1. Chlorine

(i) Occurrence:

Common salt, NaCl is most important. Chlorine is also present in sea water and as rock salt.

(ii) Preparation of Chlorine:

1. By oxidation of conc. HCl:
   4NaCl + Mn0₂ + 4H₂SO₄ → 4NaHSO₄ + MnCl₂ + 2H₂O + Cl₂
2. Weldon’s process:
   MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂
3. Deacon’s process: 
   In this process, HCl is oxidised by O₂ in the presence of CuCl₂ as a catalyst at 400^oC.
   4HCl + O₂ → 2Cl₂ + 2H₂O
4. 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)

(iii) Properties:

It is yellowish-green gas, collected by upward displacement of air poisonous in nature, soluble in water. Its aqueous solution is known as chlorine water.

(iv) Chemical Reactions:

1. Action of water:
   Coloured matter + [0] → colourless matter. The bleaching action of chlorine is due to oxidation and is permanent.
2. Action of hydrogen:
     
3. Displacement reactions:
   
4. Action of NaOH (cold):
   An aqueous solution of NaOCl is called Javelle water, 
5. Action of H₂S:
   
6. Action of dry SO_2:
   
7. Action of CO:
   
8. Oxidising properties:
   
9. Reaction with ammonia:
   
10. 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 the addition of lead acetate gives a yellow precipitate of lead chromate.

(v) Uses: 

2. Hydrochloric Acid (HCl)

(i) Preparation: 

(ii) Properties:

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].

(iii) Uses:

It is used in the manufacture of chlorides. Chlorine, in textile and dyeing industries, in medicine and in the extraction of glue from animal tissues and bones.

3. Iodine (I₂):

1. Its major source is deep seaweeds of laminaria variety. Their ashes which are called kelp contain 0.5% iodine as iodides.
2. Another source of I₂ is caliche or crude chile saltpetre (NaNO₃) which contains 0.2%, NaIO_3. Iodine is purified by sublimation.
3. It shows no reaction with water. The tincture of iodine is a mixture of I₂ and KI dissolved in rectified spirit.

4.Oxoacids of Halogens:

Due to high electronegativity and small size, fluorine forms only one oxoacid, HOF known as fluoric (I) acid or hypofluorous acid. The other halogens form several oxoacids. Most of them cannot be isolated in pure state. They are stable only in aqueous solutions or in the form of their salts.

Oxoacids of Chlorine

Oxoacids of Halogens