The p-Block Elements (Halogen Family – Group 17)

NEET Chemistry: The p-Block Elements (Halogen Family) – Detailed Notes and Practice Questions

Chapter 7C: The p-Block Elements (Halogen Family – Group 17)

1. Introduction to Group 17 Elements

  • Group 17 elements (Halogen Family) include Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At).
  • The term “Halogen” is derived from Greek words ‘halo’ (sea salt) and ‘genes’ (born), meaning ‘salt producers’.
  • The general electronic configuration is ns2np5. They are one electron short of a stable noble gas configuration, hence highly reactive non-metals.
  • Astatine is a radioactive element with a very short half-life.

2. General Characteristics of Group 17 Elements

A. Electronic Configuration:

  • ns2np5.
  • F: [He]2s22p5
  • Cl: [Ne]3s23p5
  • Br: [Ar]3d104s24p5
  • I: [Kr]4d105s25p5
  • At: [Xe]4f145d106s26p5

B. Atomic and Ionic Radii:

  • Atomic and ionic radii generally increase down the group due to the addition of new electron shells.
  • Fluorine has the smallest atomic and ionic size in its period.

C. Ionization Enthalpy:

  • Generally decreases down the group due to increasing atomic size and increasing shielding effect.
  • They have very high ionization enthalpies due to stable ns2np5 configuration.

D. Electronegativity:

  • Electronegativity generally decreases down the group.
  • Fluorine is the most electronegative element in the entire periodic table.

E. Electron Gain Enthalpy:

  • Halogens have very high negative (exothermic) electron gain enthalpies due to their strong tendency to gain one electron to achieve a stable noble gas configuration.
  • The electron gain enthalpy generally becomes less negative down the group.
  • Chlorine has the highest negative electron gain enthalpy in the entire periodic table. Fluorine has a less negative electron gain enthalpy than chlorine due to its very small size, which leads to strong inter-electronic repulsions in the compact 2p subshell when an electron is added.

F. Oxidation States:

  • Common oxidation state is −1.
  • All halogens show a −1 oxidation state.
  • Fluorine exclusively shows a −1 oxidation state because it is the most electronegative element and does not have vacant d-orbitals to expand its octet.
  • Chlorine, Bromine, and Iodine can also show positive oxidation states of +1,+3,+5, and +7 (e.g., in their oxides, oxoacids, and interhalogen compounds) due to the presence of vacant d-orbitals in their valence shell, allowing for the expansion of their octet.
  • The stability of the highest oxidation states (e.g., +7) decreases down the group.

G. Physical States and Colour:

  • Fluorine (F2​): Pale yellow gas.
  • Chlorine (Cl2​): Greenish-yellow gas.
  • Bromine (Br2​): Reddish-brown liquid.
  • Iodine (I2​): Violet-black solid (sublimes easily).
  • The colour of halogens deepens down the group. This is because their absorption maxima shift towards longer wavelengths as we go from F to I, due to decreasing energy difference between HOMO and LUMO.

H. Bond Dissociation Enthalpy:

  • Bond dissociation enthalpy generally decreases down the group.
  • However, there is an anomaly: Cl2​>Br2​>F2​>I2​.
  • The unusually low bond dissociation enthalpy of F2​ is due to the small size of fluorine atoms and strong interelectronic repulsions between the lone pairs of electrons on the two fluorine atoms.

I. Reactivity:

  • Halogens are highly reactive non-metals. Reactivity decreases down the group.
  • Fluorine is the most reactive halogen.
  • Oxidizing power decreases down the group (F2​>Cl2​>Br2​>I2​). A halogen with higher oxidizing power can oxidize halide ions of lower oxidizing power.
    • Cl2​+2KBr→2KCl+Br2​
    • Br2​+2KI→2KBr+I2​

3. Anomalous Behaviour of Fluorine

Fluorine differs significantly from other halogens due to:

  1. Small size.
  2. Highest electronegativity.
  3. Absence of d-orbitals in its valence shell.
  4. Low bond dissociation enthalpy of F2​ molecule.
  • Oxidation States: Only shows −1 oxidation state.
  • Hydrogen Bonding: Forms strongest hydrogen bonds among halogens (e.g., in HF).
  • Reactivity: Extremely reactive.
  • Compounds: Forms only one oxoacid (HFO). Unlike other halogens, it forms OF2​ where oxygen has a positive oxidation state.

4. Important Compounds of Halogens

A. Chlorine (Cl2​):

  • Preparation:
    • Deacon’s Process (Industrial): Oxidation of HCl by atmospheric oxygen in the presence of CuCl2​ catalyst at 723K. 4HCl(g)+O2​(g)CuCl2​,723K​2Cl2​(g)+2H2​O(g)
    • Electrolytic Process (Industrial): By the electrolysis of brine solution (conc. NaCl solution) in Nelson cell or Castner-Kellner cell. 2NaCl(aq)+2H2​O(l)Electrolysis​2NaOH(aq)+Cl2​(g)+H2​(g)
    • Laboratory: By oxidation of concentrated HCl with strong oxidizing agents like MnO2​,KMnO4​,K2​Cr2​O7​. MnO2​+4HCl→MnCl2​+Cl2​+2H2​O
  • Properties: Greenish-yellow gas. Pungent and suffocating smell. Soluble in water (forms chlorine water). Oxidizing agent, bleaching agent (due to nascent oxygen).
    • Bleaching action: Cl2​+H2​O→HCl+HOCl (hypochlorous acid)
    • HOCl→HCl+[O] (nascent oxygen)
    • Coloured substance + [O]→ Colourless substance. (Permanent bleaching)
  • Uses: Bleaching paper pulp, textiles; sterilization of water; in the manufacture of PVC, chloroform, DDT, bleaching powder.

B. Hydrogen Chloride (HCl):

  • Preparation:
    • Laboratory: By heating NaCl with concentrated H2​SO4​. NaCl+H2​SO4​420K​NaHSO4​+HCl
    • Industrial: By-product of chlorination of organic compounds.
  • Properties: Colourless, pungent-smelling gas. Extremely soluble in water (forms hydrochloric acid). Acidic in nature. Anhydrous HCl is not acidic.
  • Uses: Cleaning steel, purification of common salt, in medicines and laboratories.

C. Oxoacids of Halogens: (Important to remember oxidation state, structure, and acidity trend)

  • Halogens (Cl, Br, I) form several oxoacids. Fluorine forms only one, HFO.
  • Hypohalous acid (HOX): +1 oxidation state (e.g., HOCl).
  • Halous acid (HOXO or HXO2​): +3 oxidation state (e.g., HClO2​).
  • Halic acid (HOXO2​ or HXO3​): +5 oxidation state (e.g., HClO3​).
  • Perhalic acid (HOXO3​ or HXO4​): +7 oxidation state (e.g., HClO4​).
  • Acidity Order (for a given halogen): HClO4​>HClO3​>HClO2​>HOCl. Acidity increases with increasing oxidation state of the halogen due to increasing electron withdrawal, which stabilizes the conjugate base.
  • Acidity Order (for same type of oxoacid): HClO4​>HBrO4​>HIO4​. (for perhalic acids)
  • Acidity Order (for same oxidation state): HOCl>HOBr>HOI. (Acidity decreases down the group due to decreasing electronegativity, making H-O bond less polar).

D. Interhalogen Compounds:

  • Definition: Compounds formed between two different halogens (e.g., XY,XY3​,XY5​,XY7​).
    • X is the larger, less electronegative halogen. Y is the smaller, more electronegative halogen.
  • Types:
    • XY type: ClF,BrF,BrCl,ICl,IBr.
    • XY3​ type: ClF3​,BrF3​,IF3​.
    • XY5​ type: ClF5​,BrF5​,IF5​.
    • XY7​ type: IF7​. (Only this one)
  • Structure:
    • XY: Linear (e.g., ClF).
    • XY3​: Bent T-shape (e.g., ClF3​).
    • XY5​: Square pyramidal (e.g., BrF5​).
    • XY7​: Pentagonal bipyramidal (e.g., IF7​).
  • Properties: More reactive than halogens (except F2​) because X−Y bonds are weaker than X−X bonds (except F−F). They are strong oxidizing agents.
  • Uses: ClF3​ and BrF3​ are used as fluorinating agents.

NEET Chemistry: The p-Block Elements (Halogen Family) – Practice Questions

I. Multiple Choice Questions (MCQs)

1. Question: Which of the following is the most electronegative element in the periodic table? a) Chlorine b) Oxygen c) Fluorine d) Nitrogen

2. Question: Which halogen has the highest negative electron gain enthalpy? a) Fluorine b) Chlorine c) Bromine d) Iodine

3. Question: Which of the following halogens exclusively shows a −1 oxidation state? a) Chlorine b) Bromine c) Iodine d) Fluorine

4. Question: The correct order of decreasing bond dissociation enthalpy for halogens is: a) F2​>Cl2​>Br2​>I2​ b) Cl2​>Br2​>F2​>I2​ c) I2​>Br2​>Cl2​>F2​ d) Cl2​>F2​>Br2​>I2​

5. Question: Which process is used for the industrial preparation of chlorine? a) Deacon’s Process b) Haber’s Process c) Contact Process d) Ostwald’s Process

6. Question: The bleaching action of chlorine is due to: a) Reduction b) Oxidation c) Hydrolysis d) Dehydration

7. Question: The structure of ClF3​ is: a) Linear b) T-shaped c) Trigonal planar d) Tetrahedral

8. Question: Which of the following is the strongest acid among the oxoacids of chlorine? a) HOCl b) HClO2​ c) HClO3​ d) HClO4​

9. Question: Which of the following halogen oxides is formed in the solid state by the electrolysis of brine solution? a) Sodium hydroxide b) Chlorine c) Hydrogen d) All of the above

10. Question: The colour of bromine is: a) Pale yellow b) Greenish-yellow c) Reddish-brown d) Violet-black

11. Question: Which of the following is an incorrect statement regarding interhalogen compounds? a) They are more reactive than halogens (except F2​). b) They are formed between two different halogens. c) The bond between the two halogens is weaker than the halogen-halogen bond in X2​ (except F2​). d) They are always non-polar.

12. Question: Which of the following does not form any oxoacid? a) Fluorine b) Chlorine c) Bromine d) Iodine

13. Question: The reason for the lower than expected electron gain enthalpy of fluorine is: a) High bond dissociation enthalpy of F2​ b) Small size of fluorine atom c) High electronegativity of fluorine d) Presence of d-orbitals in fluorine

14. Question: The shape of IF7​ is: a) Square pyramidal b) Pentagonal bipyramidal c) Octahedral d) Trigonal bipyramidal

15. Question: Which of the following statements is true about HCl? a) It is a reddish-brown gas. b) It is extremely soluble in water. c) It is a reducing agent. d) It gives a pungent smell of rotten fish.

II. Assertion-Reason Type Questions

Directions: In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R). Choose the correct option. a) Both A and R are true and R is the correct explanation of A. b) Both A and R are true but R is NOT the correct explanation of A. c) A is true but R is false. d) A is false but R is true.

16. Assertion (A): Fluorine is the most reactive halogen. Reason (R): Fluorine has the lowest bond dissociation enthalpy among all halogens.

17. Assertion (A): HClO4​ is a stronger acid than HClO3​. Reason (R): The stability of the conjugate base increases with increasing oxidation state of the central halogen atom.

18. Assertion (A): Interhalogen compounds are generally more reactive than halogens (except fluorine). Reason (R): The X-Y bond in interhalogen compounds is stronger than the X-X bond in diatomic halogens.

19. Assertion (A): Direct nitration of aniline is not preferred. Reason (R): The amino group is a strong meta-director.

20. Assertion (A): Chlorine water loses its yellow colour on standing. Reason (R): Chlorine water contains HCl and HOCl, and HOCl decomposes to HCl and nascent oxygen.

III. Short Answer / Conceptual Questions

21. Question: Explain why fluorine shows anomalous behavior compared to other halogens. Give two reasons.

22. Question: Describe the industrial preparation of chlorine by Deacon’s process. Write the balanced chemical equation.

23. Question: Arrange the following oxoacids of chlorine in increasing order of acidic strength: HOCl,HClO2​,HClO3​,HClO4​. Give a reason for this trend.

24. Question: Differentiate between the oxidizing power of F2​,Cl2​,Br2​, and I2​. Provide a chemical equation to support your answer.

25. Question: What are interhalogen compounds? Why are they generally more reactive than individual halogens (except F2​)?

26. Question: Give the structures and hybridization of the central atom in: a) BrF5​ b) ICl3​

27. Question: Write the chemical equation for the laboratory preparation of hydrogen chloride gas. State one property and one use of HCl.

28. Question: Explain the bleaching action of chlorine. Is it permanent or temporary?

29. Question: Why does fluorine not form oxoacids with positive oxidation states like +3,+5, or +7?

30. Question: Compare the electron gain enthalpy of Fluorine and Chlorine. Give a reason for the observed trend.

Answers and Explanations

I. Multiple Choice Questions (MCQs) – Answers

1. Answer: c) Fluorine Explanation: Fluorine is the most electronegative element in the entire periodic table due to its small size and high effective nuclear charge.

2. Answer: b) Chlorine Explanation: Chlorine has the highest negative (most exothermic) electron gain enthalpy among all elements in the periodic table. Fluorine has a less negative electron gain enthalpy than chlorine due to the small size of the fluorine atom, which leads to strong inter-electronic repulsions when an electron is added.

3. Answer: d) Fluorine Explanation: Fluorine exclusively shows a −1 oxidation state because it is the most electronegative element and does not have vacant d-orbitals to expand its octet. Other halogens can exhibit positive oxidation states due to the presence of vacant d-orbitals.

4. Answer: b) Cl2​>Br2​>F2​>I2​ Explanation: The bond dissociation enthalpy generally decreases down the group. However, F2​ has an anomalously low bond dissociation enthalpy due to the small size of fluorine atoms and strong interelectronic repulsions between the lone pairs on the two fluorine atoms. So, the correct order is Cl2​>Br2​>F2​>I2​.

5. Answer: a) Deacon’s Process Explanation: Deacon’s process (4HCl+O2​CuCl2​​2Cl2​+2H2​O) is an industrial method for the preparation of chlorine by the catalytic oxidation of HCl. Haber’s process is for ammonia, Contact process for sulphuric acid, and Ostwald’s process for nitric acid.

6. Answer: b) Oxidation Explanation: The bleaching action of chlorine is due to oxidation. Chlorine reacts with water to form hypochlorous acid (HOCl), which then decomposes to release nascent oxygen ([O]). This nascent oxygen oxidizes the coloured substance, making it colourless.

7. Answer: b) T-shaped Explanation: For ClF3​: The central atom is Cl. Valence electrons of Cl = 7. It forms 3 bonds with F atoms and has 2 lone pairs. (7 – 3 = 4 electrons, so 2 lone pairs). Total electron pairs = 3 (bond pairs) + 2 (lone pairs) = 5. According to VSEPR theory, this leads to a trigonal bipyramidal electron geometry. With two lone pairs occupying equatorial positions, the molecular geometry is T-shaped.

8. Question: d) HClO4​ Explanation: For oxoacids of the same halogen, the acidic strength increases with increasing oxidation state of the central halogen atom. This is because a higher oxidation state means more oxygen atoms are directly attached to the central halogen, increasing the electron-withdrawing effect, and thereby stabilizing the conjugate base more effectively. Oxidation states: HOCl(+1)<HClO2​(+3)<HClO3​(+5)<HClO4​(+7).

9. Answer: b) Chlorine Explanation: In the electrolytic process (e.g., Nelson cell or Castner-Kellner cell) for the electrolysis of brine (concentrated NaCl solution), chlorine gas is produced at the anode. Sodium hydroxide solution is produced near the cathode. Hydrogen gas is produced at the cathode. So, chlorine gas is the specific halogen element produced.

10. Answer: c) Reddish-brown Explanation: The halogens exhibit distinct colours: Fluorine is a pale yellow gas, Chlorine is a greenish-yellow gas, Bromine is a reddish-brown liquid, and Iodine is a violet-black solid.

11. Answer: d) They are always non-polar. Explanation: Interhalogen compounds are formed between two different halogens, meaning there will always be an electronegativity difference between them. This leads to polar X-Y bonds and, for non-symmetrical structures (XY3​,XY5​), a net dipole moment, making them polar molecules. All other statements are generally correct.

12. Answer: a) Fluorine Explanation: Fluorine forms only one oxoacid, HFO (hypofluorous acid). Unlike other halogens, it does not form oxoacids with positive oxidation states of +3,+5, or +7 due to the absence of vacant d-orbitals.

13. Answer: b) Small size of fluorine atom Explanation: Fluorine has a less negative (less exothermic) electron gain enthalpy than chlorine. This is primarily attributed to the exceptionally small size of the fluorine atom. When an incoming electron approaches the compact 2p subshell of fluorine, it experiences strong inter-electronic repulsions from the already existing electrons, which makes the addition of an electron less favorable (less energy released).

14. Answer: b) Pentagonal bipyramidal Explanation: For IF7​: The central atom is I. Valence electrons of I = 7. It forms 7 bonds with F atoms and has 0 lone pairs. Total electron pairs = 7. According to VSEPR theory, 7 electron pairs result in a pentagonal bipyramidal geometry.

15. Answer: b) It is extremely soluble in water. Explanation: Hydrogen chloride gas (HCl) is extremely soluble in water, forming hydrochloric acid. a) It is a colourless gas. c) It is not a reducing agent in its common form; rather, it provides H+ for various reactions. d) Rotten fish smell is characteristic of phosphine (PH3​). HCl has a pungent smell.

II. Assertion-Reason Type Questions – Answers

16. Answer: c) A is true but R is false. Explanation: Fluorine is indeed the most reactive halogen. However, the reason is incorrect. Fluorine has an anomalously low bond dissociation enthalpy among halogens (Cl2​>Br2​>F2​>I2​), not the lowest. Its high reactivity is mainly due to its high electronegativity and the relatively weak F-F bond that is easy to break.

17. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: HClO4​ (Perchloric acid) is a stronger acid than HClO3​ (Chloric acid). This is because the central chlorine atom in HClO4​ is in a higher oxidation state (+7) than in HClO3​ (+5). A higher oxidation state means more oxygen atoms are withdrawing electron density from the Cl-OH bond, making the O-H bond more polar and easier to ionize. This also leads to greater stabilization of the conjugate base (ClO4−​ vs ClO3−​) through resonance.

18. Answer: c) A is true but R is false. Explanation: Interhalogen compounds are generally more reactive than halogens (except fluorine). This is true. However, the reason is false. The X-Y bond in interhalogen compounds is typically weaker than the X-X bond in the diatomic halogens (except F-F). This weaker bond facilitates their reactivity.

19. Answer: d) A is false but R is true. Explanation: The Assertion is incorrect. Direct nitration of aniline is not preferred, but the reason given is false. The amino group (−NH2​) is an ortho-para directing group, not a meta-director. The problem in direct nitration of aniline is that it gets protonated to anilinium ion (C6​H5​NH3+​) in acidic medium, which is meta-directing and deactivating. So, while direct nitration is problematic, the reason for the amino group’s directivity is incorrectly stated.

20. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Chlorine water loses its yellow colour on standing due to the decomposition of hypochlorous acid (HOCl) which is formed when chlorine dissolves in water. HOCl is unstable and decomposes to hydrogen chloride (HCl) and nascent oxygen ([O]), leading to the disappearance of the yellow colour of chlorine. Cl2​+H2​O⇌HCl+HOCl HOCl→HCl+[O]

III. Short Answer / Conceptual Questions – Answers

21. Question: Fluorine shows anomalous behavior compared to other halogens due to a combination of factors:

  1. Extremely Small Size: Fluorine is the smallest element in Group 17. This small size results in high charge density and strong inter-electronic repulsions in its compact 2p orbitals.
  2. Highest Electronegativity: Fluorine is the most electronegative element in the entire periodic table (Pauling scale: 3.98). This high electronegativity significantly influences its bonding and reactivity.
  3. Absence of d-orbitals: Unlike other halogens (Cl, Br, I), fluorine does not have vacant d-orbitals in its valence shell. This restricts its covalency to a maximum of 1 and prevents it from expanding its octet.
  4. Low Bond Dissociation Enthalpy: The F-F bond in F2​ has an unusually low bond dissociation enthalpy (158kJ/mol) due to strong interelectronic repulsions between the lone pairs on the two small fluorine atoms.

22. Question: Deacon’s Process: This is an industrial process used for the preparation of chlorine (Cl2​) by the catalytic oxidation of hydrogen chloride (HCl) gas using atmospheric oxygen. Balanced Chemical Equation: 4HCl(g)+O2​(g)CuCl2​ catalyst,723K​2Cl2​(g)+2H2​O(g) Conditions: The reaction takes place at about 723K in the presence of a copper(II) chloride (CuCl2​) catalyst.

23. Question: The acidic strength of the oxoacids of chlorine increases with increasing oxidation state of the central chlorine atom. Increasing order of acidic strength: HOCl(+1)<HClO2​(+3)<HClO3​(+5)<HClO4​(+7) Reason: As the oxidation state of chlorine increases, the number of oxygen atoms directly bonded to the chlorine atom (and not to hydrogen) increases. These oxygen atoms are highly electronegative and withdraw electron density from the O-H bond, making the O-H bond more polar and easier to break (release H+). Furthermore, the increasing number of oxygen atoms helps to delocalize and stabilize the negative charge on the conjugate base through resonance, making the acid stronger.

24. Question: The oxidizing power of halogens decreases down the group. This means a halogen higher in the group can oxidize the halide ions of a halogen lower in the group from its salt solution. Order of oxidizing power: F2​>Cl2​>Br2​>I2​. Chemical Equation to support: Chlorine (Cl2​) can oxidize bromide ions (Br−) to bromine (Br2​). Cl2​(g)+2KBr(aq)→2KCl(aq)+Br2​(aq) (Here, Chlorine, being a stronger oxidizing agent, displaces Bromine from potassium bromide solution.) Similarly, bromine can oxidize iodide ions to iodine: Br2​(aq)+2KI(aq)→2KBr(aq)+I2​(aq)

25. Question:

  • Interhalogen Compounds: These are compounds formed between two different halogen elements. They have the general formula XXn′​, where X is the larger halogen and X’ is the smaller, more electronegative halogen, and n can be 1, 3, 5, or 7.
  • Why generally more reactive than individual halogens (except F2​)? Interhalogen compounds are generally more reactive than individual halogens (except F2​) because the bond between two different halogen atoms (X-X’) is weaker than the bond between two identical atoms (X-X or X’-X’), except for the F-F bond. For example, the Cl−F bond in ClF is weaker than the Cl−Cl bond in Cl2​. This weaker X-X’ bond is easier to break, leading to higher reactivity. (Note: The F-F bond in F2​ is exceptionally weak due to interelectronic repulsions, making F2​ extremely reactive).

26. Question: a) BrF5​: * Structure: Square pyramidal. * Hybridization: The central bromine atom is sp$^3d^2$ hybridized. (Valence electrons of Br = 7. Forms 5 bonds with F atoms, 2 electrons remaining = 1 lone pair. Total electron pairs = 5 bond pairs + 1 lone pair = 6. This corresponds to sp$^3d^2$ hybridization.) b) ICl3​: * Structure: Bent T-shape. (Exists as a dimer, I2​Cl6​, in solid/liquid, but monomeric ICl3​ is T-shaped). * Hybridization: The central iodine atom is sp$^3$d hybridized. (Valence electrons of I = 7. Forms 3 bonds with Cl atoms, 4 electrons remaining = 2 lone pairs. Total electron pairs = 3 bond pairs + 2 lone pairs = 5. This corresponds to sp$^3$d hybridization.)

27. Question: Laboratory Preparation of Hydrogen Chloride Gas (HCl): Hydrogen chloride gas is prepared in the laboratory by heating sodium chloride (NaCl) with concentrated sulphuric acid (H2​SO4​). Chemical Equation: NaCl(s)+H2​SO4​(l)420K​NaHSO4​(s)+HCl(g) One Property: Hydrogen chloride gas is extremely soluble in water, forming hydrochloric acid. One Use: It is used as a reagent in laboratories, for cleaning steel, and in the purification of common salt.

28. Question: Bleaching Action of Chlorine: The bleaching action of chlorine is due to oxidation. When chlorine gas is dissolved in water (forming chlorine water), it reacts to produce hypochlorous acid (HOCl) and hydrochloric acid (HCl). Cl2​(g)+H2​O(l)→HCl(aq)+HOCl(aq) Hypochlorous acid (HOCl) is unstable and readily decomposes to release nascent oxygen ([O]), which is a powerful oxidizing agent. HOCl(aq)→HCl(aq)+[O] This nascent oxygen then oxidizes the coloured organic matter, converting it into a colourless substance. Permanence: The bleaching action of chlorine is permanent because the coloured substance is destroyed by oxidation and cannot be restored. This is unlike bleaching by sulphur dioxide (SO2​), which is temporary and occurs due to reduction.

29. Question: Fluorine does not form oxoacids with positive oxidation states like +3,+5, or +7 for the following reasons:

  1. Highest Electronegativity: Fluorine is the most electronegative element in the entire periodic table. In its compounds with oxygen, oxygen will always be less electronegative than fluorine. Therefore, oxygen will carry a positive oxidation state, and fluorine will carry a negative oxidation state (−1). For example, in OF2​, oxygen is +2 and fluorine is −1.
  2. Absence of d-orbitals: Fluorine is in the second period and does not possess any vacant d-orbitals in its valence shell. This means it cannot expand its octet beyond a maximum covalency of 1 and thus cannot form multiple bonds with oxygen or exhibit higher positive oxidation states by promoting electrons to d-orbitals, unlike other halogens.

30. Question: Electron Gain Enthalpy (EGE) Comparison: The electron gain enthalpy is the energy released when an electron is added to a neutral gaseous atom. It generally becomes less negative (less exothermic) down a group. However, there is an anomaly when comparing Fluorine (F) and Chlorine (Cl).

  • Chlorine (Cl) has a more negative (more exothermic) electron gain enthalpy than Fluorine (F). (e.g., Cl: −349kJ/mol, F: −328kJ/mol) Reason for the observed trend: The less negative (less exothermic) electron gain enthalpy of fluorine compared to chlorine is primarily due to the exceptionally small size of the fluorine atom. When an electron approaches the compact 2p subshell of fluorine, it experiences significant inter-electronic repulsions from the already existing electrons in the small 2p orbitals. This strong repulsion hinders the effective attraction of the incoming electron by the nucleus, making the addition of an electron less energetically favorable (i.e., less energy is released). In contrast, chlorine is larger and has vacant 3d orbitals, so the incoming electron experiences less inter-electronic repulsion and can be more easily accommodated, leading to a more exothermic electron gain enthalpy.

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