The p-Block Elements (Noble Gases)

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

Chapter 7D: The p-Block Elements (Noble Gases – Group 18)

1. Introduction to Group 18 Elements

  • Group 18 elements are known as Noble Gases or Inert Gases or Aerogens. They include Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn).
  • The general electronic configuration is ns2np6 (except Helium, which is 1s2). This complete octet (or duplet for He) in their outermost shell makes them extremely stable and unreactive.
  • They are all gases at room temperature.
  • Radon is a radioactive element and is found in very minute quantities in the earth’s crust, primarily from the decay of Radium.

2. General Characteristics of Group 18 Elements

A. Electronic Configuration:

  • General: ns2np6 (octet), except He (1s2, duplet).
  • He: 1s2
  • Ne: [He]2s22p6
  • Ar: [Ne]3s23p6
  • Kr: [Ar]3d104s24p6
  • Xe: [Kr]4d105s25p6
  • Rn: [Xe]4f145d106s26p6

B. Atomic Radii:

  • Atomic radii increase down the group due to the addition of new electron shells.
  • They have the largest atomic radii in their respective periods due to the completely filled valence shells leading to high electron-electron repulsion and absence of strong attractive forces from other atoms (van der Waals radius is usually quoted).

C. Ionization Enthalpy:

  • Have very high ionization enthalpies in their respective periods. This is due to their stable, completely filled electronic configurations, making it difficult to remove an electron.
  • Ionization enthalpy generally decreases down the group due to increasing atomic size and shielding effect.

D. Electron Gain Enthalpy:

  • Have large positive (endothermic) electron gain enthalpies. This means they have no tendency to accept an additional electron as their valence shells are already completely filled and stable.

E. Melting and Boiling Points:

  • Have very low melting and boiling points due to the presence of only weak van der Waals forces (London dispersion forces) between their atoms.
  • Melting and boiling points increase down the group due to increasing atomic size and thus stronger van der Waals forces.

F. Chemical Reactivity:

  • Generally unreactive (inert) due to their stable electronic configuration (high ionization enthalpy and positive electron gain enthalpy).
  • Until 1962, they were considered completely inert. However, Neil Bartlett prepared the first noble gas compound, XePtF6​.
  • The reactivity generally increases down the group as ionization enthalpy decreases and atomic size increases, making the outer electrons less tightly held. Xenon (Xe) and Krypton (Kr) form compounds. Argon, Neon, and Helium are generally unreactive.

3. Compounds of Noble Gases

  • The only noble gases known to form a significant number of chemical compounds are Xenon (Xe) and, to a lesser extent, Krypton (Kr). This is because their ionization enthalpies are lower, and their atomic sizes are larger, making it easier to remove electrons or induce polarity.
  • Most noble gas compounds are formed with highly electronegative elements like Fluorine (F) and Oxygen (O).

A. Xenon-Fluorine Compounds:

  • XeF2​ (Xenon difluoride):
    • Preparation: Xe(g)+F2​(g)400∘C,1bar,Ni​XeF2​(s) (Excess Xe)
    • Structure: Linear (sp$^3$d hybridized Xe, 3 lone pairs).
    • Properties: White crystalline solid, strong fluorinating agent.
  • XeF4​ (Xenon tetrafluoride):
    • Preparation: Xe(g)+2F2​(g)600∘C,6bar,Ni​XeF4​(s) (Xe:F$_2$ ratio of 1:5)
    • Structure: Square planar (sp$^3d^2$ hybridized Xe, 2 lone pairs).
    • Properties: White crystalline solid, strong fluorinating agent.
  • XeF6​ (Xenon hexafluoride):
    • Preparation: Xe(g)+3F2​(g)300∘C,50−60bar,Ni​XeF6​(s) (Xe:F$_2$ ratio of 1:20)
    • Structure: Distorted octahedral (sp$^3d^3$ hybridized Xe, 1 lone pair).
    • Properties: Colourless solid, strongest fluorinating agent among Xenon fluorides.

B. Xenon-Oxygen Compounds:

  • XeO3​ (Xenon trioxide):
    • Preparation: Hydrolysis of XeF4​ or XeF6​. XeF4​+6H2​O→XeO3​+4Xe+24HF+3O2​ (Disproportionation) XeF6​+3H2​O→XeO3​+6HF
    • Structure: Pyramidal (sp$^3$ hybridized Xe, 1 lone pair).
    • Properties: Colourless, explosive solid.
  • XeOF4​ (Xenon oxytetrafluoride):
    • Preparation: Partial hydrolysis of XeF6​. XeF6​+H2​O→XeOF4​+2HF
    • Structure: Square pyramidal (sp$^3d^2$ hybridized Xe, 1 lone pair).
  • XeO2​F2​ (Xenon dioxydifluoride):
    • Preparation: Partial hydrolysis of XeF6​. XeF6​+2H2​O→XeO2​F2​+4HF
    • Structure: Trigonal bipyramidal (sp$^3$d hybridized Xe, 1 lone pair).

C. Krypton Compounds:

  • Only one compound, KrF2​ (Krypton difluoride), is known.
  • Preparation: By combining Kr and F$_2$ at 183K under electric discharge.
  • Properties: Highly unstable.

4. Uses of Noble Gases

  • Helium (He):
    • Non-flammable, light gas.
    • Used in filling balloons for meteorological observations.
    • Used in gas-cooled nuclear reactors.
    • Used in oxygen-helium mixtures for deep-sea divers (to avoid bends).
    • Used in cryogenics (maintaining very low temperatures for scientific research, MRI machines).
  • Neon (Ne):
    • Used in neon signboards (produces brilliant orange-red glow).
    • Used in beacon lights.
  • Argon (Ar):
    • Most abundant noble gas in air.
    • Used to provide an inert atmosphere for welding and metallurgical processes.
    • Used in filling incandescent and fluorescent lamps (prevents oxidation of filament).
  • Krypton (Kr):
    • Used in high-efficiency miner’s cap lamps.
    • Used in some photographic flash lamps.
  • Xenon (Xe):
    • Used in high-intensity discharge lamps and photographic flash bulbs.
    • Has limited uses due to its rarity and cost.
  • Radon (Rn):
    • Radioactive gas.
    • Used in cancer therapy (radiotherapy) and seismic activity detection.

NEET Chemistry: The p-Block Elements (Noble Gases) – Practice Questions

I. Multiple Choice Questions (MCQs)

1. Question: The general electronic configuration of noble gases is: a) ns2np5 b) ns2np6 c) ns2np4 d) (n−1)d10ns2np6

2. Question: Which noble gas has the lowest ionization enthalpy? a) Helium b) Neon c) Argon d) Radon

3. Question: Which of the following noble gases is radioactive? a) Xenon b) Krypton c) Argon d) Radon

4. Question: The geometry of XeF2​ is: a) Linear b) Trigonal planar c) Tetrahedral d) Bent

5. Question: Which of the following noble gas compounds has a square planar geometry? a) XeF2​ b) XeF4​ c) XeF6​ d) XeO3​

6. Question: The oxidation state of Xenon in XeO3​ is: a) +2 b) +4 c) +6 d) +8

7. Question: Which noble gas is used in filling balloons for meteorological observations? a) Neon b) Argon c) Helium d) Krypton

8. Question: The first noble gas compound was prepared by Neil Bartlett using: a) Xenon and platinum hexafluoride b) Krypton and fluorine c) Argon and oxygen d) Radon and fluorine

9. Question: Which of the following noble gas compounds is explosive? a) XeF2​ b) XeF4​ c) XeOF4​ d) XeO3​

10. Question: What is the hybridization of Xenon in XeF6​? a) sp$^3$ b) sp$^3dc)sp^3d^2$ d) sp$^3d^3$

11. Question: Which noble gas is filled in incandescent and fluorescent lamps to prevent oxidation of the filament? a) Neon b) Argon c) Krypton d) Xenon

12. Question: Which of the following is the correct order of increasing boiling points for noble gases? a) He < Ne < Ar < Kr < Xe < Rn b) Rn < Xe < Kr < Ar < Ne < He c) He < Ar < Ne < Kr < Xe < Rn d) He < Ne < Kr < Ar < Xe < Rn

13. Question: Which noble gas forms the maximum number of compounds? a) Helium b) Argon c) Krypton d) Xenon

14. Question: Deep-sea divers use a mixture of helium and oxygen instead of nitrogen and oxygen because: a) Helium is more soluble in blood than nitrogen. b) Helium is less soluble in blood than nitrogen. c) Helium is cheaper than nitrogen. d) Helium is heavier than nitrogen.

15. Question: The product of complete hydrolysis of XeF6​ is: a) XeF4​ b) XeOF4​ c) XeO3​ d) Xe

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): Noble gases have very high ionization enthalpies. Reason (R): They have completely filled valence electronic configurations.

17. Assertion (A): Helium is used in gas-cooled nuclear reactors. Reason (R): Helium is chemically inert and has a high thermal conductivity.

18. Assertion (A): Xenon forms compounds readily with fluorine. Reason (R): Xenon has a relatively low ionization enthalpy and a larger atomic size among noble gases.

19. Assertion (A): Noble gases have very low boiling points. Reason (R): They are monatomic gases and held by strong covalent forces.

20. Assertion (A): XeO3​ is an explosive solid. Reason (R): It has a pyramidal structure.

III. Short Answer / Conceptual Questions

21. Question: Why are Group 18 elements called ‘noble gases’?

22. Question: Explain why the atomic radii of noble gases are the largest in their respective periods.

23. Question: Give the preparation and structure of XeF4​.

24. Question: Write balanced chemical equations for the hydrolysis of: a) XeF6​ (complete hydrolysis) b) XeF4​ (complete hydrolysis)

25. Question: List two important uses of Helium.

26. Question: How does the reactivity of noble gases change down the group? Why?

27. Question: Draw the structure of XeOF4​ and state the hybridization of Xenon.

28. Question: Explain why deep-sea divers use a mixture of Helium and Oxygen.

29. Question: Despite having stable electronic configurations, Xenon and Krypton form compounds. Justify.

30. Question: What type of intermolecular forces exist between noble gas atoms? How does this affect their melting and boiling points?

Answers and Explanations

I. Multiple Choice Questions (MCQs) – Answers

1. Answer: b) ns2np6 Explanation: All noble gases have a completely filled octet in their outermost shell (ns2np6), except Helium, which has a duplet (1s2).

2. Answer: d) Radon Explanation: Ionization enthalpy decreases down a group due to increasing atomic size and shielding effect. Therefore, Radon (Rn), being at the bottom of Group 18, has the lowest ionization enthalpy.

3. Answer: d) Radon Explanation: Radon (Rn) is a naturally occurring radioactive noble gas. All other noble gases (He, Ne, Ar, Kr, Xe) are non-radioactive isotopes in their most common forms.

4. Question: a) Linear Explanation: For XeF2​: Xenon (Xe) is the central atom. Valence electrons of Xe = 8. It forms 2 bonds with F atoms, and has 3 lone pairs (8 – 2 = 6 electrons, so 3 lone pairs). Total electron pairs = 2 (bond pairs) + 3 (lone pairs) = 5. According to VSEPR theory, this leads to a trigonal bipyramidal electron geometry. With the three lone pairs occupying equatorial positions, the molecular geometry is linear.

5. Answer: b) XeF4​ Explanation: For XeF4​: Xenon (Xe) is the central atom. Valence electrons of Xe = 8. It forms 4 bonds with F atoms, and has 2 lone pairs (8 – 4 = 4 electrons, so 2 lone pairs). Total electron pairs = 4 (bond pairs) + 2 (lone pairs) = 6. This leads to an octahedral electron geometry. With two lone pairs occupying opposite axial positions, the molecular geometry is square planar.

6. Answer: c) +6 Explanation: In XeO3​, let the oxidation state of Xenon be x. Oxygen has an oxidation state of -2. x+3(−2)=0 x−6=0⇒x=+6.

7. Answer: c) Helium Explanation: Helium is a non-flammable and extremely light gas, making it ideal for filling balloons for meteorological observations.

8. Question: a) Xenon and platinum hexafluoride Explanation: Neil Bartlett prepared the first true chemical compound of a noble gas (XePtF6​) in 1962 by reacting Xenon with platinum hexafluoride (PtF6​).

9. Question: d) XeO3​ Explanation: Xenon trioxide (XeO3​) is a powerful explosive solid. Its decomposition into more stable products (xenon and oxygen) is highly exothermic.

10. Answer: d) sp$^3d^3$ Explanation: For XeF6​: Xenon (Xe) is the central atom. Valence electrons of Xe = 8. It forms 6 bonds with F atoms, and has 1 lone pair (8 – 6 = 2 electrons, so 1 lone pair). Total electron pairs = 6 (bond pairs) + 1 (lone pair) = 7. This corresponds to sp$^3d^3$ hybridization. The geometry is distorted octahedral due to the lone pair.

11. Answer: b) Argon Explanation: Argon is commonly used to fill incandescent and fluorescent lamps. Its inert nature prevents the oxidation of the hot filament, thereby prolonging the lamp’s lifespan.

12. Answer: a) He < Ne < Ar < Kr < Xe < Rn Explanation: Noble gases are monatomic and are held together only by weak London dispersion forces (a type of van der Waals force). As atomic size increases down the group, the strength of these forces increases, leading to a gradual increase in melting and boiling points.

13. Answer: d) Xenon Explanation: Due to its relatively lower ionization enthalpy and larger atomic size among the stable noble gases, Xenon forms the most extensive range of compounds with highly electronegative elements like fluorine and oxygen.

14. Answer: b) Helium is less soluble in blood than nitrogen. Explanation: Deep-sea divers use a mixture of helium and oxygen. Nitrogen is significantly more soluble in blood than helium. At high pressures under water, nitrogen would dissolve in the blood and then form painful bubbles (bends) upon depressurization. Helium’s lower solubility in blood minimizes this problem.

15. Answer: c) XeO3​ Explanation: Complete hydrolysis of XeF6​ yields XeO3​ (xenon trioxide) and HF. XeF6​+3H2​O→XeO3​+6HF

II. Assertion-Reason Type Questions – Answers

16. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Noble gases have very high ionization enthalpies. This is because their valence shells are completely filled (ns2np6 or 1s2), which makes their electronic configuration exceptionally stable and requires a large amount of energy to remove an electron.

17. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Helium is chemically inert and non-flammable, making it safe for use in nuclear reactors. Additionally, its high thermal conductivity allows it to efficiently transfer heat, acting as an effective coolant.

18. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Xenon forms compounds more readily than lighter noble gases. This is because, moving down the group, the ionization enthalpy decreases and the atomic size increases. This means the outermost electrons in Xenon are less tightly held and can be more easily involved in bond formation, especially with highly electronegative elements like fluorine.

19. Answer: c) A is true but R is false. Explanation: Noble gases are indeed monatomic gases and have very low boiling points. However, the reason is incorrect. They are held together by only very weak van der Waals forces (London dispersion forces), not strong covalent forces.

20. Answer: b) Both A and R are true but R is NOT the correct explanation of A. Explanation: XeO3​ is a known explosive solid (Assertion A is true). It has a pyramidal structure (Reason R is true). However, the pyramidal structure itself does not explain why it is explosive. Its explosiveness is due to its thermodynamic instability and the release of large amounts of energy upon decomposition to more stable products (Xe and O2​).

III. Short Answer / Conceptual Questions

21. Answer: Group 18 elements are called ‘noble gases’ because of their exceptional chemical inertness or very low reactivity. This inertness stems from their highly stable electronic configuration: they have a completely filled outermost shell (ns2np6, an octet) or a completely filled K-shell (1s2 for Helium). This stable configuration means they have no tendency to lose, gain, or share electrons under normal conditions, making them reluctant to participate in chemical reactions.

22. Answer: The atomic radii of noble gases are the largest in their respective periods. This might seem counterintuitive since atomic radii generally decrease across a period. However, for noble gases, only their van der Waals radii are quoted (as they do not form covalent or metallic bonds easily). For other elements, covalent or metallic radii are typically considered. Van der Waals radii are generally larger than covalent or metallic radii. Additionally, the completely filled valence shell in noble gases leads to maximum electron-electron repulsion within that shell, pushing the electron cloud outwards and contributing to their larger size.

23. Answer: Preparation of XeF4​: Xenon tetrafluoride (XeF4​) is prepared by heating xenon gas with fluorine gas in a specific molar ratio (e.g., 1:5) in a nickel or Monel vessel at a temperature of about 600∘C and a pressure of about 6bar. Xe(g)+2F2​(g)600∘C,6bar,Ni​XeF4​(s) Structure of XeF4​: The central Xenon atom in XeF4​ is sp$^3d^2$ hybridized. It has four bond pairs (with fluorine atoms) and two lone pairs of electrons. According to VSEPR theory, these six electron pairs arrange themselves in an octahedral electron geometry. The two lone pairs occupy opposite axial positions to minimize repulsion, resulting in a square planar molecular geometry.

24. Answer: a) Complete hydrolysis of XeF6​: XeF6​(s)+3H2​O(l)→XeO3​(s)+6HF(aq) (Xenon trioxide, which is an explosive solid, is formed) b) Complete hydrolysis of XeF4​: 6XeF4​(s)+12H2​O(l)→4Xe(g)+2XeO3​(s)+24HF(aq)+3O2​(g) (This is a disproportionation reaction where Xenon from +4 oxidation state forms both Xe(0) and XeO3​(+6)).

25. Answer: Two important uses of Helium:

  1. Cryogenics: Helium has the lowest boiling point of any known substance (4.2K). It is extensively used to produce and maintain extremely low temperatures required for various scientific research, superconducting magnets (e.g., in MRI scanners and NMR spectrometers), and in superconducting materials.
  2. Inflating Balloons and Airships: Due to its non-flammable nature and extreme lightness (second lightest element after hydrogen), helium is used for filling observation balloons, weather balloons, and earlier, airships, as a safer alternative to hydrogen.

26. Answer: The reactivity of noble gases increases down the group. Reason: As we move down Group 18 from Helium to Radon, the atomic size of the elements increases. This leads to:

  1. Decrease in Ionization Enthalpy: The outermost electrons are further from the nucleus and experience greater shielding. This makes it easier to remove an electron from the valence shell, thus lowering the ionization enthalpy.
  2. Increased Polarizability: Larger electron clouds are more easily distorted (polarized) by the presence of an approaching highly electronegative atom. These factors make the heavier noble gases (especially Xenon and Krypton) more willing to lose or share electrons to form chemical bonds, particularly with highly electronegative elements like Fluorine and Oxygen. Hence, their reactivity increases.

27. Answer: Structure of XeOF4​: The central Xenon atom in XeOF4​ is sp$^3d^2$ hybridized. It has five bond pairs (four with F atoms and one with O atom via double bond) and one lone pair of electrons. According to VSEPR theory, these six electron pairs result in an octahedral electron geometry. With the lone pair occupying one position, the molecular geometry is square pyramidal.

Drawing:

      O
      ||
      Xe
     /|\
    F F F
     \ /
      F

(Simplified representation; the lone pair would be opposite to the oxygen, causing the pyramidal shape.)

28. Question: Deep-sea divers use a mixture of Helium and Oxygen (typically 80% He, 20% O$_2$) instead of normal air (which is mostly nitrogen and oxygen) to avoid a condition known as “bends” or decompression sickness. Explanation:

  1. Solubility of Gases under Pressure: At high pressures encountered deep underwater, gases dissolve more readily in the blood.
  2. Nitrogen’s Effect: Nitrogen is significantly more soluble in blood than helium. If air is used, a large amount of nitrogen dissolves in the diver’s blood and tissues.
  3. Formation of Bubbles (Bends): When the diver ascends (depressurizes), the dissolved nitrogen comes out of solution and forms bubbles in the blood vessels and tissues, similar to opening a fizzy drink. These nitrogen bubbles cause excruciating pain, damage to tissues and joints, and can even be fatal.
  4. Helium’s Advantage: Helium is much less soluble in blood than nitrogen. Therefore, when a diver breathes a helium-oxygen mixture, much less gas dissolves in their blood at high pressure. Upon ascent, the smaller amount of dissolved helium diffuses out more quickly and causes fewer or no bubbles, thus preventing “bends.” Helium also has lower density, which reduces the effort of breathing at high pressures.

29. Question: Despite having exceptionally stable electronic configurations (ns2np6), Xenon (Xe) and Krypton (Kr) form compounds, primarily with highly electronegative elements like Fluorine and Oxygen. This can be justified by:

  1. Lower Ionization Enthalpy: As we move down Group 18, the ionization enthalpy decreases significantly. Xenon and Krypton have sufficiently large atomic sizes, which means their outermost valence electrons are less strongly attracted by the nucleus. This makes it possible to remove an electron or sufficiently polarize the electron cloud to participate in bond formation.
  2. Larger Atomic Size: Larger atomic size means that the valence electrons are further away from the nucleus and are more diffuse. This makes them more polarizable, allowing for stronger induced dipoles and greater overlap with the orbitals of highly electronegative atoms (like F and O).
  3. Strong Oxidizing Agents: The compounds are formed with highly electronegative and oxidizing elements like fluorine and oxygen, which have the strong ability to pull electrons from even the relatively stable noble gas atoms. Neil Bartlett’s first compound (XePtF6​) highlighted that if an oxidizing agent is strong enough to oxidize oxygen (to O2+​), it might also oxidize xenon due to their similar first ionization enthalpies.

30. Question:

  • Type of Intermolecular Forces: Noble gas atoms are monatomic. The only type of intermolecular forces existing between them are very weak London Dispersion Forces (a type of van der Waals forces). These forces arise from temporary, instantaneous dipoles formed due to fluctuations in electron distribution around the atoms.
  • Effect on Melting and Boiling Points: Because these London dispersion forces are extremely weak, very little energy is required to overcome them. Consequently, noble gases have very low melting points and boiling points. They exist as gases at room temperature and need to be cooled to extremely low temperatures to liquefy and solidify. The melting and boiling points increase slightly down the group because larger atoms (like Xe) have more electrons and thus more polarizable electron clouds, leading to stronger, albeit still weak, London dispersion forces.

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