The p-Block Elements (Carbon Family – Group 14)

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

Chapter 9: The p-Block Elements (Carbon Family – Group 14)

1. Introduction to Group 14 Elements

  • Group 14 elements (Carbon Family) include Carbon (C), Silicon (Si), Germanium (Ge), Tin (Sn), and Lead (Pb).
  • The general electronic configuration is ns2np2.
  • The sum of the group number and the number of valence electrons is 14.
  • Carbon is a non-metal, Silicon and Germanium are metalloids, and Tin and Lead are soft metals.

2. General Characteristics of Group 14 Elements

A. Electronic Configuration:

  • ns2np2.
  • C: [He]2s22p2
  • Si: [Ne]3s23p2
  • Ge: [Ar]3d104s24p2
  • Sn: [Kr]4d105s25p2
  • Pb: [Xe]4f145d106s26p2

B. Atomic and Ionic Radii:

  • Atomic and ionic radii increase down the group.
  • However, there is a small increase from Si to Ge (due to poor shielding of 3d electrons in Ge) and from Sn to Pb (due to poor shielding of 4f electrons in Pb, leading to lanthanoid contraction effects). This makes Pb’s size not as large as expected.

C. Ionization Enthalpy:

  • Generally decreases down the group due to increasing atomic size.
  • Irregularities: IE1​ of Ge is slightly higher than Si, and IE1​ of Pb is slightly higher than Sn. This is due to the poor shielding effect of d and f orbitals in Ge and Pb, respectively, leading to increased effective nuclear charge.

D. Electronegativity:

  • Electronegativity values slightly decrease from C to Si, then are almost constant for Ge, Sn, and Pb.
  • Carbon is the most electronegative in the group.

E. Oxidation States:

  • Common oxidation states are +4 and +2.
  • Due to ns2np2 configuration, they can lose all four valence electrons to show +4 oxidation state.
  • The tendency to show +2 oxidation state increases down the group.
    • This is due to the inert pair effect, where the reluctance of the ns2 electrons to participate in bonding becomes more pronounced as we move down the group.
    • Stability order of +2 oxidation state: C<Si<Ge<Sn<Pb
    • Stability order of +4 oxidation state: C>Si>Ge>Sn>Pb
  • Pb2+ is more stable than Pb4+, and Sn4+ is more stable than Sn2+.
  • PbO2​ (Pb in +4) is an oxidizing agent, SnCl2​ (Sn in +2) is a reducing agent.

F. Allotropy:

  • All elements of Group 14 exhibit allotropy except Lead.
  • Carbon: Diamond, Graphite, Fullerenes (and various amorphous forms like coal, coke, charcoal, lampblack).
  • Silicon: Amorphous and Crystalline forms.
  • Germanium: Amorphous and Crystalline forms.
  • Tin: White tin (β-Sn, metallic), Grey tin (α-Sn, non-metallic, brittle), Rhombic tin (γ-Sn).

G. Catenation:

  • The ability of atoms of an element to link with each other to form long chains or rings is called catenation.
  • Group 14 elements show catenation, with carbon having the maximum tendency due to its small size and strong C-C bonds.
  • The tendency for catenation decreases down the group as bond strength decreases and atomic size increases.
  • Order: C>>Si>Ge>Sn>Pb

3. Important Compounds of Carbon

A. Oxides of Carbon:

  1. Carbon Monoxide (CO):
    • Preparation:
      • On small scale: HCOOHConc.H2​SO4​,373K​CO+H2​O
      • On large scale: By passing steam over hot coke: C+H2​O1273K​CO+H2​ (water gas)
      • By passing air over hot coke: 2C+O2​+4N2​1273K​2CO+4N2​ (producer gas)
    • Properties: Colourless, odourless, poisonous gas. Highly reducing agent.
    • Uses: Fuel (water gas, producer gas), reducing agent in metallurgy.
  2. Carbon Dioxide (CO2​):
    • Preparation:
      • Combustion of carbon-containing fuels.
      • From limestone: CaCO3​Heat​CaO+CO2​
    • Properties: Colourless, odourless, non-combustible gas. Acidic oxide.
    • Uses: Photosynthesis, fire extinguishers, carbonated beverages, refrigerants (dry ice).

B. Allotropes of Carbon:

  1. Diamond:
    • Structure: Covalent network solid, each carbon is sp$^3$ hybridized, tetrahedrally bonded to four other carbon atoms. Strong C-C bonds.
    • Properties: Extremely hard, highest melting point, electrical insulator, transparent.
    • Uses: Cutting tools, abrasives, jewelry.
  2. Graphite:
    • Structure: Layered structure, each carbon is sp$^2$ hybridized, forming hexagonal rings within layers. Layers are held by weak van der Waals forces.
    • Properties: Soft, slippery, good electrical conductor (due to delocalized π electrons), good thermal conductor, opaque.
    • Uses: Lubricant, electrodes, pencil leads.
  3. Fullerenes:
    • Structure: Cage-like structures, e.g., C60​ (Buckminsterfullerene) has a football-like structure with 20 six-membered rings and 12 five-membered rings. Both sp$^2$ and sp$^3$ hybridizations are found.
    • Properties: Soluble in organic solvents, relatively reactive.
    • Uses: Superconductors, catalysts, potential in nanotechnology.

4. Silicon and its Compounds

A. Silica (SiO2​):

  • Found in nature as sand, quartz, cristobalite, tridymite.
  • Structure: Covalent network solid. Each silicon atom is tetrahedrally bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms. Silicon is sp$^3$ hybridized.
  • Properties: High melting point, hard, chemically inert, poor electrical conductor.
  • Uses: Glass, cement, ceramics, semiconductors (pure quartz).

B. Silicones:

  • Definition: Organosilicon polymers containing R2​SiO repeating units, characterized by −Si−O−Si− linkages.
  • Preparation: Hydrolysis of alkyl or aryl substituted chlorosilanes (Rn​SiCl4−n​). For example, (CH3​)2​SiCl2​ on hydrolysis gives (CH3​)2​Si(OH)2​, which then polymerizes via condensation to form linear silicones. (CH3​)2​SiCl2​H2​O​(CH3​)2​Si(OH)2​Polymerization​−(CH3​)2​SiO−n+nH2​O
  • Properties: Water repellent, high thermal stability, low temperature flexibility, electrical insulators, non-toxic.
  • Uses: Sealants, lubricants, greases, electrical insulators, water-proofing fabrics, cosmetic and surgical implants.

C. Silicates:

  • Compounds containing silicon-oxygen anions (silicate units). Basic building unit is the SiO44−​ tetrahedron.
  • Different types based on how SiO44−​ units share oxygen atoms (e.g., orthosilicates, pyrosilicates, cyclic silicates, chain silicates, sheet silicates, 3D silicates).
  • Examples: Feldspar, Zeolites, Asbestos, Mica.
  • Uses: Ceramics, cement, glass.

D. Zeolites:

  • Aluminosilicates with 3D network structures, where some silicon atoms are replaced by aluminum atoms.
  • Have porous structures (honeycomb-like), acting as molecular sieves.
  • Uses: Catalysts (e.g., ZSM-5 converts alcohols directly into gasoline), ion exchangers (softening of hard water).

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

I. Multiple Choice Questions (MCQs)

1. Question: Which of the following elements in Group 14 is a metalloid? a) Carbon b) Silicon c) Tin d) Lead

2. Question: The general electronic configuration of Group 14 elements is: a) ns2np1 b) ns2np2 c) ns2np3 d) ns2np4

3. Question: The most stable oxidation state of Lead (Pb) is: a) +4 b) +2 c) +1 d) +3

4. Question: Which allotrope of carbon is a good conductor of electricity? a) Diamond b) Graphite c) Fullerene d) All of the above

5. Question: Catenation property is maximum in: a) Silicon b) Germanium c) Carbon d) Tin

6. Question: In the structure of diamond, each carbon atom is: a) sp hybridized b) sp$^2$ hybridized c) sp$^3$ hybridized d) dsp$^2$ hybridized

7. Question: Water gas is a mixture of: a) CO+N2​ b) CO2​+N2​ c) CO+H2​ d) CO2​+H2​

8. Question: Which of the following is an example of an organosilicon polymer with −Si−O−Si− linkages? a) Silica b) Silicates c) Silicones d) Zeolites

9. Question: The basic building unit of silicates is: a) SiO2​ b) SiO32−​ c) SiO44−​ d) Si2​O76−​

10. Question: Which of the following is used as a catalyst to convert alcohols directly into gasoline? a) V2​O5​ b) ZSM-5 c) AlCl3​ d) TiCl4​

11. Question: Which statement is incorrect regarding CO2​? a) It is a linear molecule. b) It is an acidic oxide. c) It is used in photosynthesis. d) It supports combustion.

12. Question: The anomaly in the atomic radii trend from Sn to Pb is due to: a) Inert pair effect b) Lanthanoid contraction c) Both (a) and (b) d) Diagonal relationship

13. Question: Which of the following is not an amorphous form of carbon? a) Coal b) Coke c) Charcoal d) Diamond

14. Question: Which of the following is a non-stoichiometric compound? a) H2​O b) CO2​ c) Fex​O (where x < 1) d) NaCl

15. Question: What happens to the stability of the +2 oxidation state down Group 14? a) Increases b) Decreases c) Remains same d) First increases then decreases

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): Carbon exhibits catenation to the maximum extent. Reason (R): Carbon has a very small size and strong C-C bonds.

17. Assertion (A): PbCl4​ is less stable than PbCl2​. Reason (R): The +2 oxidation state of lead is more stable than its +4 oxidation state due to the inert pair effect.

18. Assertion (A): Graphite is a good conductor of electricity. Reason (R): It has a layered structure with sp$^2$ hybridized carbon atoms, and one free electron per carbon atom.

19. Assertion (A): Silica (SiO2​) is a covalent network solid. Reason (R): Each silicon atom is tetrahedrally bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms.

20. Assertion (A): Silicones are water repellent. Reason (R): The alkyl groups in silicones are hydrophobic and oriented outwards.

III. Short Answer / Conceptual Questions

21. Question: Explain the inert pair effect with reference to Group 14 elements.

22. Question: Draw the structure of C60​ (Buckminsterfullerene) and mention its two main characteristics.

23. Question: What are silicones? Give their general preparation and two uses.

24. Question: Differentiate between diamond and graphite based on their structure and electrical conductivity.

25. Question: Why is CO a poisonous gas?

26. Question: How are water gas and producer gas formed? What is their common use?

27. Question: Explain why the atomic radius of Ge is slightly higher than Si, and Pb is slightly higher than Sn, despite a general increase down the group.

28. Question: Give two uses of ZSM-5.

29. Question: Write the balanced chemical equation for the thermal decomposition of limestone.

30. Question: What is the basic structural unit of silicates? Draw its structure.

Answers and Explanations

I. Multiple Choice Questions (MCQs) – Answers

1. Answer: b) Silicon Explanation: Carbon is a non-metal, Silicon and Germanium are metalloids, and Tin and Lead are metals in Group 14.

2. Answer: b) ns2np2 Explanation: Group 14 elements have four valence electrons, two in the s-orbital and two in the p-orbital of the outermost shell.

3. Answer: b) +2 Explanation: Due to the inert pair effect, the stability of the +2 oxidation state increases down Group 14. Lead (Pb) exhibits the most prominent inert pair effect, making its +2 oxidation state more stable than its +4 oxidation state.

4. Answer: b) Graphite Explanation: Graphite has a layered structure with sp$^2$ hybridized carbon atoms. Each carbon atom forms three covalent bonds with adjacent carbon atoms within the layer, leaving one valence electron free to move within the layers. This delocalized electron cloud makes graphite a good conductor of electricity. Diamond is an insulator.

5. Answer: c) Carbon Explanation: Carbon has the strongest tendency for catenation among Group 14 elements due to its small size and the high strength of C-C bonds. The catenation tendency decreases down the group.

6. Answer: c) sp$^3$ hybridized Explanation: In diamond, each carbon atom is sp$^3$ hybridized and tetrahedrally bonded to four other carbon atoms, forming a rigid three-dimensional network.

7. Answer: c) CO+H2​ Explanation: Water gas is a mixture of carbon monoxide (CO) and hydrogen gas (H2​), produced by passing steam over hot coke.

8. Answer: c) Silicones Explanation: Silicones are organosilicon polymers characterized by the repeating −Si−O−Si− linkages, with organic groups (R) attached to silicon.

9. Answer: c) SiO44−​ Explanation: The fundamental building unit of all silicates is the SiO44−​ tetrahedron, where a silicon atom is at the center surrounded by four oxygen atoms.

10. Answer: b) ZSM-5 Explanation: ZSM-5 is a type of zeolite (aluminosilicate) catalyst primarily used in the petrochemical industry, famously for converting alcohols directly into gasoline.

11. Answer: d) It supports combustion. Explanation: Carbon dioxide (CO2​) is a non-combustible gas and does not support combustion. This property makes it useful in fire extinguishers.

12. Answer: b) Lanthanoid contraction Explanation: The small increase in atomic radii from Sn to Pb, making Pb’s size not as large as expected, is primarily attributed to the lanthanoid contraction. The filling of 4f orbitals before 5d and 6p orbitals in elements preceding Pb leads to a stronger nuclear pull due to poor shielding by f-electrons.

13. Answer: d) Diamond Explanation: Diamond is a crystalline allotrope of carbon with a highly ordered, well-defined crystal structure. Coal, coke, and charcoal are amorphous forms, lacking a regular long-range crystal structure.

14. Answer: c) Fex​O (where x < 1) Explanation: Interstitial compounds formed by transition metals are often non-stoichiometric, meaning the ratio of elements is not a simple whole number, indicating variable composition within certain limits (e.g., Fe0.95​O). H2​O, CO2​, and NaCl are stoichiometric compounds.

15. Answer: a) Increases Explanation: The stability of the +2 oxidation state increases down Group 14 from Carbon to Lead, primarily due to the inert pair effect. The ns2 electrons become increasingly reluctant to participate in bonding.

II. Assertion-Reason Type Questions – Answers

16. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Carbon exhibits the maximum tendency for catenation among all elements. This is because carbon is very small in size, which allows for strong C-C covalent bonds, and these bonds are stable enough to form long chains and rings.

17. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: The inert pair effect becomes significant for heavier elements in p-block. For Lead (Pb), the +2 oxidation state (6s2 electrons participating) is more stable than the +4 oxidation state (6s26p2 electrons participating) due to the reluctance of 6s2 electrons to participate in bond formation. Therefore, PbCl4​ (Pb in +4) is less stable and readily decomposes to PbCl2​ (Pb in +2).

18. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Graphite has a layered structure where each carbon is sp$^2$ hybridized, forming hexagonal rings. Each carbon atom is bonded to three other carbons, leaving one valence electron unbonded. These unbonded electrons become delocalized within the layers, making graphite an excellent electrical conductor.

19. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Silica (SiO2​) exists as a giant covalent network solid. Its structure involves each silicon atom being tetrahedrally bonded to four oxygen atoms, and each oxygen atom being bonded to two silicon atoms. This forms an extended three-dimensional network, characteristic of covalent network solids.

20. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Silicones are organosilicon polymers with alkyl (R) groups attached to silicon. These alkyl groups are hydrophobic (water-repelling) and are oriented outwards from the polysiloxane chain. This arrangement creates a water-repellent surface, making silicones useful for waterproofing fabrics and other materials.

III. Short Answer / Conceptual Questions – Answers

21. Answer: The inert pair effect refers to the reluctance of the ns2 electrons (the valence s-electrons) to participate in chemical bonding as we move down a group in the heavier p-block elements (especially from Group 13 to 16). In Group 14, elements typically show +4 oxidation state by using both ns2 and np2 electrons. However, for heavier elements like Tin (Sn) and Lead (Pb), the ns2 electrons are more tightly held by the nucleus and are less available for bonding. This effect makes the +2 oxidation state (where only np2 electrons participate) increasingly stable compared to the +4 oxidation state as we go down the group. For example, for Lead, Pb2+ is more stable than Pb4+, and for Tin, Sn4+ is more stable than Sn2+.

22. Answer: Structure of C60​ (Buckminsterfullerene): (You can imagine or sketch a hollow sphere resembling a soccer ball) It has a shape of a truncated icosahedron with 20 six-membered rings and 12 five-membered rings of carbon atoms. The carbon atoms are located at the vertices of these rings. Two main characteristics:

  1. Cage-like Structure: It’s a hollow, spherical molecule, often referred to as ‘buckyball’.
  2. Aromaticity/Delocalized Electrons: It exhibits some aromatic character due to delocalized π-electrons. All carbon atoms are sp$^2$ hybridized, but bond angles are slightly distorted from 120∘ to accommodate the curvature of the sphere.

23. Answer:

  • Silicones: Silicones are a class of organosilicon polymers that contain −Si−O−Si− (siloxane) linkages as their backbone, with organic groups (alkyl or aryl) attached to the silicon atoms.
  • General Preparation: Silicones are typically prepared by the hydrolysis of alkyl or aryl substituted chlorosilanes (Rn​SiCl4−n​), followed by polymerization. For example, the hydrolysis of dimethyldichlorosilane: (CH3​)2​SiCl2​+2H2​O→(CH3​)2​Si(OH)2​+2HCl The dihydroxy derivative then undergoes condensation polymerization: n(CH3​)2​Si(OH)2​→−(Si(CH3​)2​−O−)n​−+nH2​O (Linear silicone polymer)
  • Two Uses:
    1. As sealants and adhesives due to their flexibility and stability over a wide range of temperatures.
    2. In waterproofing fabrics and paper because they are water repellent.

24. Answer: | Feature | Diamond | Graphite | | :—————— | :—————————————– | :———————————————- | | Structure | Covalent network solid, 3D lattice | Layered structure | | Hybridization | Each C is sp$^3$ hybridized | Each C is sp$^2$ hybridized | | Bonding | Each C bonded to 4 other C atoms | Each C bonded to 3 other C atoms within a layer | | Electrical Conductivity | Insulator (no free electrons) | Good conductor (delocalized π electrons) | | Hardness | Extremely hard (hardest natural substance) | Soft and slippery |

25. Answer: Carbon monoxide (CO) is a highly poisonous gas because it has a strong affinity for the iron in hemoglobin (the oxygen-carrying pigment in blood). CO combines with hemoglobin to form carboxyhaemoglobin, which is much more stable (about 200-300 times) than oxyhaemoglobin. This stable complex reduces the oxygen-carrying capacity of the blood, leading to oxygen starvation in the body’s tissues and organs, which can be fatal.

26. Answer:

  • Water gas: It is produced by passing steam over hot coke (carbon) at a high temperature (1273K). C(s)+H2​O(g)1273K​CO(g)+H2​(g)
  • Producer gas: It is formed by passing air (a mixture of O2​ and N2​) over hot coke (carbon) at high temperatures. 2C(s)+O2​(g)+4N2​(g)1273K​2CO(g)+4N2​(g)
  • Common Use: Both water gas and producer gas are important industrial fuels. They are also used as synthesis gas (syngas) for the production of methanol and other hydrocarbons.

27. Answer: The general trend for atomic radii down a group is an increase due to the addition of new electron shells. However, in Group 14, there are slight deviations:

  1. Ge vs. Si: The atomic radius of Germanium (Ge) is only slightly larger than that of Silicon (Si). This is because Germanium follows the first transition series (3d series). The filling of 3d orbitals in the elements preceding Ge (Sc to Zn) leads to poor shielding of the outermost electrons. This poor shielding results in an increased effective nuclear charge, causing a slight contraction and thus a smaller than expected increase in atomic radius from Si to Ge.
  2. Pb vs. Sn: The atomic radius of Lead (Pb) is only slightly larger than that of Tin (Sn). This is due to the lanthanoid contraction. The filling of 4f orbitals in the elements preceding Lead (lanthanoids) results in very poor shielding of the outer electrons. This causes a significant increase in the effective nuclear charge pulling the outermost electrons closer to the nucleus, leading to a smaller than expected atomic radius for Pb.

28. Answer: ZSM-5 (Zeolite Socony Mobil-5) is an important zeolite catalyst with a unique porous structure. Two uses are:

  1. Conversion of Alcohols to Gasoline: It is used directly in the petrochemical industry to convert alcohols (like methanol) into gasoline by dehydrating them to hydrocarbons.
  2. Shape-Selective Catalysis: Due to its molecular sieve property (pores of specific sizes), it acts as a shape-selective catalyst in various organic reactions, allowing only molecules of a certain size and shape to enter and react within its pores.

29. Answer: The balanced chemical equation for the thermal decomposition of limestone (CaCO3​) is: CaCO3​(s)Heat​CaO(s)+CO2​(g) Calcium carbonate (limestone) decomposes upon heating to form calcium oxide (quicklime) and carbon dioxide gas.

30. Answer: The basic structural unit of all silicates is the silicon dioxide tetrahedron, SiO44−​. Structure: In this unit, a central silicon atom is bonded to four oxygen atoms, which are arranged tetrahedrally around the silicon atom.

Drawing:

     O
     |
    Si
   /|\
  O O O

(This is a simplified representation. Imagine a central Si atom with four O atoms connected to it in a tetrahedral arrangement. Each oxygen carries a negative charge, leading to the overall 4− charge.)

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