Carbon and Its Various Forms

21. Which one of the following elements is used in pencil-lead?

Which one of the following elements is used in pencil-lead?

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Zinc
Lead
Carbon (Graphite)
Tin
This question was previously asked in
UPSC NDA-1 – 2017
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The element used in pencil-lead is Carbon in the form of graphite.
Pencil ‘lead’ is not made of the metal lead. It is primarily composed of graphite, which is an allotrope of Carbon, mixed with clay.
Graphite is a soft, black, slippery solid that conducts electricity. Its layered structure allows it to leave marks on paper. The amount of clay mixed with graphite determines the hardness of the pencil lead.

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22. Which one of the following elements forms highest number of compounds?

Which one of the following elements forms highest number of compounds?

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Oxygen
Hydrogen
Chlorine
Carbon
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UPSC NDA-1 – 2017
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Carbon forms the highest number of compounds among the given options. This is due to its unique chemical properties, forming the basis of organic chemistry.
Carbon atoms can form strong covalent bonds with other carbon atoms, creating long chains, branched structures, and rings (catenation). Carbon also forms strong bonds with many other elements, including hydrogen, oxygen, nitrogen, sulfur, and halogens. Carbon can form single, double, and triple bonds. This versatility allows carbon to form an enormous variety of stable compounds.
Hydrogen, oxygen, and chlorine form many compounds (oxides, acids, salts, hydrides, etc.), but the sheer number and diversity of organic compounds containing carbon far exceeds the number of compounds formed by these other elements. The field of organic chemistry is dedicated solely to the study of carbon compounds (with a few exceptions like carbon oxides, carbonates, and cyanides, which are often considered inorganic).

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23. Molecules of which of the following has cage like structure ? 1. Dia

Molecules of which of the following has cage like structure ?

1. Diamond

2. Graphite

3. Fullerenes

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1, 2 and 3
2 and 3 only
2 only
3 only
This question was previously asked in
UPSC NDA-1 – 2017
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Fullerenes are allotropes of carbon characterized by their spherical or cage-like structures. The most well-known fullerene is C60, which has a structure resembling a soccer ball. Diamond has a rigid, three-dimensional tetrahedral network structure. Graphite consists of layers of hexagonal carbon rings. Neither diamond nor graphite have discrete, cage-like molecules.
Fullerenes are molecular allotropes of carbon with characteristic closed cage structures.
Fullerenes were discovered in 1985 by Robert Curl, Harold Kroto, and Richard Smalley, who were awarded the Nobel Prize in Chemistry in 1996 for their discovery. They have potential applications in materials science, electronics, and medicine.

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24. Why is Graphite used in electrolytic cells ? A) Graphite is soft and c

Why is Graphite used in electrolytic cells ?
A) Graphite is soft and can be easily moulded into electrodes
B) Graphite is made of layers of carbon atoms which can slide
C) Graphite is inert to most of the chemicals and remains intact in electrolytic cells
D) Graphite is a good conductor of electricity

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Graphite is soft and can be easily moulded into electrodes
Graphite is made of layers of carbon atoms which can slide
Graphite is inert to most of the chemicals and remains intact in electrolytic cells
Graphite is a good conductor of electricity
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UPSC NDA-1 – 2015
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Graphite is used in electrolytic cells primarily because it is a good conductor of electricity. Electrodes serve as points of contact between the external electrical circuit and the electrolyte, facilitating the flow of current needed for electrolysis. Being a good conductor is a fundamental requirement for this function. While its relative inertness to many chemicals (option C) and ability to withstand high temperatures are also beneficial properties that make it a suitable material for electrodes in specific processes, the primary reason for its *use as an electrode* is its ability to conduct electricity. Options A and B describe physical properties that are not the main reasons for its use in this context.
Identifying the essential property of a material required for it to function as an electrode in an electrolytic cell.
Electrodes in electrolytic cells are either anodes or cathodes, where oxidation and reduction reactions occur, respectively. These reactions require the transfer of electrons between the external circuit and the ions in the electrolyte, which is facilitated by the electrical conductivity of the electrode material. Graphite’s conductivity stems from the delocalized electrons within its layered structure.

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25. Which one of the following statements is correct about diamond and

Which one of the following statements is correct about diamond and graphite?

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Diamond and graphite have similar physical and chemical properties.
Diamond is hard but graphite is smooth and slippery.
Diamond and graphite are both non-conductors of electricity.
Both diamond and graphite have similar structures.
This question was previously asked in
UPSC CDS-2 – 2023
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Diamond and graphite are allotropes of carbon, meaning they are different structural forms of the same element. Their vastly different physical properties stem from their different atomic structures.
In diamond, each carbon atom is bonded to four other carbon atoms in a rigid three-dimensional tetrahedral lattice, making it extremely hard and an electrical insulator. In graphite, carbon atoms are arranged in layers of hexagonal rings, with weak van der Waals forces between the layers. This layered structure allows the layers to slide past each other easily, making graphite soft, smooth, and slippery. Graphite also conducts electricity because of delocalized electrons within its layers.
While their physical properties are very different, their chemical properties (being forms of pure carbon) are similar, though reactivity can differ due to structural factors. For example, both burn in oxygen to form carbon dioxide, but diamond is much less reactive than graphite under normal conditions.

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26. Which of the following carbon allotropes is/are good conductor(s) of e

Which of the following carbon allotropes is/are good conductor(s) of electricity ?

  • 1. Diamond
  • 2. Graphite
  • 3. Fullerene

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1 only
1 and 2 only
2 only
1 and 3 only
This question was previously asked in
UPSC CDS-2 – 2021
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1. Diamond: In diamond, each carbon atom is covalently bonded to four other carbon atoms in a rigid tetrahedral structure. All valence electrons are involved in strong covalent bonds, and there are no free electrons. Hence, diamond is a very poor conductor of electricity (an insulator).
2. Graphite: In graphite, carbon atoms are arranged in layers of hexagonal rings. Within each layer, atoms are covalently bonded (sp2 hybridisation). Each carbon atom has one valence electron that is not involved in sigma bonding and is delocalized across the layers. These delocalized electrons are free to move, making graphite a good conductor of electricity.
3. Fullerene: Fullerenes, such as C60, have carbon atoms arranged in spherical or cage-like structures involving pentagons and hexagons. The bonding is predominantly sp2 hybridisation with some delocalized electrons. However, in their pure form, most fullerenes are semiconductors or insulators due to the localisation of the pi electrons compared to graphite. While some doped fullerenes can become conductors or even superconductors, pure fullerenes are not considered good conductors in the same way as graphite.
Therefore, among the listed common allotropes, only graphite is a good conductor of electricity.
– Electrical conductivity in carbon allotropes depends on the presence of delocalized or free electrons.
– Diamond has no free electrons.
– Graphite has delocalized pi electrons allowing it to conduct electricity.
– Pure fullerenes are typically semiconductors or insulators.
The differences in electrical conductivity (and other properties like hardness) among carbon allotropes arise from the different ways carbon atoms are bonded and arranged in their crystal structures. Graphite’s layered structure and delocalized electrons make it suitable for applications like electrodes and lubricants. Diamond’s strong 3D network makes it extremely hard.

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27. In graphite, each carbon atom is bonded to three other carbon atoms

In graphite, each carbon atom is bonded to three other carbon atoms

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forming a three-dimensional structure
in the same plane giving a hexagonal array
in the same plane giving a square array
in the same plane giving a pentagonal array
This question was previously asked in
UPSC CDS-2 – 2019
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Graphite has a layered structure. Within each layer, carbon atoms are arranged in a hexagonal lattice, where each carbon atom is covalently bonded to three other carbon atoms in the same plane. These layers are stacked on top of each other and held together by weak van der Waals forces.
Graphite has a layered structure with carbon atoms bonded hexagonally in flat planes.
This layered structure with weak interlayer forces explains graphite’s properties, such as its softness and lubricating ability, which contrast sharply with the hardness of diamond, where carbon atoms form a three-dimensional tetrahedral network.

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28. Which one of the following is * not * true for the form of carbon know

Which one of the following is *not* true for the form of carbon known as diamond?

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It is harder than graphite.
It contains the same percentage of carbon as graphite.
It is a better electric conductor than graphite.
It has different carbon to carbon distance in all directions.
This question was previously asked in
UPSC CDS-2 – 2018
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Option C is the correct answer. Diamond is an electrical insulator, whereas graphite is a good electrical conductor.
Diamond and graphite are allotropes of carbon, meaning they are made solely of carbon atoms but arranged differently. This structural difference leads to vastly different properties. Diamond has a tetrahedral structure where each carbon is bonded to four others, with all valence electrons tightly held in covalent bonds, making it very hard and a poor conductor of electricity. Graphite has a layered structure where each carbon is bonded to three others, forming hexagonal rings within layers. One valence electron per carbon atom is delocalized and free to move across the layers, making graphite soft and a good conductor of electricity. Therefore, the statement that diamond is a better electric conductor than graphite is false. Options A and B are true: diamond is much harder than graphite, and both are pure carbon, hence containing the same percentage of carbon (100%). Option D is generally considered true in that the distances between non-bonded carbon atoms vary depending on their relative positions in the lattice, unlike the consistent bond length. However, the distinct electrical conductivity is the most prominent difference.
Diamond is known for its extreme hardness, high refractive index, and high thermal conductivity (higher than most metals), but it is an electrical insulator. Graphite is soft, opaque, and a good electrical conductor, used in pencils and electrodes. This stark contrast in properties arises solely from the different arrangement of carbon atoms.

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29. Which one of the following is not an allotrope of carbon?

Which one of the following is not an allotrope of carbon?

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Coal
Diamond
Graphite
Graphene
This question was previously asked in
UPSC CDS-2 – 2016
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Allotropes are different physical forms in which an element can exist in the same physical state. Diamond, Graphite, and Graphene are distinct structural forms of pure carbon.
Diamond, Graphite, and Graphene are well-known allotropes of carbon with different crystal structures and physical properties. For example, diamond is very hard with a tetrahedral structure, graphite is soft with a layered structure, and graphene is a single layer of carbon atoms in a hexagonal lattice.
Coal, however, is a naturally occurring fossil fuel composed primarily of carbon but also containing varying amounts of hydrogen, oxygen, nitrogen, sulfur, and other elements, as well as mineral matter. It is a complex mixture, not a pure element existing in a specific crystalline or amorphous form.
Other allotropes of carbon include fullerenes (like Buckyballs), carbon nanotubes, and amorphous carbon. Coal contains carbon but is chemically and structurally too complex to be considered an allotrope of carbon.

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