Which one of the following allotropes of carbon is used for cutting and drilling ?

1. The Unique Nature of Carbon: Catenation and Tetravalency (basic)

To understand why the world around us—from the food we eat to the diamonds in a drill bit—is built on carbon, we must look at its two extraordinary chemical 'superpowers': Catenation and Tetravalency. Despite making up only about 0.02% of the Earth's crust, carbon forms more compounds than all other elements combined Science, Class X (NCERT 2025 ed.), Chapter 4, p. 58.

Catenation is carbon’s unique ability to form strong covalent bonds with other carbon atoms, resulting in massive, stable structures. This property allows carbon to link together in various geometries:

• Long straight chains (like those in fats and waxes).
• Branched chains (complex structures found in fuels).
• Closed rings (like those found in many medicines and pigments).

Furthermore, carbon atoms can be linked by single, double, or triple bonds. Compounds with only single bonds are called saturated, while those with double or triple bonds are unsaturated Science, Class X (NCERT 2025 ed.), Chapter 4, p. 62.

The second pillar is Tetravalency. Because carbon has four valence electrons, it has four 'hands' available to bond with other atoms (carbon, hydrogen, oxygen, nitrogen, etc.). This high bonding capacity, combined with catenation, is the reason carbon is the 'versatile element' that forms the backbone of all life Science, Class X (NCERT 2025 ed.), Chapter 4, p. 77. This versatility even extends to the physical world; for instance, the rigid, three-dimensional arrangement of carbon atoms in Diamond makes it the hardest natural substance known, essential for industrial cutting and drilling Science, Class X (NCERT 2025 ed.), Chapter 3, p. 40.

Property	Definition	Impact
Catenation	Self-linking ability with other Carbon atoms.	Forms chains, branches, and rings of various lengths.
Tetravalency	Having 4 valence electrons available for bonding.	Allows bonding with a wide variety of other elements.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.58, 62, 63, 77; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.40

2. Covalent Bonding in Carbon Compounds (basic)

To understand why carbon is the building block of life, we must first look at its valency. Carbon has an atomic number of 6, meaning it has four electrons in its outermost shell. To achieve a stable 'noble gas' configuration, it needs four more electrons. Rather than gaining or losing electrons (which would require immense energy), carbon shares its valence electrons with other atoms. This sharing of electron pairs between atoms is what we call a covalent bond Science, Class X (NCERT 2025 ed.), Chapter 4, p.60. Carbon is unique because of two specific properties that allow it to form millions of compounds:

• Tetravalency: Having four valence electrons, carbon can bond with four other atoms (like Hydrogen, Oxygen, or Nitrogen). For example, in Methane (CH₄), a single carbon atom shares electrons with four hydrogen atoms Science, Class X (NCERT 2025 ed.), Chapter 4, p.60.
• Catenation: This is carbon's 'superpower'—the ability to form strong covalent bonds with other carbon atoms, resulting in long chains, branched structures, or even rings Science, Class X (NCERT 2025 ed.), Chapter 4, p.62.

These bonds can be single, double, or triple. When carbon atoms are linked by only single bonds, the compounds are called saturated (like Ethane, C₂H₆). If they contain double or triple bonds, they are unsaturated (like Ethene, C₂H₄) Science, Class X (NCERT 2025 ed.), Chapter 4, p.63. While the covalent bonds within these molecules are very strong, the forces between the molecules (intermolecular forces) are generally weak, which is why many carbon compounds have low melting and boiling points.

Feature	Saturated Compounds	Unsaturated Compounds
Bond Type	Only single bonds between carbons	Double or triple bonds present
Reactivity	Generally less reactive	More reactive
Example	Methane (CH₄), Ethane (C₂H₆)	Ethene (C₂H₄), Ethyne (C₂H₂)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.60; Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.62; Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.63

3. Understanding Allotropy in Elements (intermediate)

In our journey through basic chemistry, we often think of an element as having a single set of properties. However, nature is more creative. Allotropy is the property by which a single element can exist in two or more different forms in the same physical state. While the atoms are the same, the way they are arranged in space differs significantly. As you'll see in Science, Class VIII (NCERT 2025 ed.), Nature of Matter: Elements, Compounds, and Mixtures, p. 131, elements are pure substances that cannot be broken down further, but allotropy shows us that even pure elements can have diverse "personalities."

Carbon is the most famous example of this. In Diamond, each carbon atom is bonded to four other carbon atoms in a rigid, three-dimensional tetrahedral structure. This makes it the hardest natural substance known, with a Mohs hardness of 10. In contrast, Graphite consists of carbon atoms arranged in hexagonal layers that slide over one another easily. This structural difference is why diamond is used for heavy-duty industrial drilling, while graphite is used as a lubricant or in pencil leads Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p. 61.

Feature	Diamond	Graphite
Structure	3D Rigid Tetrahedral	Hexagonal Layers
Hardness	Extremely Hard	Soft and Slippery
Conductivity	Insulator	Good Conductor
Primary Use	Cutting/Drilling	Lubrication/Electrodes

Beyond these, carbon also forms Fullerenes (like C₆₀), where atoms are arranged in the shape of a football. Interestingly, while the physical properties (like hardness or melting point) vary wildly between these allotropes, their chemical properties remain similar because they are all made of the same fundamental unit: Carbon. Approximately 80% of mined diamonds, known as bort, are too low-quality for jewelry and are instead utilized for their extreme hardness in industrial tools Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p. 40.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.61; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.40; Science, Class VIII (NCERT 2025 ed.), Nature of Matter: Elements, Compounds, and Mixtures, p.131

4. Amorphous Carbon: Charcoal, Lampblack, and Activated Carbon (intermediate)

In our study of carbon, we often focus on the brilliant structure of diamonds or the slippery layers of graphite. However, carbon also exists in amorphous (meaning 'without form') varieties. While they may appear like simple black powders, they are technically microcrystalline, consisting of tiny, disorganized fragments of graphite-like structures. These forms are typically produced through the incomplete combustion of organic matter or the destructive distillation (heating in the absence of air) of carbon-rich substances. Two of the most common forms are Charcoal and Lampblack. Charcoal is made from wood or animal bones and is highly porous. Lampblack, often referred to in environmental studies as Black Carbon or soot, is a fine powder collected from the smoke of burning oils or fats. Unlike CO₂, which is a gas, Black Carbon consists of solid particles or aerosols that strongly absorb sunlight, making it a significant driver of atmospheric warming Environment, Shankar IAS Academy (ed 10th), Climate Change, p.258. Because it results from incomplete combustion, it is a major component of emissions from diesel engines and wood burning Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Environmental Degradation and Management, p.54. Perhaps the most industrially significant version is Activated Carbon. By treating ordinary charcoal with steam or chemicals at high temperatures, we 'activate' it, essentially blowing open millions of tiny pores. This creates an incredibly large surface area. This structure allows it to perform adsorption—a process where molecules of gases or liquids physically stick to its surface. This makes activated carbon indispensable for water purification, gas masks, and even medical treatments to neutralize swallowed poisons.

Form	Primary Source	Key Characteristic
Charcoal	Wood or Bone	Highly porous; used as fuel and for filtration.
Lampblack (Soot)	Oils/Fats/Fossil Fuels	Fine particles; used in printing inks, rubber tires, and black paint.
Activated Carbon	Processed Charcoal	Extreme surface area; specialized for high-efficiency adsorption.

Sources: Environment, Shankar IAS Academy (ed 10th), Climate Change, p.258; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Environmental Degradation and Management, p.54

5. Graphite: Structure, Lubrication, and Conductivity (intermediate)

In our exploration of chemical principles, Graphite stands out as a fascinating example of how the arrangement of atoms—rather than the atoms themselves—dictates a material's personality. While both diamond and graphite are composed entirely of carbon (making them allotropes), their internal structures are starkly different. In graphite, each carbon atom is bonded to only three other carbon atoms within the same plane, creating a flat, hexagonal array that looks like a honeycomb Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.61. To satisfy carbon's valency of four, one of these bonds is a double bond.

This layered architecture is the secret behind graphite's role as a lubricant. These hexagonal sheets are stacked on top of one another, but the forces holding the layers together are relatively weak. As a result, the layers can easily slide over each other, making graphite smooth and slippery to the touch Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.61. This is why graphite is used in pencil leads and as a dry lubricant in heavy machinery where liquid oils might fail.

Perhaps most surprisingly for a non-metal, graphite is an excellent conductor of electricity Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.40. In most non-metals, electrons are tightly bound in bonds and cannot move. However, because each carbon in graphite is only bonded to three neighbors, the fourth valence electron is "delocalized." These free electrons are mobile and can move through the layers, allowing graphite to carry an electric current—a property that makes it invaluable for making electrodes in batteries and electrolysis.

Feature	Diamond	Graphite
Bonding	Each C bonded to 4 others	Each C bonded to 3 others
Geometry	Rigid 3D structure	Flat hexagonal layers
Physical Feel	Hardest natural substance	Soft and slippery
Conductivity	Insulator (no free electrons)	Good conductor (free electrons)

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.61; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.40

6. Modern Allotropes: Fullerenes and Graphene (exam-level)

While diamond and graphite are the most famous forms of carbon, material science has expanded our horizon with modern allotropes like Fullerenes and Graphene. Allotropes are different physical forms of the same element; they share the same chemical properties but differ vastly in their physical structure and behavior Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p. 61. While diamond forms a rigid 3D lattice and graphite forms layers, modern allotropes often exist at the nanoscale, offering revolutionary possibilities in technology and environmental science.

Fullerenes represent a unique class of carbon allotropes where atoms are arranged in closed cages. The most prominent member is C₆₀ (Buckminsterfullerene), which consists of 60 carbon atoms arranged in a spherical shape resembling a football Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p. 61. Named after the architect Buckminster Fuller because the structure looks like his geodesic domes, these molecules are being researched for drug delivery, superconductors, and specialized lubricants due to their hollow, stable cage-like geometry.

Another "wonder material" is Graphene, which is essentially a single, one-atom-thick layer of carbon atoms arranged in a hexagonal honeycomb lattice. It is the fundamental building block for other allotropes (wrapping it creates fullerenes, rolling it creates nanotubes). A remarkable derivative is Graphene Aerogel, currently recognized as the lightest material on earth Science, Class VIII (NCERT 2025 ed.), Nature of Matter: Elements, Compounds, and Mixtures, p. 129. Because it is highly porous and has a massive surface area relative to its weight, it can absorb up to 900 times its own weight in oil, making it an ideal candidate for cleaning up oil spills in oceans.

Allotrope	Structure	Key Property/Use
Buckminsterfullerene (C₆₀)	Spherical (Football-like) cage	Nanotechnology and specialized chemical delivery.
Graphene	2D Single-layer hexagonal sheet	Extreme strength and electrical conductivity.
Graphene Aerogel	Highly porous 3D carbon structure	Ultra-lightweight; used for oil spill cleanup.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.61; Science, Class VIII (NCERT 2025 ed.), Nature of Matter: Elements, Compounds, and Mixtures, p.129

7. Physical Properties and Industrial Applications of Diamond (intermediate)

At its core, a diamond is a pure form of carbon where each atom is bonded to four others in a rigid, three-dimensional tetrahedral arrangement. This specific geometry creates an incredibly strong structure, making diamond the hardest natural substance known Science, class X (NCERT 2025 ed.), Chapter 4, p. 61. While diamond and graphite are both composed of carbon, their physical properties are vastly different due to this internal bonding: diamond's atoms are locked in a cage-like lattice, whereas graphite's atoms are arranged in layers that can slide past one another, making it soft and slippery Science, class X (NCERT 2025 ed.), Chapter 3, p. 40.

Beyond its mechanical strength, diamond possesses unique optical and electrical characteristics. It has a remarkably high refractive index of 2.42, which causes light to bend sharply and reflect internally, giving it its signature brilliance Science, class X (NCERT 2025 ed.), Chapter 10, p. 150. Furthermore, despite being a record-breaking thermal conductor, it is an electrical insulator with extremely high resistivity (10¹² to 10¹³ Ωm), a stark contrast to graphite, which is an excellent conductor of electricity Science, class X (NCERT 2025 ed.), Chapter 11, p. 179.

These properties translate into significant industrial power. Approximately 80% of mined diamonds (often referred to as bort) are used for industrial applications like cutting, drilling, and grinding because no other naturally occurring material can scratch or wear them down. In India, while the Panna district of Madhya Pradesh is a primary source of raw diamonds, cities like Surat and Ahmedabad have become global leaders in the specialized industry of diamond cutting and polishing Geography of India, Majid Husain (9th ed.), Resources, p. 29.

Feature	Diamond	Graphite
Hardness	Extremely Hard (Mohs 10)	Soft and Slippery
Electrical Nature	Insulator	Good Conductor
Structural Form	Rigid 3D Lattice	Hexagonal Layers

Sources: Science, class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.61; Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.40; Science, class X (NCERT 2025 ed.), Chapter 10: Light – Reflection and Refraction, p.150; Science, class X (NCERT 2025 ed.), Chapter 11: Electricity, p.179; Geography of India, Majid Husain (9th ed.), Resources, p.29

8. Solving the Original PYQ (exam-level)

Now that you have mastered the concept of allotropy, you can see how the UPSC tests your ability to link atomic structure to real-world applications. The building blocks you just learned—specifically the three-dimensional tetrahedral arrangement of carbon atoms—are exactly what make this answer clear. In Diamond, each carbon atom is bonded to four others in a rigid, interlocking lattice. This structure makes it the hardest natural substance known, with a Mohs hardness of 10. When you see a question about cutting or drilling, your mind should immediately go to the material that no other substance can scratch: (A) Diamond.

To arrive at the correct answer, you must apply the logic of physical properties. Since the task involves cutting and drilling, the primary requirement is extreme hardness and durability. As noted in Science, class X (NCERT 2025 ed.) > Chapter 4, while all these options are made of carbon, only diamond possesses the rigid covalent network necessary to penetrate rock or metal. This is why approximately 80% of industrial-grade diamonds, often called bort, are used specifically for drill bits and saw blades.

UPSC often includes Graphite as a distractor because it is the most well-known allotrope alongside diamond; however, its hexagonal layered structure makes it soft and slippery—perfect for lubrication but useless for cutting. Activated carbon and Carbon black are common traps that focus on chemical properties like surface area (for adsorption) or pigmentation. By remembering that structure dictates function, you can easily eliminate these options and identify Diamond as the only logical choice for high-stress mechanical tasks.