Which one of the following is an element ?

1. Classification of Matter: Pure Substances vs. Mixtures (basic)

Welcome to the first step of your journey into chemical principles! To understand chemistry, we must first learn how scientists view the world around us. In everyday life, we use the word "pure" to mean something unadulterated or healthy—like "pure honey" or "pure milk." However, in science, the definition is much stricter. A pure substance consists of only one type of particle, and all its constituent particles behave identically (Science, Class VIII, Chapter 7, p.130).

Pure substances are further divided into two categories: Elements and Compounds. Elements are the simplest form of matter; they consist of only one type of atom and cannot be broken down into anything simpler (Science, Class VIII, Chapter 7, p.130). For example, a diamond is a pure element because it is made entirely of Carbon atoms arranged in a rigid structure (Science, Class X, Chapter 4, p.61). Compounds, on the other hand, are made of two or more elements chemically combined in a fixed ratio, such as water (H₂O) or aluminum oxide (Al₂O₃).

In contrast, a mixture consists of two or more substances that are physically mixed together but not chemically bonded. In a mixture, the individual components retain their own original properties (Science, Class VIII, Chapter 7, p.130). For example, air is a mixture of gases like Oxygen, Nitrogen, and Carbon Dioxide. Even things that look uniform, like milk or fruit juice, are scientifically considered mixtures because they contain water, fats, proteins, and sugars mixed together (Science, Class VIII, Chapter 7, p.121).

Feature	Pure Substance	Mixture
Composition	Only one type of particle (atoms or molecules).	Two or more different substances mixed.
Separation	Cannot be separated by physical methods.	Can be separated by physical methods (filtration, evaporation).
Properties	Fixed and constant properties.	Shows the properties of its constituent components.

Sources: Science, Class VIII (NCERT 2025 ed.), Chapter 7: Particulate Nature of Matter, p.121, 130; Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.61

2. Characteristics of Elements and Compounds (basic)

To master chemistry, we must first distinguish between the two purest forms of matter: elements and compounds. An element is a substance that consists of only one type of atom. Because of this, it cannot be broken down into simpler substances by ordinary chemical means. Even when an element exists in different physical forms—known as allotropes—it remains elemental. For example, diamond is a pure form of the element carbon, where each atom is bonded to four others in a rigid, three-dimensional structure Science, Class X (2025 ed.), Carbon and its Compounds, p.61.

In contrast, a compound is a substance formed when two or more different elements chemically combine in a fixed ratio. The most fascinating thing about compounds is that they possess entirely different properties from the elements that make them up. For instance, aluminium (a metal) and oxygen (a gas) combine to form Aluminium Oxide (Al₂O₃), which is the chemical identity of gemstones like ruby and sapphire Science, Class X (2025 ed.), Metals and Non-metals, p.40. Unlike a simple mixture, a compound can only be separated into its original elements through chemical reactions, not physical ones.

To help you visualize the differences, consider this comparison:

Feature	Element	Compound
Composition	Consists of only one type of atom.	Consists of two or more different atoms.
Chemical Formula	Represented by a single symbol (e.g., C for Carbon).	Represented by a combination of symbols (e.g., Al₂O₃).
Properties	Retains the fundamental properties of that atom.	Properties are unique and different from its components.

Understanding these building blocks is essential because the way atoms bond—whether with their own kind to form elements or with others to form compounds—determines the stability and variety of matter we see in the world Science, Class X (2025 ed.), Carbon and its Compounds, p.62.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.61-62; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.40; Science, Class VIII (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.116

3. Non-Metals and the Unique Nature of Carbon (intermediate)

In our study of chemistry, we classify elements into two main categories: metals and non-metals. While non-metals are fewer in number compared to metals, they are the essential building blocks of life and the atmosphere. Most non-metals exist as gases (like Oxygen and Nitrogen) or solids (like Carbon and Sulphur) at room temperature, with Bromine being the notable exception as a liquid Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.39. Chemically, non-metals are defined by their tendency to gain or share electrons, a trait that reaches its peak of complexity in Carbon.

Carbon is often called a "friendly" element because of its extraordinary ability to form bonds Science, class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.65. It possesses two unique properties: catenation (the ability to form long chains by bonding with other carbon atoms) and tetravalency (having four valence electrons available for bonding). These properties allow carbon to form millions of compounds—far outnumbering the compounds of all other elements combined Science, class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.62. From the fuels we burn to the very structure of our DNA, carbon compounds are everywhere.

However, it is crucial to distinguish between a compound and an element. An element consists of only one type of atom. Carbon can exist in several pure physical forms known as allotropes. For example, in a diamond, every single atom is a carbon atom bonded in a rigid, three-dimensional tetrahedral structure Science, class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.61. This makes diamond a pure element. In contrast, many other gemstones like rubies or sapphires are compounds—specifically aluminium oxide (Al₂O₃) with trace impurities—meaning they contain different types of atoms bonded together.

Feature	Metals	Non-Metals (e.g., Carbon)
State	Mostly solids (except Mercury)	Solids, gases, and one liquid (Bromine)
Bonding	Tend to lose electrons	Tend to gain or share electrons (covalent)

Sources: Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.39; Science, class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.61; 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.65

4. Mineralogy: Oxides, Silicates, and Gemstones (intermediate)

In mineralogy, we distinguish between materials based on their chemical building blocks. A mineral is a naturally occurring substance with a fixed chemical composition and a definite internal structure. Most minerals are compounds—substances made of two or more elements chemically combined—but a rare few exist as pure elements, known as 'native elements' Science, Class VIII (NCERT 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130. For example, Diamond is an allotrope of carbon; it consists solely of carbon atoms arranged in a rigid, three-dimensional tetrahedral lattice Science, Class X (NCERT 2025), Chapter 4: Carbon and its Compounds, p.61. This makes it chemically distinct from most other gemstones, which are complex compounds. Most gemstones and industrial minerals fall into two major chemical categories: Oxides and Silicates.

• Oxides: These are minerals where a metal is bonded with oxygen. Bauxite, the primary ore of aluminium, is a rock consisting mainly of hydrated aluminium oxides Geography of India, Majid Husain, Resources, p.18. Similarly, Ruby and Sapphire are varieties of the mineral Corundum, which is chemically aluminium oxide (Al₂O₃) Science, Class X (NCERT 2025), Chapter 3: Metals and Non-metals, p.40.
• Silicates: These are the most abundant minerals in Earth's crust, containing silicon and oxygen. Quartz (SiO₂) and Feldspar (which makes up half the crust) are prime examples Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. Topaz is a more complex silicate, containing aluminium and fluorine (Al₂(F,OH)₂SiO₄).

Understanding these chemical identities is crucial because it determines a mineral's physical properties, such as hardness and luster. While Diamond is a pure element, its cousins in the jewelry box—like Ruby and Topaz—are chemical compounds structured in specific ratios of multiple elements.

Mineral Name	Chemical Classification	Chemical Formula
Diamond	Pure Element	C
Ruby/Sapphire	Oxide	Al₂O₃
Quartz	Silicate (Oxide-like)	SiO₂
Cinnabar	Sulphide	HgS

Sources: Science, Class VIII (NCERT 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130; Science, Class X (NCERT 2025), Chapter 4: Carbon and its Compounds, p.61; Science, Class X (NCERT 2025), Chapter 3: Metals and Non-metals, p.40; Geography of India, Majid Husain, Resources, p.18; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175

5. The Concept of Allotropy (intermediate)

In our journey through basic chemistry, we’ve established that an element is a pure substance consisting of only one type of atom Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.123. However, nature has a fascinating way of arranging these identical atoms in different patterns to create materials with vastly different properties. This phenomenon is known as Allotropy.

Allotropy refers to the property of some chemical elements to exist in two or more different physical forms, known as allotropes. It is vital to remember that while the chemical identity remains the same (they are made of the same atoms), their physical structures differ because the atoms are bonded together in different ways. For instance, Carbon can exist as a brilliant, hard diamond or as soft, black graphite. These are not different elements or compounds; they are simply different "avatars" of the pure element Carbon Science, Class X NCERT, Carbon and its Compounds, p.61.

To understand how structure dictates function, let's look at the two most famous allotropes of Carbon:

Feature	Diamond	Graphite
Atomic Bonding	Each carbon atom is bonded to 4 others in a rigid, 3D tetrahedral structure.	Each carbon atom is bonded to 3 others in hexagonal layers/sheets.
Physical Property	Hardest natural substance known; transparent.	Soft, slippery, and opaque; used as a lubricant and in pencils.
Conductivity	Electrical insulator.	Excellent conductor of electricity.

A common point of confusion in competitive exams is distinguishing between allotropes and compounds. A substance like Diamond is an allotrope (and thus a pure element) because it contains only carbon atoms. In contrast, many other precious stones like Ruby or Sapphire are actually chemical compounds. For example, Ruby is a variety of the mineral corundum, which is chemically Aluminum Oxide (Al₂O₃) Science, Class X NCERT, Metals and Non-metals, p.40. Because they involve different types of atoms (Aluminum and Oxygen) bonded together, they cannot be called allotropes.

Sources: Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.123; Science, Class X NCERT, Carbon and its Compounds, p.61; Science, Class X NCERT, Metals and Non-metals, p.40

6. Allotropes of Carbon: Diamond and Graphite (exam-level)

To understand the behavior of carbon, we must first grasp the concept of allotropy. Carbon is a versatile non-metal that exists in several distinct physical forms, known as allotropes. While these forms are chemically identical—meaning they are all composed entirely of pure carbon atoms—their physical properties differ drastically because of how those atoms are arranged in space Science, Class X (NCERT 2025 ed.), Chapter 3, p.40. This demonstrates a core principle in chemistry: structure determines function. In Diamond, each carbon atom is covalently bonded to four other carbon atoms, creating a rigid, three-dimensional tetrahedral structure. This dense, interconnected network makes diamond the hardest natural substance known and gives it an exceptionally high melting and boiling point Science, Class X (NCERT 2025 ed.), Chapter 4, p.61. In India, such natural diamonds are notably found in the basaltic intrusions of the Panna district in Madhya Pradesh Geography of India, Majid Husain, Resources, p.29. Conversely, Graphite consists of carbon atoms arranged in hexagonal layers. Within each layer, a carbon atom is bonded to only three others. This leaves one valence electron 'free' to move, making graphite an excellent conductor of electricity—a rare trait for a non-metal Science, Class X (NCERT 2025 ed.), Chapter 3, p.40. Beyond these two, science has identified other allotropes like Fullerenes. The most famous is C₆₀ (Buckminsterfullerene), where carbon atoms are arranged in the shape of a football Science, Class X (NCERT 2025 ed.), Chapter 4, p.61. Despite their differences in appearance and utility, all these substances are pure elements because they contain no atoms other than carbon.

Feature	Diamond	Graphite
Bonding	Each C bonded to 4 others	Each C bonded to 3 others
Structure	3D Tetrahedral (Rigid)	Hexagonal Layers (Slippery)
Conductivity	Insulator (No free electrons)	Good Conductor (Free electrons)
Hardness	Hardest natural substance	Smooth and slippery

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.39-40; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.61; Geography of India, Majid Husain, Resources, p.29

7. Solving the Original PYQ (exam-level)

This question tests your ability to apply the fundamental definition of an element—a substance consisting of only one type of atom—to real-world materials. While we often think of elements as simple gases or metals from the periodic table, the UPSC frequently tests your knowledge of allotropes. You have recently learned that certain elements can exist in different physical forms; here, that conceptual building block is the key to identifying the only pure substance in the list. The trick is to look past the physical appearance and focus on the atomic composition.

To arrive at the correct answer, (B) Diamond, you must recognize it as a crystalline form of pure carbon. As explained in Science, Class X (NCERT), each carbon atom in a diamond is covalently bonded to four other carbon atoms in a rigid tetrahedral structure. Because it is composed entirely of carbon atoms, it fits the strict definition of an element. The reasoning follows a simple path: if a substance can be represented by a single chemical symbol (like 'C' for Carbon), it is an element; if it requires a formula with multiple symbols, it is a compound.

The other options—Topaz, Ruby, and Sapphire—are common UPSC traps. These are compounds, meaning they are made of two or more different elements chemically bonded together. Ruby and Sapphire are both varieties of the mineral corundum, chemically known as Aluminium Oxide (Al2O3), while Topaz is a complex Aluminium Silicate. Do not be distracted by the "purity" or "clarity" of a gemstone; in chemistry, purity refers to the absence of different atoms, not the absence of color or flaws. Always prioritize the chemical formula over the physical category of the object.