Which one of the following elements is a non-metal but is lustrous ?

1. Physical Properties of Metals (basic)

When we look at the world around us, elements are the basic building blocks of everything we see. To make sense of these elements, scientists classify them primarily into metals and non-metals based on their physical characteristics Science, Class X (NCERT 2025 ed.), Chapter 3, p.37. Metals are generally hard, solid, and possess a distinct metallic lustre, meaning they have a shining surface when pure.

Two of the most remarkable properties of metals are malleability and ductility. Malleability is the ability of a metal to be beaten into thin sheets (think of the silver foil on sweets or aluminum foil), while ductility is the ability to be drawn into thin wires. For instance, gold is so incredibly ductile that a wire about 2 km long can be drawn from just one gram of it! Science, Class X (NCERT 2025 ed.), Chapter 3, p.38. Furthermore, metals are typically excellent conductors of heat and electricity, which is why your cooking vessels and electrical wires are made of materials like copper and aluminum.

However, science is often defined by its exceptions. While most metals are solid at room temperature and have high melting points, mercury is a notable exception as it exists as a liquid at room temperature Science, Class X (NCERT 2025 ed.), Chapter 3, p.39. On the flip side, non-metals are usually brittle and dull, but iodine stands out because it is a non-metal that possesses a lustrous, shiny appearance just like a metal.

Property	Metals	Non-Metals
Appearance	Lustrous (Shiny)	Dull (Except Iodine)
Physical State	Mostly Solids (Except Mercury)	Solids, Liquids, or Gases
Ability to shape	Malleable & Ductile	Brittle (Break easily)
Conductivity	Good conductors of heat/electricity	Poor conductors (Except Graphite)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.37; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.38; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.39; Science, Class VII, NCERT(Revised ed 2025), Chapter 4: The World of Metals and Non-metals, p.54

2. Physical Properties of Non-metals (basic)

While metals often dominate our visual landscape with their shine and strength, non-metals are the essential building blocks of life (like oxygen and carbon). Unlike the uniform nature of most metals, non-metals are physically diverse, appearing as solids, gases, or even liquids at room temperature. For instance, while hydrogen and oxygen are gases, carbon and sulfur are solids, and bromine is a unique example of a non-metal that exists as a liquid Science, Class X, Metals and Non-metals, p.39.

To understand non-metals, it is best to view them as the physical opposites of metals. Where metals are strong and flexible, non-metals are often brittle—meaning if you hit a piece of coal or sulfur with a hammer, it will shatter into pieces rather than flattening into a sheet Science, Class VII, The World of Metals and Non-metals, p.44. They also act as poor conductors of heat and electricity, which is why we use non-metallic materials like plastic or rubber to insulate electrical wires and protect ourselves from shocks Science, Class VII, Electricity: Circuits and their Components, p.36.

However, chemistry is a science of exceptions. While most non-metals are dull and non-lustrous, iodine stands out because it possesses a distinct metallic-like shine or luster Science, Class VII, The World of Metals and Non-metals, p.54. This illustrates that "non-metal" is a broad category with fascinating outliers.

Comparison of Physical Properties

Property	Metals	Non-Metals
Lustre	Shiny (Lustrous)	Generally dull (Exception: Iodine)
Physical State	Mostly solids	Solids, liquids (Bromine), or gases
Malleability	Can be beaten into sheets	Brittle (shatter when struck)
Conductivity	Good conductors	Poor conductors (Insulators)

Sources: Science, Class X, Metals and Non-metals, p.39; Science, Class VII, The World of Metals and Non-metals, p.44; Science, Class VII, The World of Metals and Non-metals, p.54; Science, Class VII, Electricity: Circuits and their Components, p.36

3. Chemical Nature: Metallic vs Non-metallic Oxides (intermediate)

When elements react with oxygen, they form compounds known as oxides. The chemical nature of these oxides—whether they are acidic, basic, or neutral—is one of the most reliable ways to distinguish between metals and non-metals. At its root, this behavior is determined by how easily the element bonded to oxygen can release or attract electrons in a chemical environment.

Metallic oxides are generally basic in nature. This is because when they react with water, many form bases (hydroxides), and more significantly, they react with acids to produce salt and water, a classic neutralization reaction Science, Class X (NCERT 2025 ed.), Chapter 2, p. 22. For instance, when copper is heated, it forms copper(II) oxide (2Cu + O₂ → 2CuO). If you react this black copper oxide with hydrochloric acid, it forms copper chloride and water, demonstrating its basic character Science, Class X (NCERT 2025 ed.), Chapter 3, p. 41.

On the other hand, non-metallic oxides are typically acidic. When dissolved in water, they produce acidic solutions (like sulfur dioxide forming sulfurous acid). However, the world of chemistry isn't always binary. There are fascinating "dual-natured" oxides called amphoteric oxides. These oxides, such as Aluminium oxide (Al₂O₃) and Zinc oxide (ZnO), show both acidic and basic behavior, reacting with both acids and bases to yield salt and water Science, Class X (NCERT 2025 ed.), Chapter 3, p. 41.

Feature	Metallic Oxides	Non-Metallic Oxides
General Nature	Basic	Acidic
Reaction with Acid	Forms Salt + Water	Generally No Reaction
Examples	Na₂O, MgO, CuO	SO₂, CO₂, NO₂

Sources: Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.40; Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.41; Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.22

4. Metalloids: The Bridge Elements (intermediate)

In our journey through the periodic table, we have seen a clear divide between metals (the sturdy conductors) and non-metals (the insulators). However, nature rarely works in absolute blacks and whites. Between these two categories lies a fascinating group of elements known as metalloids or semimetals. These elements act as a bridge, possessing a unique mix of properties that don't allow them to be classified strictly as one or the other Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.123.

The defining characteristic of metalloids is their intermediate nature. For instance, elements like Silicon (Si) and Germanium (Ge) often look like metals because they have a distinct metallic luster (they are shiny). However, if you were to hit them with a hammer, they wouldn't flatten into sheets like gold or copper; instead, they would shatter. This brittleness is a classic non-metallic trait Science, Class X, Metals and Non-metals, p.39. This combination of being "shiny but fragile" makes them chemically and industrially unique, particularly in the electronics industry where their semiconducting properties are used to control electrical flow.

To keep your concepts sharp for the UPSC, it is vital to distinguish metalloids from exceptions in the other categories. For example, Iodine is a non-metal that is famously lustrous (shiny), yet it is not a metalloid because it lacks the intermediate chemical and conductive properties found in elements like Boron or Silicon Science, Class X, Metals and Non-metals, p.40. The following table helps visualize where these "bridge elements" sit in the spectrum of properties:

Property	Metals	Metalloids	Non-Metals
Appearance	Lustrous (Shiny)	Often Lustrous	Generally Dull (except Iodine)
Physical State	Malleable & Ductile	Brittle	Brittle (if solid)
Conductivity	High	Semiconductors	Poor (except Graphite)

Beyond physical appearance, metalloids also bridge the chemical gap. While metals typically form basic oxides and non-metals form acidic oxides, metalloids (and some metals like Aluminum) often show amphoteric tendencies, meaning they can react with both acids and bases Science, Class X, Metals and Non-metals, p.55. This versatility is why silicon is the backbone of modern computing and why boron is essential in heat-resistant glass.

Sources: Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.123; Science, Class X NCERT, Metals and Non-metals, p.39-40, 55

5. Carbon Allotropes: Conductors and Brilliance (intermediate)

In the study of chemistry, carbon is often called the 'king of elements' due to its sheer versatility. While it is classified as a non-metal, it doesn't always behave like one. This is best seen through allotropy — a phenomenon where a single element exists in two or more different physical forms. Even though these forms have identical chemical properties, their physical structures lead to vastly different characteristics. For carbon, the two most famous natural allotropes are diamond and graphite, and they couldn't be more different.

Diamond is a giant 3D network where each carbon atom is bonded to four others in a rigid tetrahedral structure. This arrangement makes it the hardest natural substance known with an exceptionally high melting point Science, class X (NCERT 2025 ed.), Chapter 3, p.40. Because all four of carbon’s valence electrons are locked in tight bonds, diamond cannot conduct electricity; however, it is prized for its brilliance and ability to disperse light. Interestingly, while we find them in nature, diamonds can also be synthesized by subjecting pure carbon to extreme pressure and temperature, creating stones that are virtually indistinguishable from the real thing Science, class X (NCERT 2025 ed.), Chapter 4, p.61.

On the opposite end of the spectrum is graphite. Here, carbon atoms are arranged in hexagonal layers stacked on top of each other. These layers can slide over one another, which is why graphite feels smooth and slippery — perfect for pencil leads and industrial lubricants. The most striking feature of graphite is its electrical conductivity. Unlike most non-metals, which are insulators, graphite is an excellent conductor because each carbon atom is bonded to only three others, leaving one 'free' electron to move through the structure Science, class X (NCERT 2025 ed.), Chapter 3, p.40. Beyond these two, scientists have discovered Fullerenes, such as C-60 (Buckminsterfullerene), where carbon atoms are arranged in a unique shape resembling a football Science, class X (NCERT 2025 ed.), Chapter 4, p.61.

Feature	Diamond	Graphite
Structure	Rigid 3D Tetrahedral	Hexagonal Layers
Hardness	Extremely Hard	Soft and Slippery
Conductivity	Insulator	Good Conductor

Sources: Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.40; Science, class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.61

6. Exceptions in Metal and Non-metal Rules (exam-level)

In the study of chemistry, we often categorize elements into two broad groups: metals and non-metals. Under standard conditions, metals are typically solid, hard, and shiny (lustrous), while non-metals are generally gases or brittle solids with a dull appearance. However, for a UPSC aspirant, the 'rules' are less important than the exceptions, as these anomalies are frequently tested to check a candidate's depth of understanding. Chemistry is rarely about rigid boxes; it is about the nuances of atomic behavior. One of the most prominent exceptions relates to the physical state of elements at room temperature. While almost all metals are solids, mercury (Hg) is a well-known exception as it remains liquid at room temperature Science, Class X (NCERT 2025 ed.), Chapter 3, p.55. Similarly, among non-metals, which are usually solids or gases, bromine (Br) stands out as the only liquid non-metal Science, Class X (NCERT 2025 ed.), Chapter 3, p.39. Furthermore, though we think of metals as having high melting points, gallium (Ga) and caesium (Cs) have melting points so low (around 303 K) that they will melt if held in the palm of your hand Science, Class VIII (NCERT 2025 ed.), Chapter 8, p.123. Another critical set of exceptions involves appearance and hardness. While non-metals like sulfur or phosphorus are dull, iodine (I) is a non-metal that possesses a distinct metallic luster or shine Science, Class X (NCERT 2025 ed.), Chapter 3, p.39. Conversely, while we expect metals to be hard, alkali metals like lithium, sodium, and potassium are so soft that they can be cut with a knife. Carbon also breaks several rules through its different forms (allotropes): diamond is a non-metal yet is the hardest natural substance known, while graphite is a non-metal that acts as an excellent conductor of electricity.

Property	General Rule	The "Rule-Breaker" (Exception)
Physical State	Metals are solids	Mercury (Hg) is liquid
Luster	Non-metals are dull	Iodine (I) is lustrous
Hardness	Metals are hard	Alkali metals (Na, K, Li) are soft
Conductivity	Non-metals are insulators	Graphite (Carbon) conducts electricity

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.39, 55; Science, Class VIII (NCERT 2025 ed.), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.123

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental classification of elements, you can see how UPSC tests the boundaries of those definitions. While you learned that non-metals are typically dull and non-reflective, this question specifically probes the exceptions to physical properties. As noted in Science, Class X (NCERT 2025 ed.), the beauty of chemistry lies in these outliers. The building blocks here involve identifying the chemical group of each element and recalling which one defies the standard "dullness" rule associated with its category.

To solve this, your reasoning should be two-fold: first, eliminate elements that aren't strictly classified as non-metals, and second, identify the classic exception. Silicon and Germanium are metalloids (or semimetals); while they may appear shiny, they possess properties intermediate between metals and non-metals and are not categorized as pure non-metals. Between Carbon and Iodine, Carbon in its most common forms is non-lustrous. However, Iodine is the unique non-metal that naturally possesses a lustrous, shiny appearance, making (D) Iodine the correct choice as confirmed in Science-Class VII, NCERT.

UPSC often uses metalloids like Silicon and Germanium as traps because they exhibit a metallic luster while being brittle, leading students to confuse their classification. Another common trap is Carbon; a student might think of the brilliance of a diamond, but diamond's shine is due to its high refractive index rather than metallic luster. Iodine remains the specific, textbook non-metal exception you must remember. Always prioritize the standard chemical classification over the specialized properties of allotropes when navigating these options.