Which of the following pairs is/are correctly matched? 1. Isotopes : Atoms with same atomic number but different atomic mass 2. Isobars : Atoms with same number of neutrons but different atomic number 3. Isotones : Atoms with same mass number but different atomic number Select the correct answer using the code given below : Code:

1. Fundamental Atomic Structure: Z and A (basic)

To understand the universe at its most fundamental level, we begin with the architecture of the atom. Every atom is defined by two primary numbers: the Atomic Number (Z) and the Mass Number (A). Think of the Atomic Number as the atom's "identity card." It represents the number of protons in the nucleus. Because protons carry a positive charge, this number determines which element the atom belongs to. For instance, any atom with 11 protons is always sodium Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46, and any atom with 7 protons is nitrogen Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60. In a neutral atom, the number of electrons orbiting the nucleus equals Z, balancing the electrical charge.

While the protons define the element, the Mass Number (A) tells us about the atom's weight. It is the total sum of protons and neutrons (collectively called nucleons) in the nucleus. Since electrons have negligible mass, nearly all of an atom's weight comes from these nucleons. You can calculate the number of neutrons simply by subtracting the atomic number from the mass number (Neutrons = A - Z). For example, a standard carbon atom has a mass of 12 atomic mass units (u) Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66; since its Z is 6, we know it contains 6 neutrons.

Nature, however, allows for variations in these numbers, leading to three critical classifications that are frequently tested in competitive exams:

Term	What is the SAME?	What is DIFFERENT?	Example
Isotopes	Atomic Number (Z) / Protons	Mass Number (A) / Neutrons	Carbon-12 and Carbon-14
Isobars	Mass Number (A) / Nucleons	Atomic Number (Z) / Protons	Argon-40 and Calcium-40
Isotones	Number of Neutrons (A - Z)	Atomic Number (Z) / Protons	Carbon-14 and Oxygen-16 (both have 8 neutrons)

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66

2. Chemical Identity and Periodic Law (intermediate)

In our journey to understand the building blocks of the universe, we must first establish what gives an atom its unique identity. The Atomic Number (Z), which represents the number of protons in the nucleus, is the "DNA" of an atom. No two different elements can have the same atomic number. For instance, Chlorine always has 17 protons Science class X, Carbon and its Compounds, p.60. This number determines the atom's electronic configuration, and since chemical reactivity is essentially a quest to fill the outermost valence shell Science class X, Metals and Non-metals, p.46, the atomic number dictates how an element will behave chemically. This repeating pattern of chemical behavior based on atomic structure is known as Periodic Law, much like how the phases of the moon follow a natural periodic cycle Science class VIII, Keeping Time with the Skies, p.178.

While the proton count (Z) defines the element, the Mass Number (A)—the sum of protons and neutrons—can vary. This leads to three crucial classifications that every UPSC aspirant must distinguish clearly:

• Isotopes: Atoms of the same element (same Z) but with different mass numbers (different number of neutrons). Because they have the same number of protons and electrons, their chemical properties are nearly identical.
• Isobars: Atoms of different elements (different Z) that happen to have the same mass number (same A). Their chemical properties are entirely different because their electronic structures differ.
• Isotones: Atoms of different elements that have the same number of neutrons (N = A - Z). They share neither chemical identity nor mass, but they are useful in nuclear physics studies.

Term	Atomic Number (Z)	Mass Number (A)	Neutrons (N)	Identity
Isotopes	Same	Different	Different	Same Element
Isobars	Different	Same	Different	Different Element
Isotones	Different	Different	Same	Different Element

Sources: Science class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science class X (NCERT 2025 ed.), Metals and Non-metals, p.46; Science class VIII (NCERT 2025 ed.), Keeping Time with the Skies, p.178

3. Radioactivity and Nuclear Stability (intermediate)

To understand Radioactivity, we must first understand why some atoms are restless while others are at peace. In chemistry, we learn that atoms seek stability through their electron shells, often aiming for a "stable octet" Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46. However, nuclear stability is an entirely different game played inside the nucleus. It depends on the delicate balance between the Strong Nuclear Force (which holds protons and neutrons together) and Electrostatic Repulsion (which pushes protons apart). When the ratio of neutrons to protons is skewed, the nucleus becomes unstable and seeks a lower energy state by spontaneously emitting particles or radiation—a process we call radioactivity.

Before diving into the decay itself, we must distinguish between three fundamental ways atoms can relate to one another based on their nuclear composition. These terms are essential for identifying which nuclides are likely to be radioactive:

Term	Same	Different	Example
Isotopes	Atomic Number (Protons)	Mass Number (Neutrons)	Protium (¹H) vs Tritium (³H)
Isobars	Mass Number (Total Nucleons)	Atomic Number (Protons)	⁴⁰Ar and ⁴⁰Ca
Isotones	Number of Neutrons	Atomic Number (Protons)	³¹P and ³²S (both have 16 neutrons)

When an unstable nucleus (a radio-nuclide) eventually disintegrates, it releases energy in three primary forms: alpha (α) particles (helium nuclei), beta (β) particles (high-speed electrons), and gamma (γ) rays (high-energy electromagnetic waves) Environment, Shankar IAS Academy (10th ed.), Environmental Pollution, p.82. This process is not instantaneous for all atoms; it is measured by a Half-life—the time required for half of the atoms in a sample to decay Environment, Shankar IAS Academy (10th ed.), Environmental Pollution, p.83. For instance, while some isotopes decay in seconds, Uranium and Thorium have incredibly long half-lives, providing a steady source of heat that accounts for over half of the Earth's internal temperature Physical Geography by PMF IAS, Earths Interior, p.58.

In the context of national development, these radioactive substances are strategic resources. Elements like Uranium and Thorium—found in the Aravalli ranges of Rajasthan and the Monazite sands of Kerala—are harnessed to generate nuclear power by altering their atomic structure to release vast amounts of heat NCERT, Contemporary India II (Class X), Print Culture and the Modern World, p.117.

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46; Environment, Shankar IAS Academy (10th ed.), Environmental Pollution, p.82-83; Physical Geography by PMF IAS, Earths Interior, p.58; NCERT, Contemporary India II (Class X), Print Culture and the Modern World, p.117

4. Applications of Nuclear Science (exam-level)

To understand the applications of nuclear science, we must first master the fundamental classifications of atoms based on their nuclei. The most crucial concept is that of Isotopes: atoms of the same element (same number of protons) that have different mass numbers because they possess a different number of neutrons. These unique properties allow us to use specific radioactive isotopes as 'tracers' or 'clocks.' In contrast, Isobars are atoms of different elements that share the same mass number, and Isotones are atoms of different elements that happen to have the same number of neutrons.

1. Archaeology and Dating: One of the most famous applications is Radiocarbon Dating. Living organisms maintain a constant ratio of Carbon-12 to the radioactive isotope Carbon-14. Once the organism dies, Carbon-14 begins to decay at a known rate. By measuring the remaining Carbon-14 using techniques like AMS (Accelerator Mass Spectrometry), scientists can determine the age of ancient artifacts. For example, excavations at Keeladi in Tamil Nadu used AMS dating on carbon samples to trace the site's history back to 580 BCE. History, Class XI (TN Board), Evolution of Society in South India, p.70.

2. Medicine and Health: Nuclear science plays a dual role in medicine. Radioisotopes like Iodine-131 are used to diagnose and treat thyroid conditions because the thyroid gland naturally absorbs iodine. However, this also makes it a significant health risk during nuclear accidents or weapon tests. Radioactive fallout can contaminate vegetation, pass into the milk of cattle, and eventually settle in the human thyroid, potentially causing severe damage, especially in children. Shankar IAS Academy, Environment, p.413. Additionally, long-term radioactivity from elements like strontium or radium can accumulate in the skeletal system or brain.

3. Energy and Defense: The immense energy bound within the nucleus is harnessed through fission and fusion. Nuclear fission, used in power plants and weapons, typically utilizes heavy isotopes like Uranium-235 and Plutonium-239. On the other hand, nuclear fusion (the process powering the sun) uses light elements like hydrogen or lithium. Shankar IAS Academy, Environment, p.83.

Term	Atomic Number (Protons)	Mass Number (Nucleons)	Example Application
Isotopes	Same	Different	Carbon-14 for dating fossils.
Isobars	Different	Same	Used in studying beta decay.
Isotones	Different	Different (but same Neutrons)	Nuclear structure research.

Sources: History, Class XI (TN Board), Evolution of Society in South India, p.70; Shankar IAS Academy, Environment, Environment Issues and Health Effects, p.413; Shankar IAS Academy, Environment, Environmental Pollution, p.83

5. Classifying Atoms: Isotopes, Isobars, and Isotones (basic)

To understand how we classify atoms, we must first look into the heart of the matter: the atomic nucleus. As we know, the nucleus is a small, positively charged central portion of the atom containing protons and neutrons Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Major Crops and Cropping Patterns in India, p.100. While the number of protons (the Atomic Number, Z) determines the identity of the element, the sum of protons and neutrons (the Mass Number, A) determines its weight. Depending on how these numbers vary or stay the same, we classify atoms into three primary categories: Isotopes, Isobars, and Isotones.

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Because they have the same atomic number, they occupy the same position in the periodic table and share identical chemical properties, as their electronic configurations remain consistent Science class X (NCERT 2025 ed.), Metals and Non-metals, p.47. However, they differ in mass and nuclear stability. A classic example is Uranium-235, which is used in nuclear fission alongside other radioactive substances like Iodine-131 Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.83. These are isotopes of Uranium and Iodine respectively, differing from their more common counterparts only by their neutron count.

Moving beyond single elements, we find Isobars and Isotones, which involve different elements entirely. Isobars are atoms that have the same mass number (A) but different atomic numbers (Z). Imagine two different elements that happen to "weigh" the same because the total count of nucleons is identical. On the other hand, Isotones are atoms of different elements that possess the same number of neutrons (A minus Z) but have different atomic numbers and different mass numbers. Understanding these variations is crucial for mastering nuclear physics and understanding why some atoms are stable while others are radioactive.

Category	Atomic Number (Z)	Mass Number (A)	Neutrons (A-Z)
Isotopes	Same	Different	Different
Isobars	Different	Same	Different
Isotones	Different	Different	Same

Sources: Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Major Crops and Cropping Patterns in India, p.100; Science class X (NCERT 2025 ed.), Metals and Non-metals, p.47; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.83

6. Solving the Original PYQ (exam-level)

Excellent work on completing the conceptual foundation! This question is a classic application of the definitions you just mastered regarding Atomic Number (Z) and Mass Number (A). To solve this, you must look past the familiar terminology and verify if the relationship described matches the scientific definition. As you learned, Isotopes are variants of the same element; because they are the same element, they must have the same atomic number, but their mass numbers differ due to varying neutron counts. This makes statement 1 a perfect match, providing the anchor for your answer.

Now, let's look at the logic behind the errors. UPSC often uses a "definition swap" trap to test your precision under pressure. In this question, statements 2 and 3 have been deliberately interchanged. Isobars (from the Greek baros meaning weight) are atoms with the same mass number but different atomic numbers. Conversely, Isotones are atoms that share the same number of neutrons. By swapping these two, the examiner creates a situation where the terms look familiar, but the logic is flawed. Always pause to verify the specific attribute (mass vs. neutrons) being linked to the term.

Because statements 2 and 3 are scientifically reversed, options (A), (C), and (D) are immediately invalidated. This leaves us with (B) 1 only as the correct answer. This pattern of "cross-matching" definitions is a frequent strategy in NCERT Class 9 Science (Chapter 4: Structure of the Atom) and NCERT Class 11 Chemistry topics. To avoid this trap in the future, remember the mnemonic: Isotopes (P for Protons/Atomic Number), Isobars (A for Atomic Mass), and Isotones (N for Neutrons).