Which one of the following statements is correct?

1. Introduction to Atomic Theory and Matter (basic)

At the most fundamental level, everything you see around you—from the screen you are reading to the air you breathe—is made of matter. To understand matter, we must look at its building blocks: atoms and molecules. An atom is the smallest unit of an element that retains its chemical properties. While some elements like Gold (Au) or Iron (Fe) are made of individual atoms, others like Oxygen (O₂) or Hydrogen (H₂) prefer to exist as molecules, where two or more atoms are bonded together Science, Class VIII, Particulate Nature of Matter, p.115.

While early scientists thought atoms were indivisible, we now know they have an internal structure. At the center of every atom lies a tiny, dense, and positively charged core called the nucleus. This nucleus contains two types of particles: protons, which carry a positive charge, and neutrons, which have no charge. The identity of an element is defined by its Atomic Number, which is exactly equal to the number of protons in its nucleus Science, Class X, Carbon and its Compounds, p.60. For instance, any atom with exactly 6 protons is Carbon, regardless of how many neutrons or electrons it has.

Surrounding this nucleus are electrons, which carry a negative charge. In a neutral atom, the number of electrons is equal to the number of protons, balancing the overall charge. However, electrons are much lighter than protons and neutrons; almost all of an atom's mass is concentrated in the nucleus. Therefore, the Mass Number of an atom is calculated by adding the number of protons and neutrons together.

Particle	Location	Charge	Key Role
Proton	Nucleus	Positive (+)	Determines the Element (Atomic Number)
Neutron	Nucleus	Neutral (0)	Contributes to Mass and Stability
Electron	Orbits/Shells	Negative (-)	Responsible for Chemical Bonding

Sources: Science, Class VIII (NCERT Revised ed 2025), Particulate Nature of Matter, p.115; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60

2. Discovery of Subatomic Particles (basic)

For centuries, the atom was considered the smallest, indivisible building block of matter. However, at the turn of the 20th century, a series of groundbreaking experiments revealed a bustling internal world. The first major crack in the "indivisible" theory came in 1897 when J.J. Thomson discovered the electron—a negatively charged particle much smaller than the atom itself. He imagined the atom as a "plum pudding," where these negative electrons were embedded in a uniform sphere of positive charge.

This view was shattered in 1911 by Ernest Rutherford. In his famous alpha-particle scattering experiment, he fired heavy, positively charged particles at a thin gold foil. To his surprise, while most particles passed through, some bounced straight back! This proved that the positive charge wasn't spread out like pudding; it was concentrated in an incredibly small, dense center called the nucleus. We now know the nucleus contains protons (positively charged) and neutrons (neutral), while electrons orbit this core. As noted in Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59, it is the nucleus and its specific number of protons that hold the electrons in place through electromagnetic attraction.

Understanding these particles allows us to define an element's identity. The Atomic Number (Z) is strictly the number of protons in the nucleus—this never changes during chemical reactions. For instance, a sodium nucleus always contains 11 protons, even if the atom loses an electron to become a cation Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46. The Mass Number (A), on the other hand, is the sum of both protons and neutrons. Interestingly, these particles didn't all appear at once; in the early moments of the universe, quarks clumped together to form protons and neutrons long before they could capture electrons to form stable atoms Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2

3. Early Models: Thomson's Plum Pudding Model (basic)

In the late 19th century, the scientific world viewed the atom as an indivisible 'solid billiard ball.' This changed dramatically in 1897 when J.J. Thomson discovered the electron—the first evidence that atoms were actually made of even smaller, subatomic particles. Since atoms were known to be electrically neutral, Thomson had to explain how these negatively charged electrons could exist without making the whole atom negative. This led to the birth of the Plum Pudding Model (or the Watermelon Model) Science, Class VIII (NCERT 2025), Particulate Nature of Matter, p.115.

Thomson proposed that an atom consists of a positively charged sphere in which electrons are embedded, much like seeds in a watermelon or raisins in a pudding. The most critical feature of this model was the idea of electrostatic neutrality: the magnitude of the positive charge of the 'soup' or 'pudding' was exactly equal to the total negative charge of the embedded electrons. This ensured that the atom as a whole had no net charge Science, Class X (NCERT 2025), Carbon and its Compounds, p.59.

While revolutionary, this model was 'solid' in nature—Thomson believed the mass of the atom was uniformly distributed throughout the sphere. It did not account for a central nucleus or empty space within the atom. Despite its eventual replacement, it laid the groundwork for understanding that atoms are composed of balanced charges.

Feature	Thomson’s Plum Pudding Model
Positive Charge	Spread uniformly throughout the entire atom.
Negative Charge	Individual electrons embedded like seeds/raisins.
Mass Distribution	Uniformly distributed across the atom's volume.
Structure	Solid sphere (no empty space).

Sources: Science, Class VIII (NCERT 2025), Particulate Nature of Matter, p.115; Science, Class X (NCERT 2025), Carbon and its Compounds, p.59

4. Isotopes, Isobars, and Isotones (intermediate)

To understand the variations in atoms, we must first look at the atomic nucleus. As defined in Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 12: Major Crops and Cropping Patterns in India, p.100, the nucleus is the small, positive central portion of an atom containing protons and neutrons. The identity of an element is determined solely by its Atomic Number (Z), which is the number of protons. However, nature often presents us with atoms that have the same identity but different weights, or different identities that happen to weigh the same. This is where we classify atoms into three categories: Isotopes, Isobars, and Isotones.

Isotopes are atoms of the same element (same Z) that have different mass numbers (A) because they contain a different number of neutrons. For example, Carbon-12 and Carbon-14 are isotopes. While Carbon-12 is stable and common in living structures (Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.58), Carbon-14 is radioactive and used for dating ancient artifacts, such as the samples found at the Keeladi excavation site in Tamil Nadu (History, class XI (Tamilnadu state board 2024 ed.), Evolution of Society in South India, p.70). Because isotopes have the same number of electrons, they exhibit nearly identical chemical properties but different physical properties.

Isobars and Isotones move us across different elements. Isobars are atoms of different elements that have the same mass number (A). Even though they have a different number of protons (and thus different chemical identities), the sum of their protons and neutrons is identical. Conversely, Isotones are atoms of different elements that have the same number of neutrons (N). Each of these radioactive or stable nuclides has a specific decay rate or half-life, which determines how long it remains active in the environment (Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.83).

Term	What stays the SAME?	What CHANGES?	Example
Isotopes	Protons (Atomic Number)	Neutrons (Mass Number)	¹²C and ¹⁴C
Isobars	Mass Number (Total Nucleons)	Protons (Atomic Number)	₁₈Ar⁴⁰ and ₂₀Ca⁴⁰
Isotones	Neutrons (A minus Z)	Protons and Mass Number	₆C¹⁴ and ₈O¹⁶ (both have 8 neutrons)

Sources: Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 12: Major Crops and Cropping Patterns in India, p.100; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.58; History, class XI (Tamilnadu state board 2024 ed.), Evolution of Society in South India, p.70; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.83

5. Nuclear Energy and Radioactivity (exam-level)

To understand radioactivity, we must look deep inside the atom. It is the spontaneous process where an unstable atomic nucleus loses energy by emitting radiation. Think of it as a 'balancing act'—the nucleus is trying to reach a more stable state by spitting out bits of itself. These emissions come in three primary forms: Alpha particles (which are essentially helium nuclei or protons), Beta particles (high-speed electrons), and Gamma rays (high-energy, short-wave electromagnetic waves) Environment, Shankar IAS Academy (10th ed.), Environmental Pollution, p.82. Unlike chemical reactions which involve electrons orbiting the nucleus, radioactivity is a purely nuclear phenomenon.

The rate at which this decay occurs is measured by the Half-life—the time required for exactly half of the radioactive atoms in a sample to disintegrate. This rate is a constant for each specific isotope. Understanding half-life is crucial for environmental science; radionuclides with long half-lives are particularly dangerous because they persist in the environment for thousands of years, acting as persistent sources of pollution Environment, Shankar IAS Academy (10th ed.), Environmental Pollution, p.83. Unlike many chemical pollutants, there is technically no safe dose of radiation, as even low levels can cause cumulative deleterious effects on living organisms over time Environment and Ecology, Majid Hussain (3rd ed.), Environmental Degradation and Management, p.44.

When it comes to Nuclear Energy, we distinguish between two powerful processes:

Process	Mechanism	Context
Fission	Splitting a heavy nucleus (like Uranium) into smaller parts.	Primary source for nuclear power plants and early nuclear devices.
Fusion	Joining light nuclei (like Hydrogen) to form a heavier one.	Powers the Sun; requires extreme heat/pressure not found naturally inside Earth Physical Geography by PMF IAS, Earths Interior, p.59.

India’s nuclear journey reached a peak during Operation Shakti (1998), where scientists like A.P.J. Abdul Kalam led the testing of both fission and fusion devices at Pokhran, declaring India a full-fledged nuclear state A Brief History of Modern India, Rajiv Ahir (2019 ed.), After Nehru..., p.754.

Sources: Environment, Shankar IAS Academy (10th ed.), Environmental Pollution, p.82-83; Environment and Ecology, Majid Hussain (3rd ed.), Environmental Degradation and Management, p.44; A Brief History of Modern India, Rajiv Ahir (2019 ed.), After Nehru..., p.754; Physical Geography by PMF IAS, Earths Interior, p.59

6. Rutherford's Alpha-Particle Scattering Experiment (intermediate)

In 1911, Ernest Rutherford conducted a landmark experiment that fundamentally changed our understanding of the atom. Before this, the prevailing theory was J.J. Thomson’s "Plum Pudding" model, which suggested that an atom was a sphere of uniform positive charge with electrons embedded in it like raisins in a pudding. Rutherford sought to test this by bombardment. He used alpha-particles (α-particles), which are fast-moving, positively charged particles Science, Class X, Magnetic Effects of Electric Current, p.204, and directed them at a very thin foil of gold.

Gold was chosen specifically because of its extreme malleability—the property that allows a metal to be beaten into incredibly thin sheets Science, Class VII, The World of Metals and Non-metals, p.43. Rutherford needed the target to be as thin as possible (only a few hundred atoms thick) so the alpha particles wouldn't just be absorbed. To his astonishment, while most particles passed straight through, some were deflected by small angles, and a tiny fraction—about 1 in 12,000—bounced back almost entirely. Rutherford famously remarked that this was as incredible as firing a 15-inch shell at a piece of tissue paper and having it come back and hit you!

These observations led to three revolutionary conclusions that defined the Rutherford Model of the atom:

• Most of the atom is empty space: Since most α-particles passed through the foil without deflection.
• Positive charge is concentrated: The deflection of positive α-particles indicated a strong repulsive force, proving that the positive charge is not spread out but concentrated in a tiny volume.
• The Nucleus: This tiny, dense, central portion of the atom that contains nearly all its mass and positive charge was named the Atomic Nucleus Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100.

Sources: Science, Class X, Magnetic Effects of Electric Current, p.204; Science, Class VII, The World of Metals and Non-metals, p.43; Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100

7. Understanding Atomic Number and Mass Number (exam-level)

In the study of atomic physics, two fundamental numbers define the identity and physical properties of an element: the Atomic Number (Z) and the Mass Number (A). To understand these, we must look into the Atomic Nucleus—the small, dense, positively charged central core of the atom that contains both protons and neutrons Environment and Ecology, Majid Hussain, Chapter 12, p.100.

The Atomic Number (Z) is the most critical identifier of an element. It is defined strictly as the number of protons in the nucleus. This number never changes during chemical reactions. For instance, whether sodium is in its metallic form or exists as a sodium cation (Na⁺) in salt, its nucleus still contains exactly 11 protons Science, Class X (NCERT 2025 ed.), Chapter 3, p.46. While a neutral atom has an equal number of protons and electrons, the atomic number is defined by the protons because the number of electrons can fluctuate during ionization.

The Mass Number (A), on the other hand, represents the total number of "nucleons" (protons + neutrons) present in the nucleus. Since electrons have negligible mass, the mass number effectively accounts for the weight of the atom. For example, Carbon-12 has a mass number of 12 (6 protons + 6 neutrons), whereas Hydrogen usually has a mass number of 1 (1 proton + 0 neutrons) Science, Class X (NCERT 2025 ed.), Chapter 4, p.66.

Feature	Atomic Number (Z)	Mass Number (A)
Definition	Total number of protons only.	Sum of protons and neutrons.
Significance	Determines the element's identity.	Determines the isotope of the element.
Location	Nucleus	Nucleus

Sources: Environment and Ecology, Majid Hussain, Chapter 12: Major Crops and Cropping Patterns in India, p.100; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46; Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.66

8. Solving the Original PYQ (exam-level)

Congratulations on completing the module! You have just transitioned from learning the evolution of atomic models to applying those concepts to a classic UPSC-style question. This specific PYQ tests your ability to distinguish between the fundamental definitions of atomic properties and the historical scientific discoveries that defined them. In the civil services exam, examiners often blend history and chemistry to see if you can accurately map a scientist's name to their specific contribution without getting confused by similar-sounding terms.

Let’s walk through the reasoning. The atomic number of an element is its ultimate identity card; it is defined strictly by the number of protons found in the nucleus. This makes (C) the correct statement. Even if an atom loses or gains electrons to become an ion, its atomic number remains unchanged because the proton count is constant. As explained in Science, class X (NCERT 2025 ed.), this proton count is what differentiates one element from another on the periodic table.

Now, let’s deconstruct the traps in the other options. Option (A) and (B) are classic scientist-discovery mismatches. Rutherford is famous for the gold foil experiment that discovered the atomic nucleus, while J.J. Thomson is the one who discovered the electron; UPSC flipped these to test your precision. Finally, Option (D) is a common conceptual trap: the mass number is the sum of protons and neutrons (the nucleons), not electrons. Understanding these distinctions, as detailed in Britannica: Rutherford model, ensures you won't be swayed by options that look technically plausible but are factually inverted.