Which one of the following statements is correct?

1. Subatomic Particles: The Building Blocks (basic)

At the very heart of everything we see—from the stars in the sky to the screen you are reading—are atoms. However, an atom is not the 'indivisible' unit it was once thought to be. It is built from smaller subatomic particles: the proton, the neutron, and the electron. In the earliest moments of our universe, just microseconds after the Big Bang, cooling temperatures allowed fundamental particles called quarks to clump together to form protons and neutrons Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2. It took much longer for the universe to cool enough for electrons to join them and form stable atoms.

The structure of an atom is often compared to a tiny solar system. At the center lies the nucleus, a dense core containing protons and neutrons. Protons carry a positive charge (+1), while neutrons are electrically neutral. Surrounding this nucleus are electrons, which carry a negative charge (-1) and occupy specific regions or 'shells'. A crucial point for your UPSC preparation is the mass difference: while protons and neutrons have roughly the same mass, an electron is incredibly light—approximately 1/1836th (often rounded to 1/2000th) the mass of a proton. Therefore, almost all the mass of an atom is concentrated in its nucleus.

The behavior of these particles dictates how matter interacts. For instance, the valence electrons (those in the outermost shell) determine an element's reactivity. Elements like carbon have four valence electrons and must either share, gain, or lose electrons to reach a stable state Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. When an atom loses or gains electrons, it becomes an ion; losing electrons creates a positive cation, while gaining them creates a negative anion Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.49.

Particle	Relative Mass	Relative Charge	Location
Proton	1 unit	+1	Inside Nucleus
Neutron	1 unit	0 (Neutral)	Inside Nucleus
Electron	~1/2000 unit	-1	Outside Nucleus

Sources: Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.49

2. Mass and Charge Characteristics (intermediate)

To understand the architecture of the universe, from the vast galaxies described in Physical Geography by PMF IAS, The Universe, p.1 to the smallest grain of sand, we must look at the three primary subatomic particles: protons, neutrons, and electrons. These particles are defined by two intrinsic properties: mass and electric charge. While protons and neutrons are tightly packed within the central nucleus, electrons occupy the vast space surrounding it. This arrangement is crucial because the nucleus holds nearly all the atom's mass, while the electron cloud determines its volume and chemical reactivity.

The relationship between these particles is a study in contrasts. Protons carry a positive (+1) charge, and electrons carry an equal but opposite negative (-1) charge. Neutrons are electrically neutral. However, their masses are vastly different. A proton and a neutron have roughly the same mass (approximately 1 atomic mass unit), but an electron is incredibly light—it takes about 1,836 electrons to equal the mass of a single proton. Because of this disparity, when an atom loses or gains electrons to become an ion, its electrical charge changes significantly, but its total mass remains almost exactly the same Science class X (NCERT 2025 ed.), Metals and Non-metals, p.46.

In a neutral atom, the number of protons (the Atomic Number) must equal the number of electrons. For instance, a Carbon atom is defined by having 6 protons; if it has 6 electrons, it is neutral. If it were to gain four extra electrons, the nucleus would struggle to hold onto them because the electromagnetic repulsion between the ten negative electrons would be difficult for the six positive protons to manage Science class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. This delicate balance of charge and mass is what dictates the stability and behavior of all matter.

Particle	Relative Mass	Relative Charge	Location
Proton	1	+1	Nucleus
Neutron	1	0	Nucleus
Electron	1/1836 (≈ 0)	-1	Orbits/Shells

Sources: Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.1; Science class X (NCERT 2025 ed.), Metals and Non-metals, p.46; Science class X (NCERT 2025 ed.), Carbon and its Compounds, p.59

3. Atomic Number, Mass Number, and Nucleons (basic)

To understand the structure of matter, we must look at the heart of the atom: the nucleus. Inside this tiny central core, we find two primary particles—protons and neutrons. Because these particles reside in the nucleus, they are collectively referred to as nucleons. While electrons orbit the nucleus, they are so light (approximately 1/1836th the mass of a proton) that they contribute almost nothing to the atom's total weight. Therefore, the mass of an atom is concentrated almost entirely within its nucleons.

The Atomic Number (denoted by Z) is the most fundamental property of an element. It represents the number of protons in the nucleus. This number is the 'identity card' of an atom; for instance, any atom with 6 protons is carbon, as noted in Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. In a neutral atom, the number of electrons equals the atomic number, ensuring the atom has no net charge.

The Mass Number (denoted by A) is the sum of the total number of protons and neutrons in the nucleus. Since protons and neutrons each have a mass of approximately 1 atomic mass unit (u), the mass number gives us a close approximation of the atom's actual mass. For example, carbon typically has 6 protons and 6 neutrons, giving it a mass number of 12 u Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66. You can calculate the number of neutrons in any atom simply by subtracting the Atomic Number from the Mass Number (Neutrons = A - Z).

Term	Symbol	Definition
Atomic Number	Z	Total number of protons; defines the element.
Mass Number	A	Total number of protons + neutrons (nucleons).
Nucleons	-	Collective name for particles in the nucleus (p + n).

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66

4. Isotopes: Varieties of Atoms (intermediate)

At its heart, the identity of an atom is defined by its Atomic Number (the number of protons). For instance, every Hydrogen atom in the universe has exactly one proton. However, nature allows for a bit of variation in the weight of these atoms. Isotopes are atoms of the same element that possess the same number of protons but a different number of neutrons. Because the atomic nucleus contains both of these particles (Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100), adding or removing neutrons changes the atom's Mass Number without altering its chemical identity.

While isotopes of an element behave almost identically in chemical reactions—because they have the same number of electrons—their physical properties and stability can differ significantly. Some isotopes are stable, while others are unstable and undergo radioactive decay. These unstable varieties, known as radioisotopes, spontaneously emit particles or radiation to reach a more stable state (Environment, Shankar IAS Academy, Environmental Pollution, p.82). This unique property makes isotopes incredibly useful in medicine, industry, and archaeology.

In the UPSC context, it is vital to distinguish between specific isotopes used in healthcare. For example, Iodine-131 is a standard treatment for thyroid-related disorders like goiter, whereas Cobalt-60 is frequently employed in high-energy radiotherapy to treat cancer. Other famous examples include Carbon-14 for dating ancient organic remains and Uranium-235 as fuel for nuclear reactors.

Isotope	Protons	Neutrons	Common Use
Carbon-12	6	6	Standard basis for atomic mass
Carbon-14	6	8	Radiocarbon dating (Archaeology)
Iodine-131	53	78	Treating Goiter/Thyroid issues

Sources: Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Major Crops and Cropping Patterns in India, p.100; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.82

5. Medical and Industrial Applications of Isotopes (exam-level)

To understand why isotopes are so vital in modern science, we must first look at their nature. While isotopes of an element share the same chemical properties because they have the same number of protons and electrons, their unstable nuclei (in the case of radioisotopes) emit radiation as they decay. This radiation—whether alpha, beta, or gamma—can be harnessed as a powerful tool for diagnosis, treatment, and industrial processing.

In the field of medicine, isotopes are used in two primary ways: imaging and therapy. For instance, the thyroid gland has a natural affinity for iodine to produce the hormone thyroxin. When a patient suffers from goiter or thyroid disorders, doctors use Iodine-131 (I-131). Because the thyroid selectively absorbs iodine, the radiation from I-131 can be used to either image the gland or destroy overactive/cancerous cells within it Science class X NCERT, Control and Coordination, p.110. Similarly, Cobalt-60 is a heavy hitter in cancer treatment; it emits high-energy gamma rays that are precisely aimed at tumors to kill malignant cells in a process known as radiotherapy.

Moving to industrial and agricultural applications, isotopes help us solve problems without high heat or chemical additives. A prime example is food irradiation. By exposing food items to ionizing radiation from sources like Cobalt-60, we can eliminate harmful microorganisms and insects, significantly extending shelf life. This is a "cold process," meaning it doesn't cook the food or change its fresh character, and importantly, it leaves no toxic radioactive residues Indian Economy Nitin Singhania, Food Processing Industry in India, p.410. Beyond food, isotopes like Americium-241 are found in household smoke detectors, and others are used as "tracers" to detect leaks in underground oil or water pipelines.

The following table summarizes the most common isotopes you will encounter in the exam:

Isotope	Primary Application	Field
Iodine-131	Treatment of Goiter and Thyroid disorders	Medicine
Cobalt-60	Cancer radiotherapy and Food irradiation	Medicine / Industry
Carbon-14	Carbon dating (age of fossils) and metabolic studies	Science / Archaeology
Uranium-235	Fuel for nuclear reactors	Energy

Sources: Science class X NCERT, Control and Coordination, p.110; Indian Economy Nitin Singhania, Food Processing Industry in India, p.410

6. Historical Atomic Models: Thomson and Rutherford (intermediate)

To understand the journey of atomic physics, we must look at how our perception of the "indivisible" atom evolved. After the discovery of the electron, J.J. Thomson proposed the first major atomic model in 1904, famously known as the Plum Pudding Model. He envisioned the atom as a sphere of uniform positive charge with negatively charged electrons embedded within it. Much like how various fruits like plums are distributed in a dessert (Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.87), Thomson believed the negative "plums" balanced the positive "pudding," making the atom electrically neutral.

However, this view was challenged by Ernest Rutherford in 1911 through his landmark Alpha-Particle Scattering Experiment. Rutherford bombarded a very thin sheet of gold with positively charged alpha (α) particles. Gold was the ideal choice for this experiment because it is among the most malleable of all metals, allowing it to be beaten into an incredibly thin foil only a few atoms thick (Science-Class VII NCERT, The World of Metals and Non-metals, p.43). If Thomson’s model were correct, the alpha particles should have passed through the uniform positive "pudding" with only minor deflections.

The results were startling: while most particles passed straight through, a small fraction deflected at large angles, and about 1 in 8,000 bounced back entirely. This led Rutherford to conclude that the atom is mostly empty space, with its mass and positive charge concentrated in a tiny, dense central region called the nucleus. This shifted the model from a solid sphere to a "planetary" system where electrons orbit a central sun-like nucleus.

Feature	Thomson’s Model	Rutherford’s Model
Positive Charge	Spread uniformly throughout the atom.	Concentrated in a tiny central nucleus.
Distribution of Mass	Assumed to be uniform.	Concentrated almost entirely in the nucleus.
Atomic Volume	Solid sphere of matter.	Mostly empty space.

Sources: Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.87; Science-Class VII NCERT, The World of Metals and Non-metals, p.43

7. The Nature of the Neutron and Nuclear Stability (exam-level)

For a long time, the scientific community struggled to explain why the atomic nucleus didn't fly apart. Since the nucleus contains protons (which are positively charged and repel each other), there had to be another particle providing stability. This particle is the neutron. While early 20th-century hypotheses suggested the neutron might be a composite of a proton and an electron bound together, James Chadwick proved in 1932 that the neutron is a distinct fundamental subatomic particle. It is located within the atomic nucleus along with protons Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100.

The neutron carries no electric charge (it is neutral), which allows it to reside in the nucleus without being repelled by protons. Its mass is nearly identical to that of a proton, though it is slightly heavier. To put this in perspective, both protons and neutrons are much heavier than electrons; a proton's mass is approximately 1836 times that of an electron. This concentration of mass in the nucleus is what gives atoms their structural integrity. Interestingly, while neutrons are stable inside a nucleus, a free neutron (one outside the nucleus) is actually unstable and will decay into a proton, an electron, and an antineutrino in about 10 minutes.

Property	Proton	Neutron	Electron
Charge	Positive (+1)	Neutral (0)	Negative (-1)
Location	Nucleus	Nucleus	Orbits/Shells
Relative Mass	~1 amu	~1 amu (slightly higher)	~1/1836 amu

In extreme cosmic events, such as a supernova, the gravitational pressure is so immense that it forces protons and electrons to combine, effectively creating a neutron star Physical Geography, PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.14. This demonstrates the unique relationship between these particles; under normal terrestrial conditions, they remain distinct, but under stellar pressure, they can transform to achieve a new state of stability.

Sources: Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100; Physical Geography, PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.14

8. Solving the Original PYQ (exam-level)

This question integrates your knowledge of atomic models, the physical properties of subatomic particles, and their medical applications. To solve this, you must synthesize the historical timeline of discovery—moving from J.J. Thomson’s Plum Pudding model to James Chadwick’s discovery of the neutron—with the quantitative mass ratios that define the atom's structure. The UPSC often tests your ability to distinguish between the specific roles of different scientists and the functional differences between isotopes.

Let's evaluate the options logically. While a neutron is neutral, it is a distinct fundamental particle discovered by Chadwick, not a mere combination of an electron and a proton as suggested in Option A. Moving to mass, you learned that protons and neutrons carry almost all the atom's mass, while electrons are significantly lighter. Since the actual mass ratio is approximately 1/1836, (B) The mass of an electron is about 1/2000 times that of a proton is the correct answer. This "about" terminology is a classic UPSC tactic, testing your grasp of scale and proportionality without requiring precise decimal memorization.

Finally, avoid the "Isotope Swap" trap in Option C; while Cobalt-60 is used for cancer radiotherapy, it is Iodine-131 that is specifically used in the treatment of goiter. Similarly, Option D attempts to confuse historical attributions. Always remember that Thomson proposed a diffuse positive sphere, whereas Rutherford discovered the nucleus and Chadwick later identified the neutron. Mastering these distinctions ensures you won't be misled by statements that sound scientifically plausible but are historically or factually incorrect. ScienceDirect and OSTI OpenNet confirm these fundamental historical and medical distinctions.

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