Who among the following proposed that atom is indivisible ?

1. Nature of Matter and its Classification (basic)

Welcome to your first step in mastering chemistry! To understand the universe, we must first understand Matter—which is simply anything that has mass and occupies space. From the air you breathe to the water in your bottle, everything is composed of tiny particles Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.117. While ancient Indian philosopher Acharya Kanad and the Greek Democritus theorized about indivisible particles (parmanu or atomos) centuries ago, it was John Dalton in 1803 who provided the first scientific framework, proposing that all matter is made of indivisible units called atoms.

In science, we classify matter based on its purity and composition. A pure substance isn't just something "clean"; it is matter that consists of only one type of particle and cannot be separated into other kinds of matter by physical means Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.121. These pure substances are further divided into Elements (the simplest building blocks like Gold or Oxygen) and Compounds (where elements bond chemically in a fixed ratio, like H₂O). When substances are just physically blended together without a chemical reaction, they form Mixtures.

Understanding the distinction between a compound and a mixture is a classic UPSC favorite. In a mixture, the components (like sand and salt) retain their original properties. However, in a compound, the new substance has entirely different properties from its constituents. For instance, when iron filings and sulfur powder are heated, they react to form Iron Sulfide (FeS), a compound that no longer behaves like magnetic iron or yellow sulfur Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.132.

Feature	Element	Compound	Mixture
Composition	Only one type of atom.	Two or more elements chemically combined in a fixed ratio.	Two or more substances physically mixed in any ratio.
Properties	Fundamental properties.	Entirely different from constituent elements.	Retains the properties of its individual components.
Separation	Cannot be broken down.	Separated only by chemical or electrochemical reactions.	Separated by physical methods (filtration, evaporation).

Sources: Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.117; Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.121; Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130; Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.132

2. Laws of Chemical Combination (intermediate)

In our journey to understand the building blocks of matter, we must first look at the mathematical guardrails that govern how substances interact. Chemistry is not a game of chance; it follows the Laws of Chemical Combination. The most fundamental of these is the Law of Conservation of Mass. This principle states that mass can neither be created nor destroyed during a chemical reaction. When you perform an experiment, the total mass of your reactants (the starting materials) must exactly equal the total mass of your products. As noted in Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3, this is the scientific reason why we must always balance chemical equations—to ensure that the number of atoms of each element remains the same before and after the reaction.

Closely following this is the Law of Definite Proportions (or Constant Proportions). This law tells us that in a pure chemical substance, the elements are always present in definite proportions by mass, regardless of the source or method of preparation. For example, in H₂O, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8. Whether you take water from a river in India or a lab in Europe, that ratio remains constant. This suggests that nature uses a specific "recipe" for every compound, using fixed numbers of atoms.

In 1803, the English chemist John Dalton synthesized these observations into the first modern Atomic Theory. He proposed that all matter is composed of tiny, indivisible particles called atoms. Dalton’s genius was in explaining why the previous laws worked: if atoms are indivisible and have specific masses, then a chemical reaction is simply a rearrangement of these "building blocks." While we now know that atoms can be further divided into subatomic particles like electrons and the nucleus, Dalton’s framework remains the foundation of how we visualize chemical changes today.

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3

3. Ancient Perspectives: Kanad and Democritus (basic)

Before the advent of modern laboratories and microscopes, the human mind used the power of logic and philosophy to explore the fundamental nature of the universe. In both ancient India and Greece, thinkers arrived at a strikingly similar conclusion: that matter is not infinitely divisible. They reasoned that if you keep breaking an object into smaller and smaller pieces, you must eventually reach a particle so tiny that it cannot be divided further.

In ancient India, this concept was pioneered by Acharya Kanad (roughly 6th century BCE), the founder of the Vaisheshika school of philosophy. He postulated that all matter is composed of tiny, eternal, and indivisible particles which he called Parmanu. According to his work, the Vaisheshika Sutras, these particles combine in various ways to form the visible world around us Science, Class VIII NCERT, Particulate Nature of Matter, p.101. This was a revolutionary leap in thought, suggesting a particulate nature of matter long before the scientific revolution.

Almost simultaneously in ancient Greece, philosophers like Democritus and Leucippus proposed a similar theory. Democritus called these ultimate particles atomos (meaning 'uncuttable' or 'indivisible'). He imagined these atoms moving through an empty void, differing only in shape and size, and believed that their arrangements determined the properties of different substances. While these ancient perspectives lacked the empirical evidence we demand today, they provided the essential philosophical seeds that John Dalton would eventually cultivate into the first scientific atomic theory over two millennia later.

Feature	Acharya Kanad (India)	Democritus (Greece)
Term Used	Parmanu	Atomos
Core Idea	Indivisible, eternal particles.	Indivisible particles in a void.
Source of Knowledge	Philosophical reasoning (Vaisheshika Sutras).	Philosophical reasoning and logic.

Sources: Science, Class VIII NCERT, Particulate Nature of Matter, p.101

4. Modern Atomic Structure and Subatomic Particles (intermediate)

The journey of understanding the atom began with the revolutionary proposal by John Dalton in 1803. He theorized that all matter is composed of tiny, indivisible particles called atoms. This was a monumental shift from ancient philosophical ideas, such as those of Acharya Kanad (who spoke of parmanu) and Democritus, because Dalton provided an evidence-based framework. He believed atoms were the ultimate building blocks of the universe—permanent structures that could neither be created nor destroyed. However, the concept of the 'indivisible' atom was eventually overturned by the discovery of subatomic particles. Through various experiments, scientists like J.J. Thomson and Ernest Rutherford revealed that the atom has a complex internal architecture. We now know that the atom consists of a tiny, dense, positively charged nucleus at its center, surrounded by electrons (negatively charged particles). To probe this structure, researchers often used alpha particles—positively charged particles that can be deflected by magnetic fields Science, Class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.204. The modern atomic model is built upon three primary subatomic particles, each defined by its charge and location:

Particle	Charge	Location
Proton	Positive (+)	Inside the Nucleus
Neutron	Neutral (0)	Inside the Nucleus
Electron	Negative (-)	Orbiting the Nucleus

In the context of nuclear stability, these particles play a vital role. Unstable atoms may release radiation in the form of Alpha, Beta, or Gamma particles/rays. These vary greatly in their energy; for example, while a simple sheet of paper can stop an alpha particle, gamma rays are highly penetrating and require thick concrete or lead for shielding Environment, Shankar IAS Academy, Environmental Pollution, p.82.

Sources: Science, Class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.204; Environment, Shankar IAS Academy, Environmental Pollution, p.82

5. Isotopes, Isobars, and Atomic Species (intermediate)

To understand the diversity of matter, we must look into the heart of the atom. An atom is defined as the smallest particle of an element that retains its unique characteristics Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100. While early scientists like John Dalton envisioned the atom as an indivisible sphere, we now know it consists of a nucleus (containing protons and neutrons) surrounded by electrons. Two fundamental numbers define an atom: the Atomic Number (Z), which is the number of protons and identifies the element, and the Mass Number (A), which is the sum of protons and neutrons.

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 number of protons, their chemical properties are nearly identical. However, their physical masses differ. For example, Carbon exists naturally as Carbon-12 and Carbon-14. Carbon-14 is particularly famous in archaeology; by measuring its radioactive decay, scientists can determine the age of ancient artifacts, such as the 580 BCE samples found at the Keeladi excavation site History, class XI (Tamilnadu state board 2024), Evolution of Society in South India, p.70.

Isobars, on the other hand, are atoms of different elements that share the same Mass Number (A) but have different Atomic Numbers (Z). In chemistry, isobars have completely different chemical properties because they are different elements (like Argon-40 and Calcium-40). Interestingly, in the context of UPSC Geography, the term "isobar" refers to lines on a weather map connecting points of equal atmospheric pressure—an example of an isopleth map Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305. It is vital not to confuse the chemical species with the meteorological tool!

Feature	Isotopes	Isobars
Atomic Number (Protons)	Same	Different
Mass Number (Protons + Neutrons)	Different	Same
Chemical Properties	Similar	Different
Example	¹²C and ¹⁴C	⁴⁰Ar and ⁴⁰Ca

Finally, some isotopes are unstable and exhibit radioactivity. This is the process where a nucleus spontaneously disintegrates, emitting particles like alpha (protons), beta (electrons), or gamma rays Environment, Shankar IAS Academy, Environmental Pollution, p.82. This property is what allows for medical treatments and carbon dating.

Sources: Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100; History, class XI (Tamilnadu state board 2024), Evolution of Society in South India, p.70; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305; Environment, Shankar IAS Academy, Environmental Pollution, p.82

6. Dalton's Atomic Theory: The Scientific Turning Point (basic)

For centuries, the world's greatest thinkers wondered: if you keep cutting an object into smaller and smaller pieces, would you ever reach a point where you couldn't cut any further? In ancient India, the philosopher Acharya Kanad proposed the concept of Parmanu—tiny, eternal, and indivisible particles that form the foundation of all matter Science Class VIII, NCERT, Particulate Nature of Matter, p.101. However, these early ideas were philosophical. The true scientific turning point came in 1803 when John Dalton formulated the first modern atomic theory, moving the discussion from abstract thought to evidence-based science.

Dalton proposed that all matter—whether an element, a compound, or a mixture—is composed of tiny particles called atoms. His theory was built on several key postulates: atoms are indivisible units that cannot be created or destroyed in a chemical reaction; all atoms of a given element are identical in mass and properties; and atoms of different elements combine in simple whole-number ratios to form compounds. This framework allowed scientists to understand why matter behaves predictably, such as why a sample of pure gold is always made up of the same gold atoms Science Class VIII, NCERT, Particulate Nature of Matter, p.115.

While Dalton’s theory was revolutionary, science is a process of constant refinement. Later discoveries revealed that the atom is not actually "indivisible." We now know that atoms are composed of subatomic particles—protons, neutrons, and electrons. In fact, in the very early stages of our universe, these particles existed independently before combining to form the first atoms of hydrogen and helium Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2. Today, we understand that chemical reactions involve the sharing or transfer of electrons rather than the shifting of indivisible solid spheres Science Class X, NCERT, Carbon and its Compounds, p.59. Despite these updates, Dalton’s work remains the bedrock of chemistry because it was the first to define the atom as the fundamental unit of chemical identity.

Sources: Science Class VIII, NCERT, Particulate Nature of Matter, p.101; Science Class VIII, NCERT, Particulate Nature of Matter, p.115; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2; Science Class X, NCERT, Carbon and its Compounds, p.59

7. Solving the Original PYQ (exam-level)

Now that you have explored the historical evolution of atomic models, you can see how the building blocks of chemistry align with this question. The core concept here is the Atomic Theory of Matter, which transitioned from ancient philosophical ideas to a formal scientific framework in the early 19th century. While thinkers like Acharya Kanad and Democritus theorized about the smallest particles of matter, it was John Dalton who formulated the first modern scientific theory. His postulate stated that matter is composed of indivisisible particles, which serves as the foundational (though later revised) premise of chemistry.

To arrive at the correct answer, (A) Dalton, you must focus on the phrase "proposed that atom is indivisible" as a formal scientific postulate. Think of it this way: Dalton treated the atom as a hard, solid billiard ball that could not be broken down further, created, or destroyed. While we now know that atoms contain subatomic particles like electrons and protons, for the purpose of the initial scientific proposal, Dalton is the primary figure. In UPSC, the distinction between philosophical speculation and scientific theory is a common theme, and Dalton represents the leap into evidence-based science as noted in Britannica.

UPSC often uses the names of other pioneering scientists as distractors to test the precision of your timeline. Berzelius is a trap because he focused on atomic weights and chemical symbols rather than the internal structure of the atom. Avogadro is famous for his work on gases and molecules, distinguishing between the two but not focusing on atomic indivisibility. Most importantly, Rutherford is the ultimate "counter-trap"; his gold foil experiment actually disproved Dalton’s idea of indivisibility by discovering the nucleus and proving that the atom is mostly empty space. Always remember: Dalton built the house, and the others either decorated it or looked inside the rooms to find smaller parts!