Journey Inside the Atom

8.1 Rediscovering the Roots of Atomic Theory

The concept of the atom has ancient origins, dating back more than 2,000 years to the intellectual traditions of ancient India and Greece. In India, Acharya Kanada proposed that matter (dravya) can be divided repeatedly until it reaches a stage where it cannot be divided further. He called these smallest indivisible particles 'parmanus'. According to his idea, parmanus are infinitely small and imperceptible to the senses. Parmanus combine to form dyads (two parmanus), triads (three parmanus), and so forth, and these combinations constitute the entire material universe, including living beings. However, this ancient theory did not specify the proportions in which parmanus combine to form different substances. Similarly, Greek philosophers Leucippus and Democritus proposed the idea of indivisible particles called 'atomos' (meaning indivisible in Greek). Their atomic concept was more philosophical and hypothetical rather than based on experimental evidence. Centuries later, in 1808, John Dalton formulated the first scientific atomic theory based on experiments. Dalton proposed that all matter is composed of indivisible atoms, which are the fundamental building blocks of matter. According to Dalton's atomic theory, atoms cannot be broken down into smaller parts. This theory laid the foundation for modern atomic science. Dalton's theory raised further questions: What are atoms made of? What is their internal structure? How do atoms of different elements differ? These questions motivated scientists to explore atomic structure further, leading to the development of atomic models.

• Ancient Indian philosopher Acharya Kanada proposed indivisible particles called parmanus.
• Greek philosophers Leucippus and Democritus introduced the concept of atomos (indivisible particles).
• Dalton's atomic theory (1808) was the first scientific description of atoms as indivisible particles.
• Dalton's theory was based on experiments, unlike earlier philosophical ideas.
• Dalton's theory led to questions about atomic composition and differences among elements.
• The concept of atom evolved from philosophical to experimental understanding.
• 📌 Atom: The smallest unit of matter that retains the properties of an element.
• 📌 Parmanu: The indivisible particle proposed by Acharya Kanada.
• 📌 Atomos: Greek term meaning indivisible, used by Leucippus and Democritus.

8.2 A Short Historical Journey Through Atomic Models

The understanding of atomic structure evolved through a series of models proposed by scientists over more than a century. Initially, atoms were believed to be the smallest indivisible units of matter. However, the discovery of radioactivity in the late 19th century revealed that atoms emit radiation, indicating they are composed of smaller particles. In 1897, J. J. Thomson conducted experiments using cathode ray tubes, where he observed cathode rays traveling from the negative electrode (cathode) to the positive electrode (anode). He concluded that these rays were streams of negatively charged particles, later named electrons, which have much smaller mass than atoms. This discovery proved that atoms are divisible and contain subatomic particles. Thomson proposed the 'plum pudding model' of the atom, where the atom is a positively charged sphere with electrons embedded throughout, like plums in a pudding or seeds in a watermelon. This model explained the neutrality of atoms but was later disproved. In 1911, Ernest Rutherford, along with Geiger and Marsden, performed the gold foil experiment. They directed alpha particles (positively charged helium nuclei) at a thin gold foil. Most alpha particles passed through undeflected, but some were deflected at large angles, and a few even bounced back. This showed that the positive charge and most of the mass of the atom are concentrated in a tiny central nucleus, with electrons orbiting around it. Rutherford's planetary model replaced Thomson's model. However, Rutherford's model could not explain why electrons do not spiral into the nucleus despite accelerating in circular orbits. This led to Niels Bohr's model in 1913, which introduced fixed energy levels or shells where electrons revolve without losing energy. Bohr's model explained atomic stability and the emission spectra of atoms. Later, James Chadwick discovered the neutron in 1932, a neutral particle in the nucleus contributing to atomic mass but not charge. This completed the basic understanding of atomic composition: electrons, protons, and neutrons.

• Discovery of electrons by J. J. Thomson using cathode ray experiments.
• Thomson's plum pudding model: electrons embedded in a positive sphere.
• Rutherford's gold foil experiment revealed a small, dense, positively charged nucleus.
• Rutherford's planetary model: electrons orbit nucleus like planets around the sun.
• Bohr's model introduced fixed energy levels (shells) for electrons to explain atomic stability.
• James Chadwick discovered neutrons, completing the subatomic particle picture.
• 📌 Electron: Negatively charged subatomic particle discovered by J. J. Thomson.
• 📌 Plum Pudding Model: Thomson's atomic model with electrons embedded in positive charge.
• 📌 Nucleus: Small, dense, positively charged center of the atom discovered by Rutherford.