Journey Inside

8.1 Rediscovering the Roots of Atomic Theory

The concept of the atom has a rich history dating back over 2,000 years, originating independently in ancient India and Greece. Acharya Kanada, an ancient Indian philosopher, proposed that matter (dravya) can be divided repeatedly until reaching the smallest indivisible particles called parmanus. These parmanus are infinitely small, imperceptible by the senses, and combine in groups such as dyads and triads to form all material substances. His ideas are recorded in the Vaisesika Sutras. Similarly, Greek philosophers Leucippus and Democritus introduced the idea of atomos, meaning indivisible particles, as the fundamental building blocks of matter. These early atomic concepts were philosophical and hypothetical rather than experimental. Centuries later, in 1808, John Dalton formulated the first scientific atomic theory based on experiments. Dalton proposed that all matter consists of indivisible atoms, which are the fundamental units that cannot be broken down further. Dalton's theory marked the beginning of modern atomic science and raised questions about the internal structure of atoms, their constituents, and differences among atoms of various elements. This section sets the stage for exploring how atomic models evolved through scientific inquiry and experimentation.

• Ancient Indian philosopher Acharya Kanada proposed indivisible particles called parmanus.
• Greek philosophers Leucippus and Democritus introduced the concept of atomos.
• Early atomic ideas were philosophical, not based on experiments.
• John Dalton's atomic theory (1808) was the first scientific description of atoms.
• Dalton proposed atoms as indivisible fundamental units of matter.
• Dalton's theory prompted questions about atomic structure and differences among elements.
• 📌 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 particles.

8.2 A Short Historical Journey Through Atomic Models

This section traces the evolution of atomic models from Dalton's indivisible atom to more complex structures revealed by experiments. Initially, atoms were thought to be indivisible. However, discoveries of radioactivity showed that atoms emit invisible energy and particles, indicating internal structure. In 1897, J. J. Thomson discovered electrons through cathode ray tube experiments, revealing atoms contain smaller negatively charged particles. Thomson proposed the plum pudding model, where electrons are embedded in a positively charged sphere, balancing the atom's neutrality. This model likened the atom to a watermelon or plum pudding, with electrons scattered like seeds or plums in a positive matrix. The cathode ray tube experiment involved applying high voltage across electrodes in a gas-filled tube, producing cathode rays identified as electrons. Thomson's discovery was pivotal, showing atoms are divisible and composed of subatomic particles. However, his model could not explain later experimental results, leading to further refinements.

• Radioactivity showed atoms emit energy and particles, indicating internal structure.
• J. J. Thomson discovered electrons via cathode ray tube experiments in 1897.
• Cathode rays are streams of negatively charged electrons emitted from atoms.
• Thomson proposed the plum pudding model: electrons embedded in a positive sphere.
• The atom is electrically neutral due to balanced positive and negative charges.
• Thomson's model was the first to describe internal atomic structure.
• 📌 Electron: Negatively charged subatomic particle discovered by Thomson.
• 📌 Cathode rays: Streams of electrons observed in vacuum tubes.
• 📌 Plum pudding model: Thomson's atomic model with electrons embedded in positive charge.