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Journey Inside the Atom 141 8.1 Rediscovering the Roots of Atomic Theory Let us embark on a journey that takes us back more than 2,000 years, to the intellectual landscapes of ancient India and ancient Greece. In these distant yet remarkably parallel civilisations, profound thinkers, such as Acharya Kanada in India, and Leucippus and Democritus in Greece, pondered over the same fundamental question that has continued to inspire human inquiry across centuries — What is everything made up of? Acharya Kanada suggested that if matter (dravya) is divided repeatedly, you will reach a stage where you would encounter the smallest particles that can no longer be divided. He called these particles parmanus. His ideas are recorded in the Sanskrit text Vaisesika Sutras. A parmanu is infinitely small and cannot be perceived by the senses. Combinations of these forms dyads (groups of two parmanus) and triads (groups of three parmanus), and so on. It is out of these combinations that the whole of the material universe, including the bodies of living beings, is created. However, this description does not specify the proportions in which parmanus combine to form different substances. The Greek philosophers Leucippus and Democritus also proposed a similar idea. They called these indivisible particles atomos (in Greek, atomos means indivisible). You must remember that the concept of ‘atom’ originated as an imaginary idea rather than from experimental observations. Many centuries later, in 1808, John Dalton proposed his atomic theory. It was based on scientific experiments of that time. He proposed that all matter is composed of indivisible particles called atoms. That is, the atoms are the fundamental building blocks of matter that cannot be broken down into smaller parts. Dalton’s atomic theory was the first scientific description of how matter is made. It became the starting point for the current understanding of atomic structure. You may be wondering how Dalton’s idea evolved into a model for the structure of the atom. Following Dalton’s theory, scientists were curious to know the answers of the questions: y What are atoms made up of ? y What would atoms look like if we could see them? y What makes the atoms of one element different from the atoms of another element? 8.2 A Short Historical Journey Through Atomic Models Morethanahundredyearsago,scientiststriedtoimaginewhatatomsmight look like by proposing simple models. As new experiments were performed and new evidence came to light, these models were changed and improved. Although we now know that the early models were not fully correct, they are still important because they show how science moves forward — one step at a time, driven by curiosity, questioning, and experimentation. Until the late 19th century, atoms were thought to be the smallest, indivisible units of matter. However, scientists discovered that certain elements emit invisible energy and particles called radiation,