Structure of Atom

From "Uncuttable" to Sub-Atomic

Long before modern chemistry existed, early Indian and Greek thinkers (around 400 BCE) had already speculated that matter could only be divided so many times before reaching a fundamental, indivisible unit. The Greek word for this idea — a-tomio, "uncuttable" — is the root of the word atom we still use today. These early ideas were philosophical guesses with no experimental backing, and they lay dormant for over two thousand years before being revived by nineteenth-century scientists.

It was John Dalton, a British schoolteacher, who in 1808 gave the atomic idea a firm scientific footing. Dalton's atomic theory treated the atom as the ultimate, indivisible particle of matter, and it successfully explained several laws of chemical combination — the conservation of mass, constant composition, and multiple proportions. But it could not explain a simple observation already known at the time: rubbing glass or ebonite with silk or fur makes them electrically charged. If atoms were truly indivisible, where did this electric charge come from?

This chapter picks up that thread. Working through experiments carried out at the end of the nineteenth and the beginning of the twentieth century, you will see how scientists established that the atom is not indivisible after all — it is built from three sub-atomic particles: the electron, the proton, and the neutron. From there, the chapter traces how our picture of where these particles sit inside the atom evolved — from Thomson's and Rutherford's early atomic models, through Bohr's quantised orbits, to the modern quantum mechanical model, which describes an electron not as a point moving along a fixed path but as a probability cloud governed by its own set of quantum numbers.