STRUCTURE OF ATOM
In the fifth century B.C. the Greek Philosopher Democritus proposed that all matter consists of very small indivisible particles called atomos (meaning uncuttable or indivisible). These earlier ideas were mere speculations and did not have any experimental basis. It was in 1808 that an English scientist, John Dalton formulated a precise definition of the indivisible building blocks of matter that are known as atoms. By the end of 19th century it was discovered that atoms consist of sub-atomic particles such as electrons, protons and neutrons. The protons are positively charged particles and are present in the nucleus of the atom. The electrons are negatively charged particles and are present in the extranuclear part of the atom. The neutrons are neutral particles and are present in the nucleus along with the protons. Most of the mass of the atom is concentrated in the nucleus.
During the past few years it has been found experimentally that some of the subatomic particles can be further split into two elementary particles, quarks and leptons.
In this unit, we shall discuss various experiments which led to the discovery of the various sub-atomic particles and evolution of the model for the internal structure of atom.