Discovery of Sub-atomic Particle - ElectronĪn insight into the structure of atoms was obtained from the experiments on electrical discharge through gases.These established that atoms are made of sub-atomic particles, i.e., electrons, protons, and neutrons - a concept very different from that of Dalton. In this unit, we start with the experimental observations made by scientists towards the end of the nineteenth and beginning of the twentieth century. However, it failed to explain the results of many experiments, for example, it was known that substances like glass or ebonite when rubbed with silk or fur get electrically charged. Dalton’s atomic theory was able to explain the law of conservation of mass, law of constant composition, and law of multiple proportions very successfully. His theory called Dalton’s atomic theory regarded the atom as the ultimate particle of matter (Unit 1). The atomic theory of matter was first proposed on a firm scientific basis by John Dalton, a British school teacher in 1808. These ideas remained dormant for a very long time and were revived again by scientists in the nineteenth century. These earlier ideas were mere speculations and there was no way to test them experimentally. The word ‘atom’ has been derived from the Greek word ‘ a-tomio’ which means ‘uncut-able’ or ‘non-divisible’. According to them, the continued subdivisions of matter would ultimately yield atoms that would not be further divisible. The existence of atoms has been proposed since the time of early Indian and Greek philosophers (400 B.C.) who were of the view that atoms are the fundamental building blocks of matter.
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