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The Beginning of Atomic Theory
The composition of matter is a question fundamental to our understanding of the world. Many once thought that matter could be split up into infinitesimal pieces; in fact, until the 19 th century, the answer to the question remained largely speculative. However, in the years 1803-1807, John Dalton, an English schoolteacher, carried out several experiments based on the laws of conservation of matter and of definite proportions. [1] The flaws in Dalton's conception of the atom were gradually corrected by other scientists, and the atomic model still undergoes modification as new discoveries are made.
Thomson's Model and Rutherford's Correction
The first formal model of an atom was put forth by British scientist J. J. Thomson in 1897, who proposed that an atom consisted of a sphere of positive charge, with negatively charged electrons dotted within it [Figure 1]. It was nicknamed the 'plum-pudding' model, as it represented an atom as a positively charged mass with electrons randomly laid about inside it, much like raisins in a cake [2] (Thomson is also credited with the discovery of the existence of electrons through an experiment involving the deflection of cathode rays with a magnetic field [3] ). However, this model was disproved by Ernest Rutherford in an experiment in which α-particles (positively charged products of nuclear radiation) were passed through extremely thin gold foil (only several thousand atoms thick) [Figure 2]. According to Thomson's model, all of...