![]() Thomson in 1903, the atom was represented as a positively charged sphere with the negatively charged electrons distributed around the exterior. Now that it was known that the atom had component parts, a new model was needed. His periodic table remains a major idea in chemistry today. ![]() In 1869, Dmitri Mendeleev ascertained that there were groups of elements which had the same (or very similar) chemical properties, the same valence, and similar physical properties and showed that the chemical and physical properties of the elements were periodically repeated. ![]() Dalton’s theory underwent a number of modifications in the next 150 years but many of its ideas are still part of the modern atomic theory. It did convince a number of chemists right away but several decades were required for all opposition to cease. Dalton’s theory proposed a number of basic ideas: 1) All matter is composed of indivisible, tiny particles called atoms, 2) atoms can neither be created nor destroyed, 3) all atoms of the same element are identical (have the same mass), 4) elements differ from one another because they have different types of atoms (different mass), 5) compounds are composed of combinations of different elements because the atoms of different elements are bonded to one another, and 6) chemical reactions occur when atoms in compounds are rearranged.ĭalton’s theory did not convince everyone immediately. Dalton’s theory went beyond just a philosophical statement that there are atoms. Dalton’s theory was different from previous discussions of atoms because it had the weight of careful chemical measurements behind it. ![]() John Dalton developed his atomic theory in 1803. ![]()
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