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held that everything is composed of "atoms", which are physically, but not geometrically, indivisible
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Elements are made of extremely small particles called atoms
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Mendeleev realized that the physical and chemical properties of elements were related to their atomic mass in a 'periodic' way, and arranged them so that groups of elements with similar properties fell into vertical columns in his table.
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Thomson believed electrons to be two thousand times lighter that a proton. He assumed that an atom is composed of a cloud of negative charge in a sphere of positive charges
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The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun.
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The Bohr model and all of its successors describe the properties of atomic electrons in terms of a set of allowed (possible) values.
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It consisted of a dense nucleus surrounded by a cloud of electrons at various levels in orbitals.
