The History of the Atom

Topics: Atom, Electron, Neutron Pages: 2 (549 words) Published: February 23, 2013
The History of the Atom
The History of the Atom

Democritus first proposed an atomic model. He said that all matter could be divided into smaller and smaller pieces until a single indivisible particle was reached. He called this particle an atom after the Greek word ‘atomos’ meaning not cuttable. He believed that different atoms are of different sizes, have regular geometric shapes and are always in motion. He also said that there is empty space between atoms. Aristotle severely criticized Democritus’ theory, arguing that the idea of atoms in continuous motion is a void is not logical. He supported the ‘four element’ theory of matter (proposed by Empedocles a century earlier. This theory is based on the idea that all matter is made up of four basic substances; earth, water, air and fire. He said each of these basic substances had different combinations of four qualities: dry, moist, cold and hot. Dalton expanded on the atomic theory proposed by Democritus. It consisted of the following statements: all matter is composed of tiny, indivisible particles called atoms, all atoms of an element have identical properties, atoms of different elements have different properties, atoms of two or more elements can combine in constant ratios to form new substances, in chemical reactions, atoms join together or separate from each other but are not destroyed. This theory explains the law of Conservation of Mass. He discovered that atoms are only rearranged during chemical reactions, atoms are neither created nor destroyed in chemical reaction and therefore there will be no change in mass during chemical reactions. His theory explains the Law of Constant Composition, which is that atoms combine to form molecules in a fixed ratio in a given chemical reaction. He also said that since the atoms of an element have identical properties, such as mass, and combine I constant ratios, every compound must have a fixed, definite composition. Thomson did experiments using Cathode ray...
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