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Atomic Theory Atomic Theory As early as 400 BC, Greek philosophers proposed the atomic theory. In 1808, John Dalton, an English chemist and physicist, published an atomic theory. 1. Each element is composed of extremely small particles called atoms. Atomic Theory continued 2. All atoms of a given element are identical 3. Atoms of different elements have different properties, including mass and chemical reactivity. 4. Atoms are not changed by chemical reactions, but merely rearranged into different compounds. Atomic Theory continued 5. Compounds are formed when atoms of more than one element combine. 6. A compound is defined by the number, type (element), and proportion of the constituent atoms. Over time, new models have been created and the theory has been expanded and updated. Atom Models • In a famous experiment using alpha particles and gold foil, Ernest Rutherford discovered that the atom was mostly empty space. • Niels Bohr developed a model of the atom in which electrons move around the nucleus in fixed orbits and have a set amount of energy. We often use this model when learning about atoms. Current Model of the Atoms Erwin Schrodinger, and Austrian physicist, developed the quantum mechanical model of the atom. We call it the electron cloud model because it cannot be described in words or pictures. The cloud represents the probability of electron position. The Neutron In 1932, James Chadwick discovered the neutron which explain the missing mass in an atoms nucleus. Many new technological developments have come about as a result of these discoveries. The late 20th century has become known as the “atomic age”.