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A Greek philosopher named Democritus said that all atoms are small, hard particles. He thought that atoms were made of a single material formed into different shapes and sizes. The word "_________elementcompoundmixtureatom" is derived from the Greek word "atomos" which means "not able to be divided."
In 1803, John Dalton, a school teacher, proposed his atomic theory. Dalton's theory states that elements (substances composed of only one type of _________moleculesionsatom) combine in certain proportions to form _________compoundsatomsmixtureselements.
In 1897, a British scientist named J. J. Thomson experimented with a cathode-ray tube which had a positively charged plate. The plate attracted negatively charged particles that we now call _________protonsneutronselectronsquarks. Rather than being indivisible particles, Thomson's plum-pudding atomic model states that atoms contain _________positivelynegativelyneutrally charged electrons embedded within a sea of positive charge.
In 1909, Ernest Rutherford conducted an experiment in which he aimed a beam of positively charged particles at a thin sheet of gold foil. Most of the particles went straight through the gold foil, some were deflected and others bounced straight back. Because some of the particles bounced straight back, Rutherford was able to show that the center of the atom, the nucleus, is _________neutralpositively chargednegatively chargedhollow and _________diffuselargevary small. The nucleus contains protons, which are positively charged, and neutrons, which are _________positively chargednegatively chargedneutral.
In 1913, Niels Bohr, a Danish scientist who worked with Dr. Rutherford, proposed that electrons move around the nucleus in certain paths, or energy levels. This model was improved upon by an Austrian physicist named Erwin Schröedinger and a German physicist named Werner Heisenberg. Schröedinger and Heisenberg proposed that electrons do not move in definite paths around the nucleus, but are be found in regions around the nucleus called electron _________wavescloudsstratospheres.
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