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# Atomic Theory Timeline ## Democritus (400 BC) * Proposed that matter was composed of tiny individual particles. * This was not based on experimental data. * He coined the Greek term "atom." * His ideas were not widely accepted. ## Alchemy (Next 2000 Years) * Efforts to turn base metals into go...

# Atomic Theory Timeline ## Democritus (400 BC) * Proposed that matter was composed of tiny individual particles. * This was not based on experimental data. * He coined the Greek term "atom." * His ideas were not widely accepted. ## Alchemy (Next 2000 Years) * Efforts to turn base metals into gold. * Lab procedures were developed. ## John Dalton (1805) * A British school teacher. * Developed an atomic theory based on experimental evidence. * Described the atom as a uniform, solid sphere. * **Four Postulates:** 1. Elements are made of tiny particles called atoms. 2. Atoms of the same element are identical. 3. Atoms of different elements are different. 4. Atoms combine to form compounds in whole number ratios. 5. In a chemical reaction, atoms are rearranged, but not changed or destroyed. ## Henri Becquerel (1896) * Discovered radioactivity from elements. * Identified three types of radiation: * Alpha (α): positively charged * Beta (β): negatively charged * Gamma (γ): neutral ## J.J. Thompson (1897) * Discovered the electron. * Proposed the "plum pudding" model of the atom. * Described the atom as a positively charged sphere with negatively charged electrons embedded within it, like plums in a pudding. ## Ernest Rutherford (1911) * Performed the gold foil experiment. * Proposed the nuclear model of the atom. * Described the atom as having a dense, positively charged nucleus with negatively charged electrons orbiting it. * Positively charged particles are called protons.