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Applied Chemistry Mr. Gensits Class Notes: September 30, 2014 Matter Is Made Up of Atoms Introduction to Atomic Theory Some early Greek philosophers believed that there were four elements: air, earth, fire, and water. Democritus (4th Century B.C.) Suggested the existence of tiny indestructible particles (atoms) that made up all matter Development of the Modern Atomic Theory Lavoisier (1782) Noted that the masses of the reactants before a chemical reaction was equal to the masses of the products after the reaction was completed. This became known as the Law of Conservation of Matter (Mass) Joseph Proust (1799) Observed that the composition of water was always 11% hydrogen and 89% oxygen. Law of Definite Composition The elements that compose a compound are always in a certain percentage by mass. John Dalton –first formulated the main principles of modern atomic theory Dalton’s Atomic Theory (1803) 1. All elements are composed of tiny indivisible particles called atoms. 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 3. Atoms of different elements can combine with one another in single whole number ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atomic Particles Electrons negatively charged particles discovered first (1897) by J. J. Thomson travels around the nucleus found in the electron cloud Nucleus - central part of the atom discovered in 1911 by Ernest Rutherford Protons positively charged discovered 1918 (Rutherford)? 1840 times more massive than electron found in the nucleus it is believed that protons are composed of three smaller particles called quarks Neutrons Have no charge. discovered in 1932 by James Chadwick in the nucleus it is believed that a neutron is also composed of three quarks Review of Atomic Models Dalton Model Thomson Model Rutherford Model