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The Atom: A Philosophical Idea • The idea of atoms is very old. • Democritus (Greek, 400 BC) - said that things were made up of tiny, indivisible particles (atoms) – only an idea • Aristotle - disagreed with Democritus’ idea – his opinion more popular. • No evidence to support either idea. Three Fundamental Laws of Chemistry • Law of Conservation of Mass (1789): – In ordinary chemical reactions, mass is neither created nor destroyed. – Mass of the reactants equals the mass of the products. • Law of Definite Proportions (1797): – A chemical compound contains the same elements in exactly the same proportions regardless of the size of the sample or source of the compound. – Example: water – hydrogen to oxygen ratio is always 2 to 1. • Law of Multiple Proportions (1803): – if two or more different compounds are composed of the same two elements, then the proportions can be expressed as simple, whole-number ratios. – Example: • Carbon and Oxygen – Carbon monoxide - CO – Carbon dioxide – CO2 John Dalton (1808) • English school teacher, wanted to explain the laws of conservation of mass, definite proportions and multiple proportions. • His model is called the Atomic Theory. Postulates of his theory • All matter is composed of extremely small particles called atoms. • Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. • Atoms cannot be subdivided, created, or destroyed. • Atoms of different elements combine in simple wholenumber ratios to form chemical compounds. • In chemical reactions, atoms are combined, separated, or rearranged. • Some aspects of Dalton’s atomic theory have been revised. We now know that: – Atoms are divisible into even smaller particles. – A given element can have atoms with different masses. One Minute Paper • You have one minute to answer these two questions concerning today’s lesson. – What was the most important thing you learned? – What is still muddy?