The Atom A Philosophical Idea The idea of

The Atom: A Philosophical Idea • The idea of atoms is very old. • Democritus (Greek, 400 BC) – Noncontinuous --- said that things were made up of tiny, indivisible particles (atoms) – only an idea • Aristotle - disagreed with Democritus’ idea – continuous --- his opinion more popular. • No evidence to support either idea.

Three Important Laws • Law of Conservation of Mass (1789): – Mass is neither created nor destroyed. • Law of Definite Proportions (1797): – A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound. – Example: water – 89% by mass of oxygen and 11% by mass of hydrogen

• Law of Multiple Proportions (1803): – if two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers – Example: Carbon and Oxygen • One compound – 1. 33 g oxygen combines with 1. 00 g carbon • Another compound – 2. 66 g oxygen combines with 1. 00 g carbon

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?