Objectives Explain the law of conservation of mass, the law of definite proportions, and the law of multiple proportions. Summarize the five essential points of Dalton’s atomic theory. Explain the relationship between Dalton’s atomic theory and the law of conservation of mass, the law of definite proportions, and the law of multiple proportions. Section 1 The Atom: From Philosophical Idea to Scientific Theory Chapter 3

Development of the atomic theory • The search for a description of matter began with the Greek philosopher Democritus more than 2000 years ago. • He and many other philosophers had puzzled over this question: • Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided? In other words, is matter continuous or discontinuous?

Developing the atomic theory • Democritus concluded that matter could not be divided into smaller and smaller pieces forever. Eventually the smallest possible piece would be obtained. • Democritus named this small, indivisible piece, an atom; atom comes from the Greek word atomos, which means “not to be cut” or “indivisible.”

Developing the atomic theory • Aristotle was part of the generation that succeeded Democritus. • Aristotle did not believe in atoms, instead, he thought that all matter was continuous. His opinion was accepted for nearly 2000 years

Developing the atomic theory • In the early 1800’s, the English chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms. • In 1803, Dalton combined the results of his experiments with other observations about matter and proposed an atomic theory.

Foundations of Atomic Theory The transformation of a substance or substances into one or more new substances is known as a chemical reaction. Law of conservation of mass:mass is neither created nor destroyed during ordinary chemical reactions or physical changes Section 1 The Atom: From Philosophical Idea to Scientific Theory Chapter 3

Foundations of Atomic Theory, continued Law of definite proportions: 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 Law of multiple proportions: 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 Section 1 The Atom: From Philosophical Idea to Scientific Theory Chapter 3

Section 1 The Atom: From Philosophical Idea to Scientific Theory Chapter 3 Law of Conservation of Mass

Section 1 The Atom: From Philosophical Idea to Scientific Theory Chapter 3 Law of Multiple Proportions

Dalton’s Atomic 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. Section 1 The Atom: From Philosophical Idea to Scientific Theory Chapter 3

Dalton’s Atomic Theory, continued Atoms of different elements combine in simple whole-number ratios to form chemical compounds. In chemical reaction, atoms are combined, separated, or rearranged. Section 1 The Atom: From Philosophical Idea to Scientific Theory Chapter 3

Modern Atomic Theory Not all aspects of Dalton’s atomic theory have proven to be correct. We now know that: Section 1 The Atom: From Philosophical Idea to Scientific Theory Chapter 3 • Atoms are divisible into even smaller particles. • A given element can have atoms with different masses. • Some important concepts remain unchanged. • All matter is composed of atoms. • Atoms of any one element differ in properties from atoms of another element.

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