Chapter 4 The Structure of the Atom

Section 4.1 Early Ideas About Matter

Objectives n Compare and contrast the atomic models of Aristotle, Democritus and Dalton. n Understand how Dalton's theory explains the conservation of mass.

Early Philosophers n In ancient Greece, philosophers considered the many mysteries of life. n Aristotle concluded that matter was composed of 4 elements - earth, air, fire, and water – and that it could be divided endlessly into ever smaller pieces. n Democritus was the first person to propose the idea that matter was not infinitely divisible. He believed that matter was composed of atomos or atoms; atoms were solid & indivisible.

Aristotle's idea went unchallenged for 2000 yrs. John Dalton ( ), an English schoolteacher and chemist, revised Democritus’ ideas based on careful & accurate scientific research that he conducted himself. Dalton’s Atomic Theory

The following statements are the main points of Dalton’s atomic theory (1803). Dalton’s Atomic Theory 1. All matter is made up of atoms. 2. Atoms cannot be created or destroyed and are indivisible. 3. All atoms of one element are exactly alike, but are different from atoms of other elements. 4. Different atoms combine in small, whole number ratios to form compounds. 5. Atoms are separated, combined, or rearranged in a chemical reaction. The last statement easily explained conservation of mass in a reaction as the result of the combination, separation, or rearrangement of atoms.