Evolution of Atomic Theory: From Democritus to Modern Science

1. The Atom: From Philosophical Idea to Scientific Theory Section 3.1

2. Defining the Atom • The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”) • He believed that atoms were indivisible and indestructible • His ideas did agree with later scientific theory, but did not explain chemical behavior, and was not based on the scientific method – but just philosophy

3. Foundations of the Atomic Theory • Better equipment for the study of matter became available in the 1790s • This lead to the discovery of several basic laws • There was no real explanation for the laws until Dalton proposed his atomic theory

4. Law of Conservation of Mass • Law of conservation of mass: mass is neither created nor destroyed during ordinary chemical reactions or physical changes

5. Law of Definite Proportions • Law of definite proportions: chemical compounds contain the same elements in exactly the same proportions by mass, regardless of the size of the sample

6. Law of Multiple Proportions • 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

7. Visual for Law of Multiple Proportions

8. Dalton • John Dalton was a schoolteacher in the early 1800s • He proposed an explanation for the three laws • He said that elements were composed of atoms and only whole numbers of atoms could combine to form compounds

9. Dalton’s Atomic Theory • 1) all matter is composed of extremely small particles called atoms • 2) atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties • 3) atoms cannot be subdivided, created, or destroyed

10. Continued • 4) atoms of different elements combine in simple whole-number ratios to form chemical compounds • 5) in chemical reactions, atoms are combined, separated, or rearranged

11. Modern Atomic Theory • Dalton’s theory was scientific, but not entirely correct • We now know that atoms are divisible into smaller particles • A given element can have atoms with different masses • Dalton’s model is called the “Solid Sphere” model.