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.