1. A.1 - In which you will learn about: Democritus Lavoisier Dalton Thomson Unit 3: Quantum Mechanics Section A: History of Atomic Theory

2. Early Humans Believed…

3. Empedocles (492-432 BCE) Four Elements Theory: Earth, air, fire, water Main Problem: No matter how many times you break down a stone (or other piece of matter), the pieces never resemble the four elements of earth, air, fire, or water Main Importance: Among the first theories to suggest that pure materials are made up of a combination of “elements”

4. Democritus (460-370 BCE) “Atomos” (Greek for “indivisible”) Theory: If you take a stone and cut it in half, each half has the same properties as the original stone. If you continued to cut the stone into smaller and smaller pieces, at some point you would reach a piece so tiny that it could no longer be divided. Democritus suggested that atomos were eternal and could not be destroyed. Democritus theorized that atomos were specific to the material that they made up, EX: The atomos of stone are unique to stone and different from the atomos of other materials, such as fur.

5. Antoine Lavoisier (1778) Conducted many experiments with air Burned substances such as phosphorus and sulfur in air, and showed that they combined with air to make new materials. These new materials weighed more than the original substances, and Lavoisier showed that the weight gained by the new materials was lost from the air in which the substances were burned. From these observations, Lavoisier established the Law of Conservation of Mass, which says that mass is not lost or gained during a chemical reaction.

6. John Dalton (1803) Dalton’s Atomic Theory: All matter is composed of indivisible particles called atoms. Dalton pictured atoms as tiny billiard-ball-like particles in various states of motion. This postulate is violated by subatomic particles – protons, neutrons, and electrons. All atoms of a given element are identical. Dalton characterized elements according to their atomic weight; however, when isotopes of elements were discovered in the late 1800s this concept changed. Chemical reactions involve the combination of atoms, not the destruction of atoms. This is largely based on Antonie Lavoisier’s law of conservation of matter. When elements react to form compounds, they react in defined, whole-number ratios. The experiments that Dalton and others performed showed that reactions are not random events; they proceed according to precise and well-defined formulas.

7. Dalton’s Model of the Atom: Billiard Ball

8. J.J. Thomson’s Discovery of the Electron Used a cathode ray tube to conduct experiments

9. Cathode Ray Tube Vocabulary

10. What is a cathode ray? “Cathode rays” occur when electricity is applied to the cathode of the tube and the electricity moves through the gas inside the tube to get to the anode. The beam of electricity causes the gas in the tube to glow. When a magnet is placed near the beam, the beam bends towards the magnet! This means the beam is negative! (We now know that electricity is the flow of electrons, so this makes perfect sense)

11. Thomson’s Fame His experiments prompted him to make a bold proposal: these mysterious rays are streams of particles much smaller than atoms He called these particles "corpuscles," and suggested that they might make up all of the matter in atoms. It was startling to imagine a particle residing inside the atom-- most people thought that the atom was indivisible (according to Dalton), the most fundamental unit of matter. Thomson also theorized that if the tiny particles were negative, then they must be surrounded by positive charge for balance

12. Thomson’s Model of the Atom: Plum Pudding

13. HOMEWORK QUESTIONS 1) Define atom using your own words. 2) Summarize Dalton’s atomic theory. 3) Explain how Dalton’s theory of the atom and the conservation of mass are related. 4) Describe the structure of a typical atom. Identify where each subatomic particle is located. 5) Evaluate the experiments that led to the conclusion that electrons are negatively charged particles found in all matter (you may want to use more outside resources for this).