Download Unit 3: Quantum Mechanics Section A: History of Atomic Theory

Unit 3: Quantum Mechanics
Section A: History of Atomic Theory
A.1 - In which you will learn about:
•Democritus
•Lavoisier
•Dalton
•Thomson
Early Humans Believed…
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”
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.
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.
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.
Dalton’s Model of the Atom: Billiard
Ball
J.J. Thomson’s Discovery of the
Electron
 Used a cathode ray tube to conduct experiments
Cathode Ray Tube Vocabulary
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)
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
Thomson’s Model of the Atom:
Plum Pudding
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).