This document provides an overview of the development of the atomic theory from ancient Greek philosophers to modern scientific discoveries. It describes early atomic models proposed by Democritus, Dalton's initial theory, Thomson's discovery of electrons, Rutherford's gold foil experiment which led to the proposal that atoms have a small, dense nucleus, and Bohr's addition of electron energy levels. The modern atomic theory incorporates the nucleus and electron clouds based on probabilities of finding electrons in different regions surrounding the atomic nucleus.

1. Physical Science
Chapter 4.1

2. Objectives
1. Describe some of the experiments that led to
the current atomic theory.
2. Compare the different models of the atom.
3. Explain how the atomic theory has changed
as scientists have discovered new
information about the atom.
Bellringer
The following is a quote by Democritus (c. 460–c. 370 bce).
“Color exists by convention, sweet by convention, bitter by convention; in reality
nothing exists but atoms and the void.”
What do you know about Democritus? And why are his thoughts important?

3. The Beginning of the
Atomic Theory
 440 BCE, a Greek philosopher, Democritus
 thought that you would eventually end up with a
particle that could not be cut.
○ He called this particle an atom.
 Aristotle, another Greek philosopher,
 disagreed with Democritus
 He believed that you would never end up with a
particle that could not be cut.
 atom
 smallest particle into which an element can be
divided and still be the same substance.

4. Dalton’s Atomic Theory
 Published in1803. Stated that
1. All substances are made of atoms.
2. Atoms are small particles that cannot be
created, divided, or destroyed.
3. Atoms of the same element are exactly alike
4. Atoms of different elements are different.
5. Atoms join with other atoms to make new
substances.
 Not Quite Correct
 The atomic theory was then changed to
describe the atom more correctly.

5. Thomson’s Discovery of
Electrons
 Negatively Charged Particles
 Used cathode-ray tube
 Discovered negatively charged particles
○ electrons.
 Plum – Pudding model
 New model of the atom.
○ Electrons were mixed throughout an atom, like
plums in a pudding.

7. Rutherford’s Atomic “Shooting
Gallery”
 Negatively Charged Particles
 1909
○ Ernest Rutherford aimed a beam of small, positively
charged particles at a thin sheet of gold foil.
 Surprising Results
 Expected the particles to pass right through the
gold in a straight line.
 Surprise, some of the particles were deflected!!

8. Rutherford’s
Model of atom

9. Where are the electrons?
 Far from the Nucleus
 Rutherford proposed that in the center of the
atom is a tiny, positively charged part
○ called the nucleus
○ Nucleus is 100,000 times smaller than atom
 Bohr’s Electron Levels
 1913
○ Niels Bohr proposed that electrons move
around the nucleus in certain paths, or
energy levels

10. Size of the Nucleus

11. Modern Atomic Theory
 The Modern Atomic Theory
 Regions inside the atom where electrons
are likely to found
○ These regions are called electron clouds