1. The History of Atoms
Not the history of the atom itself, but the history of the idea of the atom.

2. 1. Democritus 460-370 B.C Democritus (Greek Philosopher 384-322 B.C.
Matter is made of Atomos
(He thought it looked like this!)

3. Some weaknesses of Democritus's idea
No empirical (experimental) evidence, just a well thought out idea.
“What holds the Atomos together?” was one important question that Democritus could not answer.

4. 2. Aristotle 384-322 B.C. Completely rejected Democritus’s idea
Aristotle was so well regarded and influential that his rejection of the Atomos caused the idea of Atoms to be dismissed for nearly 2000 years .

5. 3. John Dalton 1766-1844 He revisited and revised Democritus’s ideas.
He made careful observations on numerous chemical reactions that helped him refine his theories.

6. Dalton’s Atomic Theory
All atoms of an element are identical having the same size, mass and chemical properties. Atoms of an elements are different from all other atoms of different elements.

7. Dalton’s Atomic Theory
Atoms cannot be created, destroyed or divided in to smaller particle.

8. Dalton’s Atomic Theory
Atoms cannot be created, destroyed or divided in to smaller particle.
Different atoms combine in simple whole number ratios to form compounds.
Atoms are neither created nor destroyed in chemical reactions.
CO2

9. Law of Conservation of Mass
The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place.

10. Is Dalton’s Theory Completely Accurate?
It was a major step in the right direction.
No, atoms of the same element can have slightly different masses (Isotopes).
No, atoms are divisible into neutrons protons and electrons.

11. 4. J. J. Thomson
Around 1900
“Plum Pudding Model”

12. 5. Rutherford’s Discovery of the Nucleus
Ernest Rutherford shot  particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.
Figure 2.10
Protons were discovered by Rutherford in 1919.

13. 6. Niels Bohr “Planetary Atomic Model” 1913
Electrons only occupy well-defined orbits with fixed energy levels
Discovered that electrons may absorb energy by jumping to higher energy levels and fall back to the original level or orbit. This release of energy is called a photon.

14. 7. Modern View of the Atom Tiny nucleus surrounded by electron “cloud”
Nucleus accounts for all of the mass

15. Subatomic Particles
Protons (p+) - positive charge – inside the nucleus – weight 1 amu (atomic mass unit)
Neutron (n0) – neutral charge – inside the nucleus – weight 1amu
Electron (e-) - Negative Charge – in Electron Cloud – weight 1/1840 amu

16. Using the Periodic Table to Determine the Subatomic Particles
Equals the Number of Protons (Also the Number of Electrons in a neutral atom)
Equals the Number of Protons + Neutrons (since the nucleus is where all the weight is located)