1. You think you know, but do you???
The Atom
You think you know, but do you???

2. Objectives
Compare and contrast the atomic models of Democritus and Dalton
Define an atom
Distinguish between the subatomic particles in terms of relative charge and mass
Describe the structure of the nuclear atom, including the locations of subatomic particles

3. What does the atom look like?

5. Atoms
Smallest particle of an element that retains the properties of that element
All matter is made of atoms

6. History of the Atom Democritus, 400 B.C. Greek philosopher
Universe is made of invisible units called atomos
Matter is predominantly empty space
Atoms differ in their size, shape and movement
Changes in matter result from changes in groupings of atoms, and not changes in the atoms themselves
Atom is derived from the Greek word meaning “unable to be divided”

7. History of the Atom Aristotle, 300 B.C.
Rejected Democritus’ Atomos Theory
Disagreed with the “Nothingness of empty space” in the Theory of Atomism
Hated Democritus so much ‘he wished all his books burnt’

8. History of the Atom John Dalton, 1808, English schoolteacher
Atomic Theory built on Democritus’ Theory of Atomism
Every element is made of tiny particles that cannot be subdivided (atoms)
Atoms of the same element are identical
Atoms cannot be created, divided, or destroyed
Atoms can combine in whole number ratios to form molecules
Chemical reactions are atoms being separated, combined, or rearranged

9. Dalton’s Atom would have looked like this:

10. Subatomic Particles
Sir William Crookes (1875) & J.J. Thomson (1890) & others
Cathode Ray Tube experiments
Explain that the cathode ray was produced by heating a metal in a vacuum. These were being ‘played with’ all over the world. Cathode Ray Tubes became the long TV’s and computer monitors.
Thomson used Cathode Ray Tube Experiments and the oil drop experiment to determine the charge to mass ratio of cathode ray
Found the mass was much smaller than a hydrogen atom…it had to be a subatomic particle that was negatively charged.
Showed that Dalton’s theory was flawed: atoms can be divided into smaller components
It did not matter what element he got the cathode ray tube from; the cathode rays were identical every time.

11. Thomson’s Atom would have looked like this:
They knew that atoms were capable of being neutral. Since he knew there were some electrons, he proposed the plum pudding model.
Uniform base of positive charge with negative charges mixed within
This attempted to explain why matter is not always charged

12. Subatomic Particles - Electrons
Robert Millikan, American, 1909
Oil Drop Experiment
Determined the charge (1.59E-19 Coulombs) and mass of electrons with amazing accuracy

13. The Nucleus
Ernest Rutherford, 1909

14. Rutherford’s Atom would have looked like this:
Ernest Rutherford, 1909
Gold Foil Experiments
Determined that the atom is mostly empty space (most alpha particles went right through the gold foil)
Nucleus is a tiny, dense positive region (deflects some alpha particles)
Contains all of an atom’s mass
Later concluded that nucleus contains protons
Developed new Atomic Model
“It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you!"

15. Completing the Atom James Chadwick, a student of Rutherford, 1932
“extra mass” problem of isotopes…
His experiment is a little tough to follow. He fired alpha particles at beryllium which fired out neutrons which hit wax and forces protons to fly out.

16. Chadwick’s Atom would have looked like this:
Neutrons were tough to identify because they were neutral. Many experiments were based on the ideas that likes repel likes and opposites attract. With no charge, the neutron was more difficult to detect…
Why were these last particles so tough to detect???

17. Disclaimer….
This isn’t the perfectly correct, all-inclusive picture of the atom. We’ll talk more about electrons later. We’ll talk about wave-particle duality, schrodinger’s equation, orbitals, and more.