1. Atomic Theory
The History of the Atom

2. Democritus (400 B.C.) Greek Philosopher…..named the atom
no science back then.
so no controlled experiments
Idea #1..matter can not be cut forever….
There is a smallest piece of everything.
That smallest piece is called an atom.
In Greek atom means indivisible

3. Democritus defines atoms
Smallest piece of matter
All atoms made of same thing just different
shapes and sizes
Atoms are always moving
Atoms can join together with other atoms

4. Lavoisier (1798) Scientist! Established Law of Conservation of Mass
= matter can not be created nor destroyed
Matter is made of atoms so …..
atoms can not be created or destroyed
Law but only under normal Earth conditions

5. Dalton (1803) Scientist! Assembled first Atomic Theory
Based on many facts determined by many experiments
1) all matter is made of atoms
2) atoms are indestructible
3) all atoms of same element are the same
4) all atoms of different elements are different
5) joining atoms of different elements makes compounds

6. Thomson (1897)
Plum pudding model
Named electrons
Know his experiment

7. Thomson’s Experiment 1) Thomson invented the cathode ray tube
thompson video
1) Thomson invented the cathode ray tube
ray tube production

8. 2) When metal at the end of the tube was heated particles shot out of it and down the tube
3) He knew the particles were doing this because when they hit the phosphorous at the end of the tube it glowed making the beam visible

9. 4) Thomson saw that he could bend the beam of particles by running an electrical current through the tube
5) The beam bent toward the positive electrode …….so the particles must be…
Negative

10. Stuck in positive dough
6) The atoms of metal that the particles came from were neutral …..so……
There must be something positive in the atom to balance out the negative charge of the particles
Plum pudding model
Negative electrons
Stuck in positive dough
thomson video

11. Rutherford (1911)
Geiger counter….measures radioactivity
Alpha particles :positive particles given off by some radoiactive materials

12. Rutherford’s discoveries:
Solar system or planetary model of atomic theory
Discovered protons
Later helped discovered neutrons (1932)

13. Rutherford’s experiment
1) Shot positive particles at gold foil
2) Detector behind foil flashed where the particles hit.
3) If Thomson’s model was correct most particles should
go straight through

14. Rutherford’s Results 1) Most particles did go straight through
This supported Rutherford’s idea that atoms are ….
Mostly empty space
2) A few particles bounced back, this supported…
The idea that there is a small hard positive core
3) A few were slightly deflected showing that they…
were pulled off track by nearby electrons.

16. Rutherford & Chadwick
Found that the protons + the electrons did not add up to the mass of the atom
Hypotheses were proposed to explain this mystery mass
Most popular was gamma ray hypothesis until……

17. Chadwick (1932) Disproved gamma ray hypothesis
Found neutral particles about same mass as a proton
Particles were neutral so named Neutrons
New atomic model included protons and neutrons in nucleus

18. Chadwick’s formula

19. Bohr Model
Electron’s distance from the nucleus determined by how much energy they have
Farther out =
More energy

20. Electrons can change energy levels
To move up an energy level they must …….
gain a set amount of energy
To move down an energy level they must…….
give off a set amount of energy.
The energy give off is a packet of light energy called a photon.

21. Wave model: Quantum Theory of Atoms
Electrons must stay a set distance from the nucleus
How far away depends on how much energy they have
More energy =
Electron could be anywhere around the atom at that distance

22. Up for debate??