1. History at a Glance
Atoms

2. Early Models
Early Philosophers and scientist could not observe individual ideas but were still able to propose ideas about the structure of the atom.
Black Box Activity –
Can you propose a structure without seeing what is inside the black box?

3. Democritus
( B.C.)
Matter is not infinitely divisible—smallest particles called atomos (later changed to atoms).
All atomos are the same just different sizes.

4. Principles
The law of conservation of mass was formulated by French chemist Antoine Lavoisier.
The law of constant composition was proposed by French chemist Joseph Louis Proust.

5. (studied chemical reactions)
John Dalton
1803 Created Atomic Theory
(studied chemical reactions)
All elements are composed of tiny indivisible particles called atoms. Atoms cannot be divided (wrong idea.)
Atoms of the same element are identical; the atoms of different elements are different.
Atoms combine in predictable ratios to make compounds.
Atoms are rearranged in chemical reactions.

6. J.J. Thomson Experimented with cathode ray tube.
Bent negative particles with an electromagnet
Discovered that every atom has negative particles— electrons.

7. Thomson’s Experiment Voltage source + -
By adding an electric field, he found that the moving pieces were negative
Animaion for how cathode ray tubes work.

8. Proton (+ charge)
Electron (- charge)
Plum Pudding Model J. J. Thomson proposed this model of the atom after the discovery of the proton, a positively charged particle in the atom.

9. Robert Millikan 1909 –measured the mass of an electron
Electron mass =1/1840 the mass of a hydrogen atom
Oil Drop Experiment
Atomizer
Animation for Milikan's Oil Drop Experiment

10. Bowling Ball Experiment
What observations can you make?

11. Ernest Rutherford 1911- Gold Foil Experiment
What if I shot alpha particles
(radiation- helium nuclei) at an atom?
What will I see?
Prediction:

12. What he got:
Animation for Rutherford's Gold Foil experiment.

13. Gold Foil Experiment (nuclear Atom model)
Rutherford shot alpha particles(+) at a thin piece of gold foil and captured particles on a screen around the foil. The screen helped him trace the path of the alpha particles.
Most of the alpha particles passed straight through the foil, but some alpha particles were deflected by the center of the
atom (+ nucleus). +

15. James Chadwick
1932
Showed there was a second particle in nucleus (neutron—no charge)

16. Conclusions:
There is a central, tiny, dense, positive region of the atom called the nucleus.
The atom consists mostly of empty space through which electrons move about.
The size of the atom is large compared to the size of the nucleus.
Ex. Ant (nucleus) in the middle of football field.

17. Subatomic particles
1. What is the charge of the electron? 2. Where is the electron located in the Bohr model of the atom? 3. What is the charge of the proton? 4. Where is the proton located in the Bohr model? 5. What is the charge of the neutron? 6. Where is the neutron located? 7. What particles are in the nucleus? 8. What is the charge of the nucleus?