1. History of the Atomic Model 2.1
Unit 2
History of the Atomic Model
2.1

2. Atomic Structure

3. Democritus 400 B.C.

4. John Dalton 1808

5. Dalton’s view of atoms of elements Can you see B. 1-4 in this picture?

6. J.J. Thomson (1897) with the Cathode Ray Tube

7. Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

8. Actual Cathode Ray in action

9. Thomson’s Atomic Model
Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

10. Robert A. Millken 1909

11. Millikan’s Oil Drop Experiment Helps to calculate the mass & charge of an electron

12. How it worked inside

13. Ernest Rutherford (1911) on New Zealand Money

14. Hans Geiger (1911)

15. Ernest Marsden (1911)

16. Rutherford’s Gold Foil Experiment
Alpha () particles are helium nuclei
Particles were fired at a thin sheet of gold foil
Particle hits on the detecting screen (film) are recorded

17. Rutherford’s Findings
Most of the particles passed right through
A few particles were deflected
VERY FEW were greatly deflected
“Like howitzer shells bouncing off of tissue paper!”
Conclusions:
The nucleus is small
The nucleus is dense
The nucleus is positively charged

18. Atomic Particles Particle Charge Mass # Location Electron -1
Electron cloud
Proton +1 1
Nucleus
Neutron

19. Niels Bohr (1913) His comment on the structure of atoms
Niels Bohr (1913) His comment on the structure of atoms. (Which are mostly empty space.)

20. Bohr Model of an Atom

21. Dmitri Ivanenko (1930)

22. Victor Ambartsumian (1930)

23. Ivanenko & Ambartssumian Model of a Nucleus

24. James Chadwick 1932

25. His Experiment using alpha particle radiation