1. Chapter 8 The Composition of the atom
Atomic Theory
Chapter 8
The Composition of the atom

2. John Dalton 1800 -Dalton proposed a modern atomic model
based on experimentation not on pure reason.
All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine in constant ratios to form compounds.
Atoms are rearranged in reactions.
His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

3. 1803 John Dalton Dalton’s Model of the Atom:
Uniform Shape
Uniform Density
Indivisible
So what did Dalton’s model look like?
A simple design that promoted future research into atomic theory…

4. Discovering the Electron
He noticed a flash of light within one of the tubes.
Flash was produced by some form of radiation striking the light at the end of the tube.
There were rays (radiation) traveling from the cathode to the anode in the tube.
They called it the cathode ray b/c the ray of radiation originated from the cathode end of the tube.
Thank you Sir William Crookes!!!!!

5. The Cathode Ray Tube (CRT)
Developed by Sir William Crookes in the late 1800s

6. Thank you Sir William Crookes!!!!! For your Accidental Discovery!!!!! 1879
Discovery of cathode rays led to invention of the TV!!!!
TV and computer monitor images are formed as radiation from the cathode strikes the light producing chemicals that coat the backside of the screen.

7. Electromagnets (turned off)
JJ Thompson 1897
Using the CRT…
Fluorescent Screen
Electromagnets (turned off)
Cathode (--)
Anode (+)
Beam of “light”

8. JJ Thompson 1897
When the magnets were turned on, the beam was pulled towards the + plate
What could Thompson conclude?
+ side
-- side

9. AKA The Chocolate-Chip Cookie Model
JJ Thompson 1897
What did his experiment discover?
ELECTRONS! (negatively-charged particles)
How did the model of the atom change?
AKA The Chocolate-Chip Cookie Model

10. Robert Millikan 1909
Determined the charge of an electron.
Mass of an electron = 9.1 x g
Mass of an electron is extremely small.

11. Millikan’s Experiment
Measured the time it took for an electrically charged oil drop to rise a measured distance when subjected to an electric field
Measured the time it took electrically charged droplets to fall with no electric field

12. Oil Drop Experiment

13. Millikan’s Outcomes Determined the charge on a single electron
Using the charge from Millikan and the charge-to-mass ratio from Thomson, many scientists determined the mass of an electron.

14. Discoveries from the CRT
Wilhelm Roentgen (1895) ?
●Discovered x-rays
●Killed his wife

15. Henri Becquerel 1896
Roentgen’s Discovery of X-rays provoked further research by many others, including Becquerel
Since fluorescent materials could be charged by X-rays, Henri wondered if naturally fluorescent minerals could give off X-rays….
Like any good scientist, he set up a test…

16. Photographic film sealed away
Henri Becquerel 1896
Photographic film sealed away

17. Henri Becquerel 1896 The photographic paper was still exposed!
The mineral contained Uranium!
Serendipity- accidental discovery

18. Becquerel’s Outcomes
● Left a sample of uranium ore in a drawer with photo plates and there was fogging on the plates
Concluded something in the sample itself must be capable of “fogging” the plates; the sample must give off some kind of ray without the influence of the sun
Rays were the products of radioactivity
credited with the discovery of radioactivity

19. Marie and Pierre Curie French Scientists
Intrigued with Becquerel’s work
Studied radioactivity
Marie discovered the element POLONIUM

20. Ernst Rutherford 1898
After the discoveries of Becquerel and Thompson, Rutherford was dissatisfied with the atomic models and set out to disprove it.
Using Uranium, he and his lab assistants (Geiger and Marsden) set out…

21. Ernest Rutherford 1911
Studied how positively charged alpha particles interacted with solid matter.
Conducted an experiment to see if alpha particles would be deflected as they passed through a thin foil of gold. Also known as Rutherford’s Gold Foil Experiment.

22. Ernest Rutherford’s Gold Foil Experiment
1. A narrow beam of alpha particles was aimed
at a thin sheet of
gold foil.
2. A zinc sulfide coated screen surrounding the gold foil produced a flash of light whenever it was struck by an alpha particle.
1. Most all of the alpha particles passed straight through the gold foil, without deflection.
A few times they were deflected at very large angles.
2. Some alpha particles were scattered at small angles.

23. Rutherford’s Gold Foil Conclusions
1. Rutherford said plum pudding model was incorrect.
2. An atom consisted mostly of empty space through which the electrons move.
3. There was a tiny, dense region, called the nucleus, centrally located within the atom that contained all of an atom’s positive charge and almost all of its’ mass.

24. Planetary Motion Model
Electrons orbit around the nucleus like planets orbit around the sun
The nucleus held only protons (now)

25. Hans Geiger Student of Rutherford
Spent long hours counting the particles that hit the screen
Created the first electric particle counter - The GEIGER counter
Determined that alpha particles had 2 p+ and 2n0 (1908)

26. Irene Curie-Joliot & Frederic Joliot
Bombarded Be with alpha particles - formed a beam of energy with great power which was able to penetrate metals
Thought it was new form of energy - thought it was gamma rays

27. James Chadwick Credited with the discovery of the NEUTRON
Found new beam of energy was unaffected by magnetic fields - so it was neutral
Particles moved at 1/10 the speed of light - so it was NOT radiant energy - but must be particles
The new particles had approximately the same mass as a proton - but they had no charge
Credited with the discovery of the NEUTRON

28. After Chadwick
After neutrons were discovered - isotopes of various elements were discovered
We will come back and pick up our story in a little bit.