1. Discover of Radioactivity
Chemistry 332

2. Where are we?
This story starts in 1895 with the work of the German physicist Wilhelm Roentgen.
He was working with a Cathode Ray tube and noticed something very interesting...

3. Cathode Ray Tube?
Electric current passed through two electrodes and an invisible beam of energy is made
(With a fluorescent screen the energy path glows green)

4. So What?
He was working on his experiment on this side of the room and had a different experiment across the room
Chem Rules!

5. He then flipped the switch...
And the Cathode ray tube turned on...
Chem Rules!
And a fluorescent screen across the room started to glow!

6. So he turned it off... And the Cathode ray tube went dead
Chem Rules!
The screen stopped glowing!

7. So he tried it again! And the Cathode ray tube turned on...
Chem Rules!
And again the screen glowed even though the tube wasn’t even aimed at it!

8. Conclusion?
The Cathode Ray tube was giving off some sort of energy in all directions!!!!
Roentgen next thought, “I wonder how powerful these energy rays are?”
So he took the fluorescent screen into the next room, came back and flipped the switch….

9. The Set-Up Fluorescent screen And the switch was flipped and...
Sure enough, the screen glowed!
Cathode ray tube

10. Conclusion Fluorescent screen
The tube must be giving off POWERFUL invisible energy
Cathode ray tube

11. What are these? The rays of energy were invisible
They were strong enough to penetrate the wall
They were unprecendented
So he called them….
X-Rays

12. So Roentgen Discovered X-Rays
X-Ray machines began popping up everywhere
Even shoe stores had them so you could see your feet bones
Some thought they were the Devil’s tool
More importantly they led to further studies

13. Science is moving faster now...
It’s now 1896, one year after Roentgen’s work
Antoine Henri Becquerel has heard of the X-Ray and it’s link to fluroescence and decides to do his own experiments with phosphorescence

14. His Hypothesis
Some phosphorescent materials emit X-rays when exposed to high energy
These x-rays can penetrate substances but are ultimately just like visible light (just stronger)

15. His Set-Up
Sunlight
Photographic film
Would the film be exposed?
Yes

16. But…. Would the film be exposed? No Sunlight Photographic film
Thick black paper
Would the film be exposed? No

17. But, but….
Sunlight
Photographic film wrapped in thick black paper
Phosphorescent material
It glows!
What happens to the phosphorescent material in the sunlight?

18. This glowing gives off energy
If the energy was just visible light, would the photographic film be exposed?
No, it’s wrapped in black paper the normal light can’t penetrate
If the energy also contained X-Rays, would the photographic film be exposed?
Yes, X-rays are just like light but can easily penetrate the papter.

19. What was the result? Sure enough, the photographic paper was exposed!
This meant that phosphorescent materials, when glowing due to the energetic effects of the sun, give off X-rays!

20. So what? Really nothing new happened there with that experiment
But on Feb 26th, Becquerel decided to repeat the experiment but couldn’t due to cloudy skies over Paris.

21. Drats!
Becquerel took the phosphorescent materials and the photofilm wrapped in black paper and stuck them in a drawer.
They stayed there, undisturbed in the dark until March 1st; four full days

22. Let’s Review
Becquerel’s conclusion was that sunlight made the phosphorescent material glow
This gave off penetrating x-rays
This caused the photofilm to be exposed even though it was wrapped in black paper
Should the photofilm, after sitting the dark, wrapped up in black paper be exposed?
No, there’s no light to cause the phosphorescent material to glow and thus give off x-rays

23. Just out of curiosity
Becquerel developed the photographic film anyway.
What did he find?
The photofilm was exposed just as much as his previous experiment!
What does this mean?

24. Sunlight doesn’t matter
The phosphorescent material was spontaneously giving off energy regardless of the sun
This energy was invisible
It was also strong enough to penetrate the black paper
Were these X-rays or some other energy?

25. How many substances do this?
Marie Curie (a student of Becquerel by the way) and Pierre Curie spent years isolating substances that exhibit these properties
But what are they?

26. One last experiment
Ernest Rutherford, using Becquerel’s ideas and material isolated by the Curies performed an experiment on the X-rays found by Roentgen as well as the energy found by Becquerel

27. His Set-Up
Strong magnet + -
X-ray or other energy source

28. The results! - Passed straight through with no deflection +
Strong magnet + -
What charge are X-rays, then?
X-rays
Neutral, just like light

29. The results! - Split into three different beams! + Strong magnet
What charges are these?
The energy discovered by Becquerel and the Curies
Positive, negative, and neutral

30. What is this?
This mysterious energy given off by the fluorescent substances was of three different types
It was given off spontaneously
It was invisible
It was of very high energy
What is it?

31. I’ll give you a hint The fluorescent ore used by Becquerel was:
K2UO2(SO4)2
Notice anything unusual about that formula?
Check out the center atom.
What do we know about that element?

32. Radioactivity!
So what did this have to do with X-rays or fluorescent materials?
Nothing!
Except that without those experiments, radioactivity would not have been discovered so early.