Discover of Radioactivity

Chemistry 332

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...

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)

So What?

He was working on his experiment on this side of the room and had a different experiment across the room

Chem Rules!

He then flipped the switch...

Chem Rules!

And the Cathode ray tube turned on...

And a fluorescent screen across the room started to glow!

So he turned it off...

Chem Rules!

And the Cathode ray tube went dead

The screen stopped glowing!

So he tried it again!

Chem Rules!

And the Cathode ray tube turned on...

And again the screen glowed even though the tube wasn’t even aimed at it!

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….

The Set-Up

Cathode ray tube

Fluorescent screen

And the switch was flipped and... Sure enough,

the screen glowed!

Conclusion

Cathode ray tube

Fluorescent screen

The tube must be giving off POWERFUL invisible energy

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

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

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

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)

His Set-Up

Photographic film

Sunlight

Would the film be exposed? Yes

But….

Photographic film

Sunlight

Would the film be exposed? No

Thick black paper

But, but….

Photographic film wrapped in thick black paper

Sunlight

What happens to the phosphorescent material in the sunlight?

Phosphorescent material

It glows!

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.

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!

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.

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

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

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?

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?

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?

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

His Set-Up

Strong magnet

X-ray or other energy source

+

-

The results!

Strong magnet

X-rays

Passed straight through with no deflection

What charge are X-rays, then?

Neutral, just like light

+

-

The results!

Strong magnet

The energy discovered by Becquerel and the Curies

What charges are these?

Positive, negative, and neutral

Split into three different beams!

+

-

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?

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?

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.