1. Unit 2 Notes – Radioactivity
Mr Nelson

2. Facts about the nucleus
Radius = 10-13cm ( cm)
Radius = 1/100000th of an atom
Density = 1.6x1014g/cm3
That’s g/cm3
A sphere the size of a ping pong ball = 2.5 billion tons

3. Why does the nucleus stay together?

4. 4 basic forces
Gravity
Weak Nuclear
Electromagnetic
Strong Nuclear

5. What holds nucleus together?
The answer was proposed by Hideki Yukawa in 1935, he called it a STRONG NUCLEAR FORCE.
It is exerted by the p+ and n0 on each other
SNF overwhelms the electrostatic charge (++)

6. SNF
The SNF acts over very short distance, so larger nuclei the force does not reach far enough
If the SNF is not enough, the electrostatic forces will dominate and the nucleus will break apart.
Instability occurs when nucleus is too big

7. Band of Stability
Predicts whether or not an atom’s nucleus will be stable.
For atoms less than 20 p+, n0:p+ are 1:1
n0/p+ ratio greater than 1.5/1 will be unstable

8. Nuclear symbols
Chapter 25 requires that you understand nuclear symbols and how to manipulate them. A X Z

9. Nuclear equations & Laws
Notice that in the equation, atomic mass (A) and atomic # (Z) are the same on both sides of the arrow.
Law of conservation of Mass!

10. Nuclear vs Chemical Reactions
In a chemical reaction:
Atoms are rearranged to form new substances
In nuclear reactions:
We are creating new elements!
ALCHEMY!

11. Radioactive Decay
Radioactive – something is radioactive if the nucleus spontaneously decomposes
When it does this a different nucleus is formed
Extra particles are usually formed also
Example:
The symbol represents a beta (b) particle

12. Types of Radioactive Decay
Alpha (a) particle: An alpha particle is a Helium Nucleus.
The nuclear symbol is:

13. b particle production
The mass of a beta particle is 0, and the atomic number is –1:
So, beta particles change neutrons to protons.

14. Where did the e- come from?
A decaying neutron!!!

15. Gamma Rays (g) A gamma ray is a high energy photon of light.
Released along with other types of radiation

16. Positrons
A particle with the same mass as an e- but a positive charge:
Positrons are anitmatter!
Example:

17. Electron capture A nucleus CAPTURES an inner electron
Gamma rays are always produced.
Explanation:
NOTE that in electron capture the beta particle is always on the LEFT of the arrow!!

18. Decay Series
An unstable nucleus undergoes a series of changes until it becomes a stable nucleus

19. Changing one element into another
Radioactive decay is when a nucleus decomposes and turns in to another nucleus
Bombarding nuclei with particles can form new nuclei also.

20. Making new elements
Scientists make new elements by bombarding existing nuclei to create elements with larger and larger atomic numbers.

21. Transuranium elements & Radioactivity
Transuranium elements are just elements # (anything after Uranium!)
All elements after Bismuth (Z = 83) are radioactive

22. Half Life ½ life: the time for ½ of a sample of nuclei to decay
For U-238, it is 4.5 billion years
Pa-234 has a ½ life of 1.2 minutes!!

23. Carbon Dating (Radioactive Dating)
Based on the radioactivity of carbon-14, which decays through b-particle production.
Carbon-14 is continuously made in the atmosphere when neutrons from space hit the nitrogen in the air.

24. More on carbon dating:
Living things always have the same amount of C-14 because they take it in through the air.
As soon as a living thing dies it stops taking in carbon from the atmosphere. The C-14 decays.
The ½ life of C-14 is 5730 years.
If a piece of wood has ½ the C as a living tree, it is 5730 years old.
What are some of the problems with this method?

25. Fission and Fusion
Combining two light nuclei to make a heavier nucleus is called fusion
Splitting a heavy nucleus to form two nuclei with smaller masses is called fission.

26. Fission Releases a lot of energy!
26 million times that of burning methane
Can cause chain rxns

27. Chain Rxns
At least one neutron must go on to split another nucleus

28. Nuclear Reactors

29. Nuclear Fusion Produces more energy than fission!
Takes very high temperatures to get protons close enough to “stick” to each other
Why is “cold fusion” a hollywood fad?

30. Effects of Radiation Different particles penetrate in different ways
The energy they have
The mass they have
How easily they make ions
Chemical properties
These things make radiation more or less harmful

31. Detecting Radiation
High energy particles from radioactive decay make ions when they travel through matter.
The fast particles “knock” electrons off the atoms they hit.
A geiger counter can detect these ions through the flow of electricity

32. A geiger counter