1. Radiation and Radioactivity
Nuclear Change
Radiation and Radioactivity

3. Change We have learned about We are now going to learn about
Physical change – when atoms retain their identities and chemical compositions
Chemical change – when atoms are rearranged forming new substances
We are now going to learn about
Nuclear change – when atoms form new atoms with different identities and properties. A change in the atom’s nucleus occurs!

4. Comparison of Chemical & Nuclear
Chemical Reaction
Nuclear Reaction
Occur when bonds are broken and formed
Involve only valence electrons
Small energy changes
Atoms keep the same identity
Occur when nuclei combine, split, and emit radiation
Can involve protons, neutrons, and electrons
Large energy changes
Atoms are converted into different elements

5. Check for understanding
What type of change are the following? H2O (l)  H2O (s) 2H2 (g) + O2  2H2O (l) 21 H (g) + 21H (g)  42He (g) He (g)  He (l)

6. Discussion What do you think of when we talk about radioactivity?
Can you give some examples of materials that are radioactive?
What are some uses for radioactivity?

7. Common radioactive elements
Uranium
Plutonium
Radon
Radium
Bismuth
Carbon

8. Uses of Radioactivity Medicine Weaponry Sun Industry Power
X-rays, PET scans, MRI
Bombs
Sterilizing bandages
Sun
Cancer treatment
Industry
Power
Mass spectroscopy
Nuclear power plants generate 1/5 of our electricity
Carbon dating

9. Essential Questions Who discovered radiation or radioactivity?
Why do we have radiation?
What are the different types of radiation?

10. Who discovered of radioactivity?
In 1895, Wilhelm Roentgen discovered the existence of X-rays, though the mechanism behind their production was not yet understood. He named the radioactive rays after the traditional unknown X.

11. Radioactivity
In 1896, Henri Becquerel discovered that uranium salts emitted rays that resembled X-rays in their penetrating power.

12. Radioactivity
Marie Curie was a French-Polish physicist and chemist, famous for her pioneering research on radioactivity.
She coined the word, radioactivity.
Marie continued work on uranium salts and in 1898, determined that the radiation was dependent on the quantity of the substance not an interaction with other molecules.

13. Radioactivity
In 1899, Ernest Rutherford, a British scientist, began to classify radiation: alpha (a), beta (b), and gamma (g).

14. Why do we have radiation?
For most stable atoms, the number of protons roughly equals the number of neutrons.
But, if an isotope has too many more neutrons than protons, the nucleus becomes unstable.
When this happens, the nucleus needs to shed some neutrons in the form of decay.

15. Remember - The Atom The atom consists of two parts:
1. The nucleus which contains:
protons
neutrons
2. Orbiting electrons.

16. Because of isotopes, there are many types of uranium:
235 92 U
238 92
Mass Number
Number of protons
Number of neutrons
235
Mass Number
Number of protons
Number of neutrons
238 92 92
143
146

17. Band of Stability
Graph of the number of protons against the number of neutrons.
Stable atoms have the ideal ratio of protons to neutrons
Unstable will release radiation until they are stable

18. Isotopes
Unstable isotopes can become stable by releasing radiation in the form of energy and different types of particles.

19. Types of Radioactive Decay
Radioactive decay results in the emission of either:
Alpha decay – When the nucleus spins off 2 protons and 2 neutrons
Beta decay – When a neutron in the nucleus converts to a proton and electron and then keeps the additional proton while shooting off the electron.
Gamma decay – accompanies the other two types of decay but does not change the mass or charge.

20. Types of Radioactive Decay
Radiation Type
Symbol(s)
Charge

21. Radiation

22. Types of Radioactive decay
Rutherford’s experiment
(1:44)

23. Alpha Decay
When an alpha particle, containing two protons and two neutrons, is ejected from the nucleus.
An alpha particle is identical to the nucleus of a helium atom
The mass number decreases by 4 and the atomic number decreases by 2.

24. Alpha Decay Rn
222 86 He 4 2 Ra
226 88

25. Beta Decay
Beta particle is a fast moving electron which is emitted from the nucleus of an atom undergoing radioactive decay.
Beta decay occurs when a neutron changes into a proton and an electron.

26. Beta Decay
As a result of beta decay, the nucleus has one less neutron, but one extra proton.
The mass number stays the same and the atomic number increases by 1 and

27. Beta Decay b -1 At
218 85 Po
218 84

28. Gamma Decay
Gamma rays are not charged particles like a and b particles.
Gamma rays are electromagnetic radiation with high frequency.
When atoms decay by emitting a or b particles to form a new atom, the nuclei of the new atom formed may still have too much energy to be completely stable.
Gamma decay does not result in the creation of a new element!

29. Other Types of Nuclear Reactions
positron –
proton -
neutron -

30. Nuclear Equations
Because we know that mass is conserved in every chemical reaction, we know that the conservation of matter is maintained even in radioactive decay!
So, can we use this information to write balanced nuclear equations?

31. X Y + He Ra Rn + He Alpha Decay A Z A - 4 Z - 2 4 2 226 88 222 86 4 2
unstable atom
alpha particle
New more stable element Ra
226 88 Rn
222 86 + He 4 2

32. Alpha Decay Rn
222 86 He 4 2 + Po
218 84 He 4 2 U
234 92 + Th
230 90

33. Alpha Decay He 4 2 + Pb
214 82 Po
218 84 He 4 2 + Ra
226 88 Th
230 90

34. Beta Decay X A Z Y
Z + 1 + e -1 Po
218 84 Rn 85 + e -1

35. Beta Decay Bi
210 83 Po 84 + e -1 Th
234 90 Pa 91 + e -1

36. Beta Decay Pb
214 82 Bi 83 + e -1 Tl
210 81 Pb 82 + e -1

37. Check for understanding
Work on problems on worksheet: Radioactive Decay Worksheet
Then with a partner, use the cards to solve the decay series of uranium. Be sure to indicate the type of decay in each step.

38. Radioactive Decay Rates
Calculating half lives

39. Radioactive decay Radioactive decay rates are measured in half-lives
Half life – the time required for one half of the nuclei to decay into its products
The decay continues until there is a negligible amount of the radioactive isotope remaining.

40. Calculating Half–lives Example
An Isotope of cesium-137 has a half-life of 30 years. If 1.0 g of cesium-137 disintegrates over a period of 90 years, how many grams of cesium-137 would remain?

41. The solution
Time (yrs)
Mass (g)
1.0 30
0.5 60
0.25 90
0.125

42. Essential Question
So you’ve talked about carbon-14 dating, how does it actually work?

43. Carbon Dating
In the 1950s W.F. Libby and others (University of Chicago) devised a method of estimating the age of organic material based on the decay rate of carbon-14.
Carbon-14 dating can be used on objects ranging from a few hundred years old to 50,000 years old.

44. How does Carbon-14 dating work? C 14 6 N 7 + b -1

46. Decay Rate Formula N = N0(1/2)t/T Where: N = the remaining amount
N0 = the initial amount
t = elapsed time
T = duration of the half life

47. Practice
1. The half life of cobalt-57 is 270 days. How much of a 5.000mg sample will remain after 810 days?
N = ??
N0 = mg
t = 810 days
T =270 days

48. Practice
2. Krypton-85 is used in indicator lights of appliances. The half-life of kypton-85 is 11 years. How much of a mg sample remains after 33 years?
N = ??
N0 = mg
t = 33 years
T =11 years