1. Nuclear Chemistry

2. The Nucleus
Remember that the nucleus is comprised of protons and neutrons.
The number of protons is the atomic number.
The number of protons and neutrons together is effectively the mass of the atom.

3. Isotopes
Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms.
There are three naturally occurring isotopes of uranium:
Uranium-234
Uranium-235
Uranium-238

4. Radioactivity
It is not uncommon for some isotopes of an element to be unstable, or radioactive.
These unstable isotopes are called radioisotopes
Radioactive isotopes undergo changes in the nucleus to gain a more stable arrangement.
When the nuclei undergo these changes they emit rays and particles this process is called radioactivity.
The penetrating rays and particles emitted by a radioactive source are called radiation.

5. Nuclear Reactions
Nuclear reactions, which account for radioactivity, differ from chemical reactions.
Chemical Reactions
Involve the gain, loss and sharing of electrons
Involve relatively small changes in energy
Nuclear Reactions
Involve changes in the nucleus
Give off very large amounts of energy
Are not effected by temperature, pressure, catalysts, presence of other elements
Nuclear reactions cannot be sped up, slowed down or turned off

6. Types of Radiation

7. Types of Radiation (Radioactive Decay)
Alpha radiation
Can penetrate up to 0.5mm
Can be blocked by a piece of paper & clothing
Beta radiation
Can penetrate up to 4 mm
Can be blocked by metal foil
Gamma radiation
Can penetrate paper, wood and the body easily
Can be blocked by several meters of concrete or several centimeters of lead
There are other types of radiation but we will not be studying these in this chapter.

8. Types of Radiation (Radioactive Decay)
Radio waves, microwaves, visible light, ultraviolet light, and X-rays are all forms of radiation like gamma radiation.
How does a gamma ray differ from these other types of electromagnetic radiation?
The other types of radiation are not directly produced by the
Radioactive decay of a nucleus.

9. He U Th He Alpha Radiation: + Loss of an -particle (a helium nucleus)4 2 U
238 92 Th
234 90 He 4 2 +
Uranium -238
Thorium -234
Alpha particle
Remember a helium nucleus has 2 protons and 2 neutrons and positive charge.

10. Beta Radiation:
Loss of a -particle (a high energy electron)  −1 e or C 14 6 N 14 7 e −1 +
Carbon-14 radioactive
Nitrogen-14 stable
Beta Particle
In this nuclear reaction a neutron is converted into a proton and an electron is released which has a negative charge.

11.  Th Ra He  Gamma Emission:
Loss of a gamma-ray (): high-energy electromagnetic radiation that almost always accompanies the loss of a nuclear particle)  Th
230 90 Ra
226 88 He 4 2 + 
Alpha particle
Gamma Ray
Thorium -230
Radon -226
Gamma rays have no charge or mass –
this is why they can penetrate matter so easily

12. e C B e Positron Emission:
Loss of a positron (a particle that has the same mass as but opposite charge than an electron) e 1 C 11 6 B 5 + e 1
Carbon-11
Boron-11
positron
In this nuclear reaction a proton is converted into a neutron and a positron is released which has a positive charge.

13. Radiation Summary Alpha decay Beta decay Gamma Rays Positron emission
↓ # of protons by 2
↓ # of neutrons by 2
Beta decay
↑ # of protons and ↓ # of neutrons
Gamma Rays
Pure energy released no nuclear particles gained/lost
Positron emission
↑ # of neutrons and ↓ # of protons

14. QNA Q: How does an unstable nucleus release energy?
A: An unstable nucleus releases energy by emitting radiation during radioactive decay
Q: What are the 4 main types of radiation?
A: Alpha, Beta, Gamma & Positron Emission

15. QNA Q: What part of an atom undergoes change during radioactive decay?
A: The nucleus
Q: How is the atomic number of a nucleus changed by alpha decay? By beta decay? By gamma decay?
A: In alpha decay the atomic number decreases by 2, in beta decay the atomic number increases by 1, in gamma radiation there is no change in the atomic number.

16. QNA
Q: Between alpha, beta & gamma radiation which is the most penetrating?
A: Gamma radiation

17. Nuclear Stability & Decay
A radioisotope undergoes changes as it emits radiation
Radioisotopes have unstable nuclei
The stability of a nucleus depends on the ratio of protons and neutrons
Atomic Number < 20 the most stable ratio is 1:1
Atomic Number > 20 the most stable arrangements have more neutrons than protons.
But too many or too few neutrons can create an unstable nucleus

18. Neutron-Proton Ratios
Any element with more than one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus.
A strong nuclear force helps keep the nucleus from flying apart.

19. Neutron-Proton Ratios
Neutrons play a key role stabilizing the nucleus.
Therefore, the ratio of neutrons to protons is an important factor.

20. Neutron-Proton Ratios
For smaller nuclei (Z  20) stable nuclei have a neutron-to-proton ratio close to 1:1.

21. Neutron-Proton Ratios
As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

22. Stable Nuclei
The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

23. Stable Nuclei Nuclei above this belt have too many neutrons.
They tend to decay by emitting beta particles.

24. Stable Nuclei Nuclei below the belt have too many protons.
They tend to become more stable by positron emission or electron capture.

25. Stable Nuclei
There are no stable nuclei with an atomic number greater than 83.
These nuclei tend to decay by alpha emission.

26. Half Life
Half-life, t1/2, is the time required for half the atoms of a radioactive nuclide to decay.
Each radioactive nuclide has its own half-life.
More-stable nuclides decay slowly and have longer half-lives.
Animation (hopefully these animations work)

27. Half-Life

28. Half Life Calculations
Sample Problem
Phosphorus-32 has a half-life of 14.3 days. How many milligrams of phosphorus-32 remain after 57.2 days if you start with 4.0 mg of the isotope?
Given: original mass of phosphorus-32 = 4.0 mg
half-life of phosphorus-32 = 14.3 days
time elapsed = 57.2 days
Unknown: mass of phosphorus-32 remaining after 57.2 days

29. Half Life Calculations
Solution

30. Radioactive Series
Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation.
They undergo a series of decays until they form a stable isotope (often a isotope of lead).
Animation

31. Transmutation
The conversion of an atom of one element to an atom of another element is called transmutation.
Transmutation can occur by radioactive decay. Transmutation can also occur when particles bombard the nucleus of an atom.

32. Transmutation
The elements in the periodic table with atomic numbers above 92, the atomic number of uranium, are called the transuranium elements.
All transuranium elements undergo transmutation.
None of the transuranium elements occur in nature, and all of them are radioactive.

33. Transmutation Reactions
Transuranium elements are synthesized in nuclear reactors and nuclear accelerators.
Fermilab is a major accelerator center located in Batavia, Illinois. The main accelerator is a ring that has a radius of 1.0 km.

34. Nuclear Transformations
Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.

35. Particle Accelerators
These particle accelerators are enormous, having circular tracks with radii that are miles long. Fermilab, CERN

36. Energy in Nuclear Reactions
There is a tremendous amount of energy stored in nuclei.
Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy.

37. QNA Q: What determines the type of decay a radioisotope will undergo?
A: The neutron-to-proton ratio
Q: How much of a sample of radioisotope remains after 1 half life? After 2 half lives?
A: After 1 half life 50% of the sample remains. After 2 half lives 25% remains.

38. QNA Q: What are 2 ways transmutation can occur?
A: Radioactive decay & particle bombardment of a nucleus
Q: A radioisotope has a half life of 4 days. How much of a 20.0 gram sample will be left at the end of 4 days? At the end of 8 days?
A: After 4 days: 10.0 grams remain; after 8 days: 5 grams remain.

39. QNA
Q: The mass of cobalt-60 in a sample is found to have been decreased from grams to grams in a period of 10.5 years. Calculate the half-life of cobalt-60?
A: 5.25 years

40. Nuclear Fission How does one tap all that energy?
Nuclear fission is the type of reaction carried out in nuclear reactors.
When the nuclei of certain isotopes are bombarded with neutrons, they undergo fission, the splitting of a nucleus into smaller fragments.
In a chain reaction, some of the neutrons produced react with other fissionable atoms, producing more neutrons which react with still more fissionable atoms.

41. Nuclear Fission Nuclear Fission
In nuclear fission, a uranium-235 nucleus breaks into two smaller nuclei and releases neutrons. Predicting What happens when the released neutrons strike other uranium-235 nuclei?

42. Nuclear Fission
Bombardment of the radioactive nuclide with a neutron starts the process.
Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

43. Nuclear Fission
This process continues in what we call a nuclear chain reaction.

44. Nuclear Fission
If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.

45. Nuclear Fission
Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.

46. Nuclear Reactors
In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.

47. Nuclear Reactors
The reaction is kept in check by the use of control rods.
These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.

48. Nuclear Fusion
Fusion occurs when nuclei combine to produce a nucleus of greater mass. In solar fusion, hydrogen nuclei (protons) fuse to make helium nuclei and two positrons.

49. Nuclear Fusion Fusion would be a superior method of generating power.
The starting materials are inexpensive and readily available,
The products of the reaction are not radioactive or highly polluting,
Such energy could eliminate dependence on resources from other governments,
The bad news is that in order to achieve fusion, the material must be in the plasma state at several million Kelvin‘s, and we just can’t sustain that or control it safely (yet?). We might have problems storing that kind of energy as well.

50. Nuclear Fusion
Tokamak apparati like the one below show promise for carrying out these reactions.
They use magnetic fields to heat the material.

51. QNA Q: Explain what happens in a nuclear chain reaction.
A: Neutrons produced by fissionable atoms react with other fissionable atoms, that produces more neutrons that react with more fissionable atoms.
Q: Why are spent fuel rods from a nuclear reaction stored in water?
A: Water cools the spent fuel rods and provides a radiation shield.

52. QNA Q: How are fusion reactions different from fission reactions?
A: Fission reactions involve splitting nuclei. In fusion reactions, small nuclei combine and release much more energy.
Q: What does nuclear moderation accomplish in a nuclear reactor?
A: Slows down neutrons.

53. A: Unused nuclear fuel and fission products.
Q: What is the source of the radioactive nuclei present in the spent fuel rods?
A: Unused nuclear fuel and fission products.
Q: If we can overcome the technical issues, what are the advantages of using a fusion reactor to produce electricity?
A: Potential fuels are inexpensive and readily available. I.E. Hydrogen