1. Chapter 21 Nuclear Chemistry

2. Radiocarbon Dating
Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation
14N + 1on ---> 14C + 1H
The C-14 is oxidized to CO2, which circulates through the biosphere.
When a plant dies, the C-14 is not replenished.
But the C-14 continues to decay with t1/2 = 5730 years.
Activity of a sample can be used to date the sample.

3. Nuclear Medicine: Imaging
Thyroid imaging using Tc-99m

4. Nuclear Reactions vs. Normal Chemical Changes
Nuclear reactions involve the nucleus
The nucleus opens, and protons and neutrons are rearranged
The opening of the nucleus releases a tremendous amount of energy that holds the nucleus together – called binding energy
“Normal” Chemical Reactions involve electrons, not protons and neutrons

5. Food Irradiation
Food can be irradiated with g rays from 60Co or 137Cs.
Irradiated milk has a shelf life of 3 mo. without refrigeration.
USDA has approved irradiation of meats and eggs.

6. Mass Defect Some of the mass can be converted into energy
Shown by a very famous equation!
E=mc2
Energy
Mass
Speed of light

7. The Nucleus
Remember that the nucleus is comprised of the two nucleons, 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.

8. 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

9. Radioactivity
It is not uncommon for some nuclides of an element to be unstable, or radioactive.
We refer to these as radionuclides.
There are several ways radionuclides can decay into a different nuclide.

10. Types of Radiation Beta (β) – an electron
Alpha (ά) – a positively charged helium isotope - we usually ignore the charge because it involves electrons, not protons and neutrons
Beta (β) – an electron
Gamma (γ) – pure energy; called a ray rather than a particle

11. Penetrating Ability

12. Other Nuclear Particles
Neutron
Positron – a positive electron
Proton – usually referred to as hydrogen-1
Any other elemental isotope

13. Types of Radioactive Decay

14. He U He Alpha Decay: Loss of an -particle (a helium nucleus) + 4 2
238 92

234 90 He 4 2 +

15.  e I Xe e Beta Decay: Loss of a -particle (a high energy electron) +−1 e or I
131 53 Xe 54
 + e −1

16. 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

17. Gamma Emission:
Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) 

18. Electron Capture (K-Capture)
Addition of an electron to a proton in the nucleus
As a result, a proton is transformed into a neutron. p 1 + e −1
 n

19. Learning Check
What radioactive isotope is produced in the following bombardment of boron?
10B He ? n

20. Write Nuclear Equations!
Write the nuclear equation for the beta emitter Co-60.

21. 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.

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

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

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

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

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

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

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

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

30. Some Trends
Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons.

31. Some Trends
Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.

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

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

34. Half-Life
HALF-LIFE is the time that it takes for 1/2 a sample to decompose.
The rate of a nuclear transformation depends only on the “reactant” concentration.

35. Kinetics of Radioactive Decay
Nuclear transmutation is a first-order process.
The kinetics of such a process, you will recall, obey this equation:
= -kt A A0 ln

36. Half-Life
Decay of 20.0 mg of 15O. What remains after 3 half-lives? After 5 half-lives?

37. Kinetics of Radioactive Decay
The half-life of such a process is:
= t1/2
0.693 k
Comparing the amount of a radioactive nuclide present at a given point in time with the amount normally present, one can find the age of an object.

38. Learning Check!
The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 39 hours?

39. Kinetics of Radioactive Decay
A wooden object from an archeological site is subjected to radiocarbon dating. The activity of the sample that is due to 14C is measured to be 11.6 disintegrations per second. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5715 yr. What is the age of the archeological sample?

40. Kinetics of Radioactive Decay
First we need to determine the rate constant, k, for the process.
= t1/2
0.693 k
= 5715 yr
0.693 k
= k
0.693
5715 yr
= k
1.21  10−4 yr−1

41. Kinetics of Radioactive Decay
Now we can determine t:
= -kt A A0 ln
= -(1.21  10−4 yr−1) t
11.6
15.2 ln
= -(1.21  10−4 yr−1) t
ln 0.763
= t
6310 yr

42. Measuring Radioactivity
One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample.
The ionizing radiation creates ions, which conduct a current that is detected by the instrument.

43. Effects of Radiation

45. 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.

46. Energy in Nuclear Reactions
In the types of chemical reactions we have encountered previously, the amount of mass converted to energy has been minimal.
However, these energies are many thousands of times greater in nuclear reactions.

47. Energy in Nuclear Reactions
For example, the mass change for the decay of 1 mol of uranium-238 is − g.
The change in energy, E, is then
E = (m) c2
E = (−4.6  10−6 kg)(3.00  108 m/s)2
E = −4.1  1011 J

48. Nuclear Fission How does one tap all that energy?
Nuclear fission is the type of reaction carried out in nuclear reactors.

49. 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.

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

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

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

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

54. Nuclear Fission & POWER
Currently about 103 nuclear power plants in the U.S. and about 435 worldwide.
20% of the world’s energy comes from nuclear.

55. Figure 19.6: Diagram of a nuclear power plant.

56. 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.

57. Nuclear power plants
All power plants produce steam which turns turbines to produce energy. Sources of heat to produce steam: heat from fission, coal, oil, natural gas, water
Nuclear power plant
1 pellet of U-235 produces as much energy as 149 gallons of oil, 157 gallons gasoline, 1780 pounds of coal
A pellet is about as big as a pencil eraser
One reactor contains 193 assemblies of rods,
260 rods / assembly w/ 230 pellets per rod

58. 1 million gallons of steam go through the turbine per minute
1 million gallons of steam go through the turbine per minute. Turbine spins at 1800 rpm’s.
395,000 gallons of water in the core are heated to 619 F with a pressure of 2235 psi
Every 1.5 years 1/3 of the core is replaced. Takes 40 days to refuel – carried down man made “canals” to a huge swimming pool on site
Standing outside of a containment building you would get 5 milirem exposure of radiation: 10 mrem exposure from x-rays, 15 mrem from 2 packs of cigarettes

59. Nuclear Fusion Fusion would be a superior method of generating power.
The good news is that the products of the reaction are not radioactive.
The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.

60. Nuclear Fusion
Tokamak apparati like the one shown at the right show promise for carrying out these reactions.
They use magnetic fields to heat the material.