1. Nuclear Chemistry

2. Nuclear Reactions vs. Normal Chemical Reactions
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

3. The Nucleus
Remember that the nucleus is comprised of the two nucleons, protons and neutrons.
The nucleons are bound together by the strong force.
The number of protons is the atomic number.
The number of protons and neutrons together is effectively the mass of the atom.

4. Atoms of a given element with: same #protons but different # neutrons
Isotopes
Atoms of a given element with: same #protons but different # neutrons
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

5. H H H

6. Radioactivity
Isotopes of certain unstable elements that spontaneously emit particles and energy from the nucleus.
So, some nuclides of an element are unstable, or radioactive.
We refer to these as radionuclides.
There are several ways radionuclides can decay into a different nuclide.

7. Marie Curie a Pioneer of Radioactivity
Lived in France
1898 discovered the elements polonium and radium.
Winner of 1903 Nobel Prize for Physics with Henri Becquerel and her husband, Pierre Curie.
Winner of the sole 1911 Nobel Prize for Chemistry.

8. Types of Radioactive Decay

9. He U He Alpha Decay: Loss of an -particle (a helium nucleus) +
helium nuclei
two protons and two neutrons
charge +2e 
can travel a few inches through air
can be stopped by a sheet of paper, clothing. He 4 2
Loss of an -particle (a helium nucleus) U
238 92

234 90 He 4 2 +

10. Alpha Decay

11.  e I Xe e Beta Decay: Loss of a -particle (a high energy electron) +
Beta particles β: electrons ejected from the nucleus when neutrons decay ( n -> p+ + β - )
Beta particles have the same charge and mass as "normal" electrons.
Can be stopped by aluminum foil or a block of wood.
Loss of a -particle (a high energy electron)  −1 e or I
131 53 Xe 54
 + e −1

12. Gamma Emission:
Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) 
Gamma radiation γ : electromagnetic energy that is released. 
Gamma rays are electromagnetic waves.
They have no mass.
Gamma radiation has no charge.
Most Penetrating, can be stopped by 1m thick concrete or a several cm thick sheet of lead.

13. 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

14. 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

15. 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

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

17. Examples of Radioactive Decay
Alpha Decay
Po  Pb + He
Beta Decay p n e
n  p e
C  N e
Gamma Decay
Ni  Ni g
(excited nucleus)

18. Penetrating Ability

21. Balancing Nuclear Reactions
In the reactants (starting materials – on the left side of an equation) and products (final products – on the right side of an equation)
Atomic numbers must balance
and
Mass numbers must balance
Use a particle or isotope to fill in the missing protons and neutrons

22. Nuclear Reactions Alpha emission
Note that mass number (A) goes down by 4 and atomic number (Z) goes down by 2.
Nucleons (nuclear particles… protons and neutrons) are rearranged but conserved

23. Nuclear Reactions Beta emission
Note that mass number (A) is unchanged and atomic number (Z) goes up by 1.

24. Other Types of Nuclear Reactions
Positron (0+1b): a positive electron
207
Electron capture: the capture of an electron

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

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

27. Nuclear Stability Half-Life
Part II
Nuclear Stability
Half-Life

28. Nuclear Stability Depends on the neutron to proton ratio.
Neutrons play a key role stabilizing the nucleus.
Therefore, the ratio of neutrons to protons is an important factor.
For smaller nuclei (Z  20) stable nuclei have a neutron-to-proton ratio close to 1:1.

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

30. What happens to an unstable nucleus?
Band of Stability
Number of Neutrons, (N)
Number of Protons (Z)
What happens to an unstable nucleus?
They will undergo decay
The type of decay depends on the reason for the instability

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

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

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

34. 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).

35. 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.

36. 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.

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

38. Kinetics of Radioactive Decay
For each duration (half-life), one half of the substance decomposes.
For example: Ra-234 has a half-life of 3.6 days If you start with 50 grams of Ra-234
After 3.6 days > 25 grams
After 7.2 days > 12.5 grams
After 10.8 days > 6.25 grams

39. Amount of 14C left after 2000 years would be 123 grams.
Half Life Calculation
AE = Ao × 0.5t/t1/2
AE = amount of substance left Ao = original amount of substance t = elapsed time t1/2 = half-life of the substance If you are given 157 grams of 14C, how much of 14C would be left after 2000 years? (The half-life of 14C is 5730 years.) AE = 157 × 0.5(2000/5730)
Amount of 14C left after 2000 years would be 123 grams.

40. 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?

41. Kinetics of Radioactive Decay
Nuclear transmutation is a first-order process.
The kinetics of such a process, you will recall, obey this equation:
= kt Nt N0 ln

42. 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.

43. 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?

44. 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

45. Kinetics of Radioactive Decay
Now we can determine t:
= kt Nt N0 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

46. 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.

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

49. Nuclear Fission Fission is the splitting of atoms
These are usually very large, so that they are not as stable
Fission chain has three general steps:
1. Initiation. Reaction of a single atom starts the chain (e.g., 235U + neutron)
2. Propagation. 236U fission releases neutrons that initiate other fissions
3. ___________ .

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

51. 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.

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

53. Nuclear Fission & POWER
Currently about 103 nuclear power plants in the U.S. and about 435 worldwide.
17% of the world’s energy comes from nuclear.

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

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

56. Nuclear Reactors
In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.
Diagram of a nuclear power plant

57. 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.

58. Fusion Nuclear Fusion small nuclei combine 2H + 3H 4He + 1n + 1 1 2 0
Occurs in the sun and other stars
Energy

59. Nuclear Fusion
Fusion
Excessive heat can not be contained
Attempts at “cold” fusion have FAILED.
“Hot” fusion is difficult to contain

60. 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.

61. 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.

62. 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.

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

64. 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.