Chapter 28

1. Chapter 28 Nuclear Chemistry

2. Nuclear Reactions • Unstable isotopes, radioisotopes, gain stability by releasing large amounts of energy. • Nuclear reactions are not affected by changes in temperature, pressure, or catalysts. • They cannot be slowed down, speeded up, or turned off.

3. Antoine Henri Becquerel • Studied how uranium salts that were exposed to sunlight fogged photographic film plates. • He found that the salts caused the same results even when they were unexposed to sunlight.

4. Uranium salts caused the photographic film to fog.

5. Marie and Pierre Curie • The Curies showed that the fogging of the plates was caused by rays emitted by the U atoms. • The penetrating rays and particles emitted by a radioactive source are called radiation.

7. What about Dalton’s Theory? • Dalton described atoms as indivisible. • Radioactive decay is a process by which an unstable nucleus loses energy by emitting radiation. • An unstable nucleus is the result of an too many or too few neutrons, relative to the number of protons.

8. Radioactivity is the emission of tiny, energetic, invisible particles by the nuclei of certain atoms.

9. Types of Radiation An electric field can be used to separate three different types of radiation. Alpha particles have a positive charge and are attracted to the negative plate. Beta particles, which have a negative charge, are attracted toward the positive plate. Gamma rays are not deflected by an electric field.

10. Alpha Radiation Alpha particles contain two protons and two neutrons and have a double positive charge.

11. When an atom loses an alpha particle, the atomic number is lower by two and the mass number is lowered by four.

12. Balancing Nuclear Reactions Like other chemical equations, nuclear equations must be balanced as well. The sum of the mass numbers and atomic numbers on both sides of the equation must be equal. Once the atomic number is known, the element can be determined. What element has been produced here?

13. In Beta radiation, a fast moving electron is released by the decomposition of a neutron. A proton is also formed. Notice the increase in the atomic number. Beta particles have less charge and less mass than an alpha particle. Beta Radiation

14. Gamma radiation This radiation is high-energy electromagnetic radiation. Visible light is also an example of electromagnetic radiation, but of a much lower energy. Gamma radiation is often emitted along with alpha or beta radiation. Gamma rays have no mass and no charge.

15. Alpha particles have the least penetrating power due to their large mass and charge. Gamma rays are the most penetrating because they have no mass or charge. Gamma radiation and x-rays are able to easily pass through paper, wood, and the human body. They can be mostly stopped by several meters of concrete or several cm of lead. Ever get an x-ray at the dentist?

16. Types of Radiation Review!!

17. Nuclear Stability For elements with a low atomic number, the ratio of neutrons to protons for stability is 1 to 1. However, above atomic number 20, stable nuclei have more neutrons than protons. For the heaviest elements, the ratio reaches 1.5 to 1. The stable nuclei are shown above in the band of stability.

18. Some nuclei have too many neutrons relative to the number of protons. These nuclei became stable by changing a neutron into a proton and emitting a beta particle. The beta decay of I-131 is shown below. How has the atomic number changed for the reaction below?

19. Some nuclei are unstable because they have too few neutrons relative to protons. These nuclei convert a proton to a neutron by capturing an electron. How has the atomic number changed?

20. A positron is a particle with the mass of an electron, but having a positive charge.

21. A positron may be emitted as a proton changes to a neutron. When a positron is emitted, the atomic number decreases by one and the number of neutrons increases by one. What happens to the mass number?

22. All atoms with atomic numbers of 83 or greater have unstable nuclei. The nuclei are too heavy and too many neutrons and protons. These nuclei usually become stable by undergoing alpha emission, which causes the mass number to decrease by four and the atomic number to decrease by two. In the chemical reactions that we have studied so far, we have noticed that there is a conservation of mass from the reactants to the products. However, in nuclear reactions, mass is not necessary conserved. The loss of a very small amount of mass is the reason for the emission of a large amount of energy. E=mc2

24. Half-life • Half-life is the time required for one half of the isotope to decay to products. • After one half-life, half of the original radioactive atoms remain. • After two half-lives, one quarter of the original radioactive atoms remain.

25. Number of Th-232 atoms in a sample initially containing 1 million atoms. Th-232 has a half life of 14 billion years. Half-life is constant, but is different for each isotope.

26. Half-lives can be as long as billions of years or as short as fractions of a second. Can you find the radioisotope in the chart with the longest half-life? The shortest?

27. Scientists can use the half-life of carbon, 5730 years, to determine the age of ancient fossils. Carbon-14 dating can be used to determine the ages of organic objects between 200 and 50,000 years old.

28. The decay of Uranium-238 • U-238 decays through a series of intermediates to the stable isotope of Pb-206. • Because the half-life of U-238 is so long it is possible to use it to date rocks nearly as old as the solar system.

30. Half-life Problem Nitrogen-13 emits beta radiation and decays to oxygen-13 with a half-life of 10 minutes. Assume a starting mass of 2.00 grams of nitrogen-13. How long is three half-lives? How many grams of the isotope will still be present at the end of three half-lives?

31. Transmutation Reactions Transmutation is when an atom of one element is changed to an atom of a different element. It can occur because of radioactive decay or when high energy particles bombard the nucleus of an atom.

32. Transuranium elements • All elements above atomic number 92 undergo transmutation. • None of them occur in nature, and all of them are radioactive. • These elements have been synthesized in nuclear accelerators.

33. The Stanford Linear Accelerator • Operated by Stanford University in CA. • The accelerator is about 2 miles long. • The first artificial elements were synthesized in 1940 in Berkeley, CA.

34. Since 1940, more than 20 elements have been synthesized.

35. A synchrotron is a circular accelerator. Magnets are used to control the path and acceleration of the particles.

36. Fission • Fission is the splitting of a nucleus into smaller fragments. • When the nucleus is struck with a slow moving neutron, it breaks into two fragments that are roughly the same size.

37. When the nucleus splits, it also gives off more neutrons which in turn strike other nuclei. Those nuclei splint and release more neutrons. This is a chain reaction that is continues to occur, similar to the dominos show.

38. Nuclear Fission • The fission of 1kg of U-235 releases the same amount of energy as 20,000 tons of dynamite. • In a nuclear reactor, energy is generated in the form of heat to create steam which turns a turbine to make electricity.

39. Neutron moderation reduces the speed of neutrons so that they can be captured by the reactor fuel. Water and carbon are good moderators.

40. Neutron absorption decreases the number of slow moving neutrons. This prevents the reaction from occurring too quickly. Neutron absorption uses a material such as cadmium to absorb neutrons. As the rods are pulled out, the fission process will speed up.

42. Nuclear Waste • Spent fuel rods are high level nuclear waste. • They are usually stored at the bottom of giant “pools” of water located onsite. The water continues to cool the rods and acts as a shield to reduce radiation.

43. Nuclear waste is being stored in temporary sites awaiting permanent disposal. After the spent fuel rods spend a decade or more in the holding tank, they must be moved to off-site storage facilities to make room for new ones. The storage is overlooked by the DOT and NRC. Nuclear plants can only operate for a certain number of years and then they must be decommissioned.

44. Fusion occurs when nuclei combine to produce a nucleus of greater mass. In solar fusion, hydrogen nuclei fuse to produce helium nuclei. Fusion reactions only occur at extremely high temperatures (like 40,000,000 degrees Celsius). Fusion as an energy source on earth is appealing because of the lack of radioactive materials produced. The fuel for the reaction is also inexpensive.

45. At the high temperatures involved in fusion, matter exists as a plasma. No known structural material can withstand the hot, corrosive plasma. Scientists are experimenting with magnetic fields to contain the plasma.

46. Ionizing radiation is radiation with enough energy to knock electrons off atoms which produce ions. A Geiger counter is able to measure ionizing radiation. The tube has a central wire electrode connected to a power supply. When exposed to radiation, the gas inside the tube becomes ionized, able to conduct an electric current. The bursts of current drive an electronic counter and an audible click. Geiger counters detect beta radiation. Alpha particles cannot pass through the end window. Gamma rays pass through the gas quickly, causing little ionization.

47. Geiger Counter

48. Agricultural Applications • Radioisotopes can be used to help understand chemical and biological processes in plants. • This information helps scientists understand the detailed mechanisms of how plants grow and reproduce.

49. Scintillation Counter • A device that uses a phosphorus coated surface to detect radiation. • The number of flashes are detected electronically and recorded.

50. Film Badge • An important piece of equipment for people who work near sources of radiation. • At the end of the work day, the film is processed and the amount of radiation exposure is recorded. • Unfortunately, the film badge does not protect against radiation.