1. Nuclear Reactions OPENER

2. The Nucleus Recall that atoms are composed of protons, neutrons, and electrons. The nucleus of an atom contains the protons, which have a positive charge, and neutrons, which have no electric charge. Atoms usually contain the same number of protons as electrons.

3. The Strong Force A strong force, causes protons and neutrons to be attracted to each other within the nucleus. The strong force is one of the four basic forces in nature and is about 100 times stronger than the electric force.

4. Attractions and Repulsion If a nucleus has only a few protons and neutrons, they are all close enough together to be attracted to each other by the strong force.

5. Forces in a Large Nucleus If nuclei have many protons and neutrons, each proton or neutron is attracted to only a few neighbors by the strong force.

6. Forces in a Large Nucleus Because only the closest protons and neutrons attract each other in a large nucleus, the strong force holding them together is about the same as in a small nucleus. All the protons in a large nucleus exert a repulsive electric force on each other. Thus, the electric repulsive force on a proton in a large nucleus is larger than it would be in a small nucleus.

7. Different than Chemical Reactions Chemical ReactionsNuclear Reactions Part of the atom involved Outermost electrons – Energy Levels Protons and Neutrons (Nucleons) - Nucleus How the reaction is started Atoms are brought close together with high temperatures and pressure High temperatures are required or atoms are bombarded with high- speed particles Outcome of the reaction Atoms form ionic or covalent bonds The number of protons and neutrons in an atom usually change How much energy is released or absorbed Small amountHuge amount ExamplesBurning Fossil fuels, digesting food, medicines Nuclear Energy, taking x-rays, treating cancer, the sun generating heat and light

8. Radioactivity When the strong force is not large enough to hold a nucleus together tightly, the nucleus can decay and give off matter and energy. This process of nuclear decay is called radioactivity. It is a process where the nucleus emits particles or energy Unstable nucleus – when a nucleus contains too many or too little numbers of neutrons the nuclei is unstable or radioactive.

9. Radioactivity All nuclei that contain more than 83 protons are radioactive. However, many other nuclei that contain fewer than 83 protons also are radioactive. Almost all elements with more than 92 protons don’t exist naturally on Earth They have been produced only in laboratories and are called synthetic elements. These synthetic elements are unstable, and decay soon after they are created.

10. The Discovery of Radioactivity In 1896, Henri Becquerel left uranium salt in a desk drawer with a photographic plate. Later, when he developed the plate, he found an outline of the clumps of the uranium salt. He hypothesized that the uranium salt had emitted some unknown invisible rays, or radiation, that had darkened the film.

11. The Discovery of Radioactivity Two years after Becquerel’s discovery, Marie and Pierre Curie discovered two new elements, polonium and radium, that also were radioactive. After more than three years, they were able to obtain about 0.1 g of radium from several tons of pitchblende.

12. Nuclear Radiation When an unstable nucleus decays, particles and energy called nuclear radiation are emitted from it. The three types of nuclear radiation are alpha, beta, and gamma radiation. Alpha and beta radiation are particles. Gamma radiation is an electromagnetic wave.

13. Alpha Particles (α) When alpha radiation occurs, an alpha particle  made of two protons and two neutrons is emitted from the decaying nucleus. (Helium)

14. Compared to beta and gamma radiation, alpha particles are much more massive. They also have the most electric charge. When alpha particles pass through matter, they exert an electric force on the electrons in atoms in their path. This force pulls electrons away from atoms and leaves behind charged ions. Alpha particles are the least penetrating form of nuclear radiation (weakest type of radiation). Alpha particles can be stopped by a sheet of paper. Alpha Particles

15. Beta Particles (β) A second type of radioactive decay is called beta decay. a neutron decays into a proton and emits an electron. The electron that is emitted from the nucleus is called a beta particle

16. Damage from Beta Particles Proton stays inside nucleus to increase the # of protons by one Atomic # increases by one and element changes Beta particles are much faster and more penetrating than alpha particles. Beta particles can damage cells when they are emitted by radioactive nuclei inside the human body. EX: Iodine-131 (treats cancer) EX: Strontium-91 radioactive tracer.

17. Transmutation = changes from one element to another element during α and β decay. Alpha Decay = mass number decreases by 4 and the atomic number decreases by 2 Beta Decay = mass number does not change and the atomic number increases by 1

18. Alpha Decay Uranium-238 has 92 protons After alpha decay (2 protons & 2 neutrons leave the nucleus) Becomes Thorium-234 and has 90 protons Beta Decay Carbon-14 has 6 protons & 8 neutrons After beta decay (a neutron is split into a proton and electron & the electron is expelled from the nucleus) Becomes Nitrogen-14 and has 7 protons and 7 neutrons

19. Gamma Rays (γ) The most powerful penetrating form of nuclear radiation is gamma radiation. Gamma rays are electromagnetic waves with the highest frequencies and the shortest wavelengths in the electromagnetic spectrum. A packet of energy

20. Gamma Rays They have no mass and no charge and travel at the speed of light. Thick blocks of dense materials, such as lead and concrete, are required to stop gamma rays. However, gamma rays cause less damage to biological molecules as they pass through living tissue.

21. Gamma radiation is very high-energy ionizing radiation. (aka high frequency)ionizing radiation Gamma photons have about 10,000 times as much energy as the photons in the visible light range of the electromagnetic spectrum. Gamma photons have no mass and no electrical charge they are pure electromagnetic energy. Because of their high energy, gamma photons travel at the speed of light and can cover hundreds to thousands of meters in air before spending their energy. They can pass through many kinds of materials, including human tissue. Very dense materials, such as lead, are commonly used as shielding to slow or stop gamma photons. Their wave lengths are so short that they must be measured in nanometers, billionths of a meter.

22. Uses of Cesium-137: cancer treatment measure and control the flow of liquids in numerous industrial processes investigate subterranean strata in oil wells measure soil density at construction sites ensure the proper fill level for packages of food, drugs and other products. Uses of Cobalt-60: sterilize medical equipment in hospitals pasteurize certain foods and spices treat cancer gauge the thickness of metal in steel mills. Uses of Technetium-99m: TC-99m is the most widely used radioactive isotope for diagnostic studies. (Technetium-99m is a shorter half-life precursor of technetium- 99.) Different chemical forms are used for brain, bone, liver, spleen and kidney imaging and also for blood flow studies.

23. Radioactive Half-Life Some radioisotopes decay to stable atoms in less than a second. However, the nuclei of certain radioactive isotopes require millions of years to decay.

24. Radioactive Half-Life The half-life of a radioactive isotope is the amount of time it takes for half the nuclei in a sample of the isotope to decay. The nucleus left after the isotope decays is called the daughter nucleus.

25. Dating Fossils Carbon Dating- Carbon 14 is used. Uranium Dating- use of uranium to determine the age of rocks

27. Carbon Dating Carbon-14 has a half-life of 5,730 years and is found in molecules such as carbon dioxide. Plants use carbon dioxide when they make food, so all plants contain carbon-14. The ratio of the number of carbon-14 atoms to the number of carbon-12 atoms in the organism remains nearly constant. When an organism dies, its carbon-14 atoms decay without being replaced. C-12 to C-14 Ratio decreases with time. Ratio helps determine age of organisms remains.

28. Radiation Detectors = devices to detect radiation A cloud chamber can be used to detect alpha or beta particle radiation and is filled with water or ethanol vapor. When a radioactive sample is placed in the cloud chamber, it gives off charged alpha or beta particles that travel through the water or ethanol vapor. As each charged particle travels through the chamber, it knocks electrons off the atoms in the air, creating ions. Beta particles leave long, thin trails, and alpha particles leave shorter, thicker trails.

29. Bubble Chambers A bubble chamber holds a superheated liquid, which doesn’t boil because the pressure in the chamber is high. When a moving particle leaves ions behind, the liquid boils along the trail. (Removes Electrons) The path shows up as tracks of bubbles.

30. Electroscopes Use positive and negative charges to detect radioactivity

31. Measuring Radiation Large doses of radiation can be harmful to living tissue. A Geiger counter is a device that measures the amount of radiation by producing an electric current when it detects a charged particle.

32. Background Radiation Background radiation, is not produced by humans, instead it is low-level radiation emitted mainly by naturally occurring radioactive isotopes found in Earth’s rocks, soils, and atmosphere. Largest sources – 1) Radon gas –produced in Earth’s curst by the decay of Uranium 2) Cosmic Rays – produces showers of nuclear radiation 3) Carbon-14 : decays to emit beta particles

33. Nuclear Reactions Nuclear Fission Nuclear Fusion

34. Two types of Nuclear Reactions 1. Fission - breaking up the nucleus of an atom to form new atoms 2. Fusion - the process of combining the nuclei of atoms to make different atoms a fused particle and a lot of energy are released

35. Nuclear Fission In 1938, Otto Hahn and Fritz Strassmann found that when a neutron strikes a uranium-235 nucleus, the nucleus splits apart into smaller nuclei. In 1939 Lise Meitner, she proposed that the uranium-235 nucleus is so distorted when the neutron strikes it that it divides into two smaller nuclei.

36. Nuclear Fission The process of splitting a nucleus into several smaller nuclei is nuclear fission to produce energy.

37. Mass and Energy Albert Einstein proposed that mass and energy were related in his special theory of relativity. According to this theory, mass can be converted to energy and energy can be converted to mass. E= mc 2 E = energy M = Mass C= constant for speed of light (299,792,458 m/s

38. Chain Reactions When a nuclear fission reaction occurs, the neutrons emitted can strike other nuclei in the sample, and cause them to split.

39. Chain Reactions The series of repeated fission reactions caused by the release of neutrons in each reaction is a chain reaction.

40. Chain Reactions For a chain reaction to occur, a critical mass of material that can undergo fission must be present. The critical mass is the amount of material required so that each fission reaction produces approximately one more fission reaction. If less than the critical mass of material is present, a chain reaction will not occur.

41. Chain Reactions Controlled Chain Reaction = many of the neutrons that are produced are absorbed in “control rods” prevent excess energy to be released. Ex. Nuclear Power Plants Uncontrolled Chain Reaction = all the neutrons are allowed to continue to hit/split other nuclei causing massive amounts of energy to be released all at once. Ex. Atomic Bomb (used on Japan in 1945)

43. Atomic Bomb Video/Manhattan Project http://www.youtube.com/watch?v=4IqKdf6I n_khttp://www.youtube.com/watch?v=4IqKdf6I n_k

44. Nuclear Fusion Nuclear Reactions In nuclear fusion, two nuclei with low masses are combined to form one nucleus of larger mass. Fusion fuses atomic nuclei together, and fission splits nuclei apart. Tremendous amounts of energy can be released during the process of nuclear fusion. Also known as a thermonuclear reaction ex: H-bomb and the sun/stars

45. Temperature and Fusion For fusion to occur, two positively charged nuclei must come in contact. This is impossible without extremely high temperatures, too high to maintain on Earth. Only at temperatures of millions of degrees Celsius are nuclei moving so fast that they can get close enough for fusion to occur.

46. Nuclear Fusion and the Sun Most of the energy given off by the Sun is produced by a process involving the fusion of hydrogen nuclei. This process occurs in several stages, and one of the stages is shown. As this occurs, a small amount of mass is changed into an enormous amount of energy. An isotope of helium is produced when a proton and the hydrogen isotope H-2 undergo fusion.

47. Nuclear Fusion and the Sun As the Sun ages, the hydrogen nuclei are used up as they are converted into helium. So far, only about one percent of the Sun’s mass has been converted into energy. It is estimated that the Sun has enough hydrogen to keep this reaction going for another 5 billion years.

48. How do we use Nuclear Power? Energy from the Sun - plants convert this to sugars - we use this energy for food and for fuel for our cars Nuclear reactors to form heat to generate steam to run turbines, the turbines generate electricity for our homes Radioactive isotopes are used a tracers in medicine and science - followed as it travels through a system (the digestive tract) Carbon dating — used to determine the age of artifacts - measure the amount of carbon-14 that is left in an artifact - this can be used to determine the age of the artifact by relating it to the half-life of carbon-14

50. Nuclear Waste Radioactive elements have a half-life - amount of time for half of the radioactive element to decay Start with 100 atoms ofcarbon-14 in 5,730 years will have 50 atoms ofcaron-14 - 5,730 years is the half-life for carbon-14 As radioactive elements decay they emit harmful radiation and this radiation is harmful to living things Uranium is the radioactive element in nuclear fuel

51. How a Power Plant Works? (Video – 7 minutes)How a Power Plant Works? (Video – 7 minutes) How Nuclear Reactors work http://www.youtube.com/watch?v=1U6Nzc v9Vwshttp://www.youtube.com/watch?v=1U6Nzc v9Vws

52. Review Questions 1) What distinguishes one atom from another? The number of protons There are no 2 elements with the same # of protons

53. 2) What is a nuclear reaction? A change in an element that involves protons and neutrons. A chemical reaction involves electrons

54. 3) What is fusion? Combining of nuclei of atoms to make different atoms A great deal of energy is released A particle is emitted Use a particle accelerator to bombard particles and atoms Requires a lot of heat but does not produce any waste

55. 4) Is nuclear Fusion possible yet on Earth? Nuclear fusion is not possible yet, like what takes place in the sun

56. 5) What is fission? Splitting of a nucleus Started when a neutron hits a nucleus Each collision releases another neutron Causes a chain reaction Nuclear reactors; produces harmful waste

57. 6. What is radioactivity? Any process where the nucleus emits particle or energy Happens because nucleus is unstable Too many neutrons When it emits it is called decay

58. 7) What types of decay are there? Alpha (α ) – release of two p+ and 2 neutrons Stopped by a piece of paper (harmless) Beta (β ) – nucleus splits into a p+ and e- Protons stays behind but e- is released Can penetrate about 1 cm of flesh Can cause damage Gamma (γ ) = release of high energy Can penetrate concrete Very harmful Alpha has the least amount of energy and the longest wavelength, then beta, then gamma Examples: The sun, Nuclear power plants, Nuclearsubmarines, x-rays

59. 8) What are the drawbacks of nuclear power? The waste products – its harmful All radioactive elements decay and emit radiation Some take years and years to decay

60. 9) What is half-life? The amount of time it takes for half of a radioactive sample to lose half of its mass Example An isotope decreased to one-forth its original amount in 18 months. What is the half-life of this radioactive isotope? 18/4 = 4.5 months