Nuclear Fission and Fusion Chapter 10.2 Notes

Nuclear Forces Protons and neutrons are tightly packed inside the nucleus Remember that unstable nuclei will undergo radioactive decay in order to form more stable nuclei Elements can have stable and unstable nuclei. For example, carbon-12 is stable but carbon-14 is unstable. What do the “12” and “14” mean? How stable a nucleus is depends on the nuclear forces that hold the nucleus together—the forces act between the protons and neutrons

Strong Nuclear Force Remember that protons have a positive charge, and like charges repel one another—so how are protons and neutrons able to stay in the nucleus without flying out? They are able to exist together because of the strong nuclear force The force is stronger than the protons repulsion towards one another—so it keeps them within the nucleus

The Role of the Neutrons Neutrons have no charge, so they do not repel other neutrons or the protons In stable nuclei, the attractive force between protons and neutrons is stronger than the repulsion, preventing nuclear decay So, neutrons contribute to an atom’s nuclear stability

Can there be too many protons or neutrons? Although a greater number of neutrons within a nucleus actually helps to hold the nucleus together, there is a limit to how many neutrons a nucleus can have Nuclei with either too many or too few neutrons will be unstable and will undergo decay Any nucleus that has more than 83 protons is unstable, no matter how many neutrons it has—they will always decay and release large amounts of energy and nuclear radiation

Nuclear Fission The process of splitting heavier (bigger) nuclei into lighter (smaller) nuclei, is called fission It was discovered by Otto Hahn and Fritz Strassman in 1939 In their experiment, uranium-235 was bombarded with neutrons One set of products produced barium-140 and krypton-93 During fission, neutrons and energy are released

Nuclear Fission During fission, each nucleus releases about 3.2 x J of energy In comparison, one molecule of the explosive TNT releases only 4.8 x J Hahn and Strassman also discovered that the overall mass decreases after the reaction—so some of the mass must have been changed into energy This equivalence of mass and energy is explained by the special theory of relativity, which Albert Einstein had presented in 1905 E=mc 2 Where E is energy, m is mass, and c is the speed of light Matter can be converted into energy and energy can be converted into matter

Nuclear Chain Reaction When a nucleus is split by a neutron, it releases smaller nuclei The smaller nuclei need fewer neutrons to stay together, so they release any excess neutrons Excess neutrons can collide with other large nuclei, which will then release more neutrons So a nuclear chain reaction is a continuous series of nuclear fission reactions

Nuclear Chain Reaction

Nuclear Bomb The chain-reaction principle is used to make bombs Inside the bomb, two or more masses of uranium-235 exist, and they are surrounded by a chemical explosive When the explosive is detonated, all of the uranium is pushed together to form a critical mass—which is the minimum amount of a substance that can undergo a fission reaction and can also sustain a chain reaction If the amount of fissionable substance is less than the critical mass, a chain reaction will not occur

Nuclear Fusion Energy is also obtained when very light (small) nuclei are combined to form heavier (bigger) nuclei—this process is called fusion In stars, and the sun, energy is produced when hydrogen atoms combine, or fuse, together--tremendous amount of energy is released when this occurs In the stars, the temperature is high enough that it provides enough energy to bring the hydrogen atoms close together and overcomes the repulsion between the protons in the nuclei

Nuclear Fusion