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Transcript
Structure of the nucleus It is now known that the nucleus consists of: o protons: positive charge: charge +1, mass number 1 o neutrons: no charge: charge 0,mass number 1 o protons and neutrons are collectively known as nucleons. The total number of protons and neutrons in the nucleus is called the mass number A. The number of protons in the nucleus is called the atomic number Z. In a neutral atom the number of protons equals the number of electrons. The mass numbers, charges and symbols for protons, neutrons and electrons are given below. The type of nucleus, and the corresponding atomic number and mass number is normally written using the standard representation A Z X for example, 14 6 C means that this is a carbon nucleus of atomic number 6 and mass number 14. This means that this carbon nucleus has 6 protons and 8 neutrons (14 – 6 = 8). The table below provides information on protons, neutrons and electrons with the last column showing the standard representation described above. Particle Mass number Charge Symbol Proton 1 +1 1 1 p Neutron 1 0 1 0 n Electron 0 -1 0 1 The mass number of an electron is taken to be zero since mass of electron = proton. e 1 mass of 1840 Isotopes Each element in the periodic table has a different atomic number and is identified by that number. It is possible to have different versions of the same element, called isotopes. An isotope of an atom has the same number of protons but a different number of neutrons, i.e. the same atomic number but a different mass number. An isotope is identified by specifying its chemical symbol along with its atomic and mass numbers. For example: Carbon-12 12 6 C contains 6 protons and 6 neutrons (12 – 6) Carbon-14 14 6 C contains 6 protons and 8 neutrons (14 – 6) Radioactive decay Many nuclei are unstable. In order to achieve stability, they can emit nuclear radiation: alpha, beta or gamma. o Such unstable nuclei are called radioisotopes or radionuclides. The process of emitting radiation is called decay. Alpha, beta and gamma or 24 He An alpha particle is the nucleus of a helium atom 4 2 A beta particle is a fast-moving electron 0 1 o or 10 e In beta decay, a neutron within the unstable nucleus changes into a proton, with the emission of a fast-moving electron from with the nucleus (discussed in more detail in Key Area - Standard Model) n 11 p 01n 1 0 Gamma rays are photons of high-energy electromagnetic radiation 0 0 Representation of radioactive decay by symbols and equations In the following equations, both mass number and atomic number are conserved, o i.e. the totals are the same before and after the decay. The original radionuclide is called the parent and the new radionuclide produced after decay is called the daughter product (sometimes this may go on to decay further). Alpha decay Uranium 238 decays by alpha emission to give Thorium 234. 238 92 U 234 90 Th + 42 He Mass number decreases by 4, atomic number decreases by 2 (due to loss of 2 protons and 2 neutrons). Beta decay Lead 210 decays by beta emission to give Bismuth 210. 210 82 Pb 210 83 Bi + 01e Mass number is unchanged, atomic number increases by 1 Gamma decay Only energy is emitted from within the nucleus and the daughter product is the same as the parent. Example Thorium-230 decays by alpha emission. Write the equation for this decay and state the name of the daughter nucleus produced. You need a Periodic Table to identify the daughter nucleus Th 230 90 Ra 24 226 88 The daughter nucleus produced is Radon-226 (the equation is balanced, giving unknown daughter nucleus atomic number 88 and mass number 226. The periodic table is used to identify the element with atomic number 88 as Radon). Nuclear Fission Nuclear fission is when a heavy nucleus breaks up into two nuclei of smaller mass number, usually along with several neutrons. o Fission can occur spontaneously, i.e. a heavy nucleus can break up randomly and of its own accord o In nuclear power stations, fission is induced by bombarding the heavy nucleus with neutrons. The following equation represents the fission of Uranium-235 235 92 141 1 U + 01n 92 36 Kr + 56 Ba + 3 0 n + energy Nuclear Fission and E=mc2 Mass number and atomic number are both conserved during the fission reaction shown above. Even though the mass number is conserved, when the masses before and after the fission are compared accurately, there is a mass difference. o The total mass before fission is greater than the total mass of the products. Einstein suggested that mass and energy are equivalent, and that when there was a decrease in mass, an equivalent amount of energy was produced. Einstein’s famous equation shows how mass and energy are related: In fission reactions, the energy released is carried away as kinetic energy of the fission products. Example The equation shown below represents a fission reaction 235 92 U + 01 n 137 56 Ba + 97 36 Kr + 2 01 n + energy The masses of the nuclei are: 235 92 U Ba 2273 10-27 kg Kr 1609 10-27 kg 137 56 97 36 1 0 3902 10-27 kg n 1675 10-27 kg Mass before fission (kg) 235 -27 92U 3902 × 10 1 0 n Mass after fission (kg) Ba 2273 × 10-27 Kr 1609 × 10-27 137 56 1675 × 10-27 97 36 ___________________ 2 01n 2 1.67510-27 = 3350 × 10-27 391875 × 10-27 ______________________________________________ 391550 × 10-27 Decrease in mass = (391875 – 391550) × 10-27 = 0325 × 10-27 = 325 10-28 kg Energy released during this fission reaction, using E = mc2 E mc 2 E 3 25 1028 (3 108 ) 2 E 2 8 1011 J This is the energy released by fission of a single nucleus. Nuclear fission reactor Fuel Rods: Generate heat by nuclear fission Moderator: fission Slows neutrons down since slow neutrons have more chance of inducing Control Rods: Absorb excess neutrons. Control the rate of heat production Coolant: Removes heat from the reactor and passes the heat to the boiler Containment Vessel: Absorbs radiation. Shields the outside from radiation produced in the reactor Nuclear Fusion Fusion is when two light nuclei combine to form a nucleus of larger mass number, o e.g. the fusion of two deuterium nuclei (an isotope of hydrogen) to form helium – as shown below Like fission, there is a decrease in mass in the fusion process and the energy released is produced as kinetic energy of the fusion products. Unlike fission however, there are no radioactive fission fragments left over from the reaction. The energy released by the sun and other stars is produced by nuclear fusion. Fusion of deuterium requires incredibly high temperatures found at the core of stars o Producing a working fusion reactor would give society an amazingly efficient energy source which uses only hydrogen fuel and no radioactive waste o To recreate the conditions that occur at the core of stars on Earth presents a huge scientific and engineering challenge Example The equation shown below represents a fusion reaction 2 1 H + 31 H 4 2 He + 01 n + energy The masses of the nuclei are: 2 1 H 3345 10-27 kg 5008 10-27 kg 3 1 4 2 He 6647 10-27 kg 1 0 n H 1675 10-27 kg Mass before fusion (kg) 2 3345 10-27 kg 1H 10-27 kg 3 5 008 1 ___________________ H 8353 × 10 Mass after fusion (kg) 4 6647 10-27 kg 2 He -27 1.67510-27 ______________________________________________ 8322 × 10-27 Decrease in mass = (8353 – 8322) × 10-27 = 0031 × 10-27 = 31 10-29 kg Energy released during this fission reaction, using E = mc2 E mc 2 E 3 11029 (3 108 ) 2 E 2 8 1012 J This is the energy released by one fusion reaction. Fusion reactor – the goal of plasma physicists The fourth state of matter which is a high-temperature, ionised gas called plasma o Physicists who study fusion are called plasma physicists Fusion experiments involve heating a very small container of hydrogen fuel to phenomenally high temperatures usually only found at the core of stars. A self-sustaining fusion reactor would generate its own incredibly high temperatures which would then continue to generate more and more fusion reactions. o The point where the fusion reactions generate the conditions to sustain further fusion reactions is called ignition. The challenge is in heating the fuel to very high temperatures – and in confining the plasma within itself to prevent any plasma touching the walls of its container – any contact with the container results in cooling. For ignition to occur, three conditions must be met o Extremely high plasma temperature (over 100 million Kelvin) o A stable period of heat energy production via fusion lasting over 2 seconds o Stable plasma density (mass per unit volume) of 1020 nuclei/m3 This is where the confinement by huge magnetic fields is required 250,000 times less dense than the earth's atmosphere Teams of physicists have come close to achieving ignition and a self-sustaining fusion reactor. o The leading centre for fusion research is the Joiunt European Torus (JET) facility in Oxfordshire. JET is the largest fusion fusion exeperiment in the world involving a huge team of plasma physicists and engineers. o The product of temperature, density, and thermal insulation attained is only a factor of 5 short of the conditions required for ignition. Diagram of a magnetic confinement fusion reactor. The plasma is confined and stopped from touching the walls of the reactor by the huge magnetic fields generated by the complex magnet system.