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Published byHugh Rice Modified over 9 years ago
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Nucleon: anything you find in the nucleus, includes protons and neutrons
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Number of protons determines element Symbol: p + Positive charge: +e Mass = m p = 1.6726e-27 kg Number of protons in nucleus = atomic number, symbol = Z
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Symbol is n 0 No charge Mass = m n = 1.6749e-27 kg Number of neutrons in nucleus = neutron number, symbol N Total number of nucleons (protons + neutrons) in a nucleus is call the atomic mass number, symbol A A = Z + N
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Atoms of the same element can have different number of neutrons in the nucleus (even though same number of protons), called ISOTOPES Isotopes react almost identically when compared to each other, but in physics we’re concerned with different isotopes Masses on periodic table are weighted averages based on natural abundances
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Ex, since most carbon is carbon-12, the number is pretty close to 12
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The number of neutrons strongly affects the stability of the nucleus In unstable isotopes, the number of neutrons partly determines the rate at which the nucleus decays and releases radiation
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Masses of atoms are sometimes given in atomic mass units (amu), which has the unit “u” Not an SI unit, but measuring small things in kg can seem silly, so it’s common Based on neutral carbon-12 atom, 12.000000u 1 u = 1/12 the mass of carbon-12 1 u = 1.660539e-27 kg
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Since the strong nuclear force holds nucleons together, energy must be added to separate them… this is binding energy Separated nucleons have more energy Nucleons bound in nucleus haven’t had energy added yet, so they have more energy
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Since separated nucleons have more energy, they must have more mass (energy is directly related to mass) Nucleons bound in the nucleus have less energy and therefore less mass
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Mass defect = difference between the mass of the nucleus and its individual nucleons Directly related to the binding energy added to break apart the nucleus
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1896: leaves uranium in a drawer with a photographic plate and accidentally identifies another part of the electromagnetic spectrum
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Isolated two other radioactive elements: polonium and radium Put them under different stresses, but the elements always emitted radiation, so concluded radioactivity comes from deep within the atom (i.e., the nucleus)
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Results from the decay of an unstable nucleus Decay happens because it results in a more stable nucleus
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Ernest Rutherford found 3 distinct forms of radiation & divided based on ability to pass through material and deflection in magnetic field Alpha (α): could barely pass through a single sheet of paper. Deflected as a positive particle in a magnetic field. Beta (β): can pass through about 3mm of aluminum. Deflected as a negative particle in a magnetic field. * Gamma (γ): can pass through several centimeters of LEAD! Not deflected in a magnetic field.
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Alpha radiation is a Helium atom, but we call it an alpha particle since it comes from radiation With protons and neutrons leaving the nucleus it gets smaller, often more stable Alpha particle: charge +2e, since no electrons Use conservation of nucleons to write-out decay
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Total mass of the daughter nucleus plus the alpha particle is less than the mass of the original nucleus Missing mass was turned into energy : E = mc 2 Works with our understanding of conservation of mass and energy being interchangeable Energy found mostly in kinetic energy of alpha particle and daughter nucleus moving away from one another
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A neutron falls apart and becomes a proton and an electron Leaving electron is the beta particle That’s why a neutrons mass is a little bigger than a protons
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Particles emitted are opposite from beta negative decay Positive positron, sometimes called an anti electron (antimatter version of an electron) Same mass as an electron, but positive charge
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Emits a form of EMR, not a particle =>much harder to stop (it’s pretty high-up in frequency of the EM spectrum) Happens most often after alpha and beta decay Nucleus has been through a lot and needs to release excess energy Since it’s a release of energy A and Z stay the same
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Half life of an element: the time it will take half of the parent atoms to transmutate into something else Through alpha or beta decays, or another process Total number of atoms stays constant Based on statistics
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The half life of C-14 is 5730 years. Explain what you would expect to happen over a long period of time.
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Activity measures the number of nuclei that decay per second Measured in Becquerels (Bq) = decays/second. Geiger counter clicking in movies measures the activity of the sample. As time passes, the number of nuclei available decrease and sample activity does too
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You have 75 g of lead-212. If it has a half life of 10.6 hours, determine how long it will take until only 9.3 g remains.
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What do you think of when I say nuclear energy?
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There are 2 types of nuclear reactions that release energy Fission Fusion
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The process of causing a large nucleus (A > 120) to split into multiple smaller nuclei, releasing energy in the process. Can start when large nuclei absorbs a neutron, causing it to become unstable to the point that it falls apart Reaction that we use in nuclear power plants and early nuclear weapon Pretty easy and cheap energy Lots of nuclear waste stored for a long time
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The process of causing small nuclei to stick together into a larger nucleus, in the process releasing energy. Process that drives our sun and all other suns We can duplicate in a lab, but use more energy than we get out Left over products are safe, so lots of research goes into trying to develop fusion reactors
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The most typical fuel used in a fission reactor is uranium-235. 1939: 4 German scientists discovered that uranium- 235 would become very unstable if it gained an extra neutron, forming uranium-236. Uranium-236 is so unstable that a fraction of a second later it will split to form two smaller atoms, and in the process release energy.
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If one neutron gives rise to another reaction, the self sustaining reaction that results is called critical. Each reaction leads to one reaction afterwards. This is a “chain reaction”. If 2+ neutrons give rise to more reactions, the increasing rate of reactions is called supercritical. Each reaction leads to multiple reactions afterwards. Generations of reactions increase exponentially
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There are a few situations when we want this to happen... Nuclear bomb, since we want one reaction we kick off to result in a cascade of exponentially more and more reactions within a split second When a nuclear power plant is first being started up ▪ Then stepped down to a critical reaction. ▪ If the nuclear reactor is melting down then supercritical reactions are BAD
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You need subcritical reactions Less than a neutron gives rise to other reactions
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Reactors use control rods to control the rate of the reaction. Made from elements such as boron and cadmium, control rods are very good at absorbing neutrons. If a reaction is going supercritical, drop the control rods further into the core to absorb extra neutrons and the reaction slows. If the reaction is going subcritical, pull the control rods out further, which lets more neutrons react and get more reactions going again.
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