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Nuclear Changes. Section 1: What is Radioactivity?. Vocabulary. Radioactivity Nuclear radiation Alpha particle Beta particle Gamma Ray Neutron emission Half-life. Nuclear Radiation. To discuss radiation, we need to define radioactivity - PowerPoint PPT Presentation


  • Nuclear ChangesSection 1: What is Radioactivity?

  • VocabularyRadioactivityNuclear radiationAlpha particleBeta particleGamma RayNeutron emissionHalf-life

  • Nuclear RadiationTo discuss radiation, we need to define radioactivityRadioactivity is a PROCESS by which an unstable nucleus emits one or more particles or energy in the form of electromagnetic radiationThis means two things can happen because of radioactivityThe nucleus of an atom gives off some particleEnergy (electromagnetic energy) is given off by the atom

  • Changes in NucleusRadioactive materials have unstable nucleiThese nuclei undergo changes by emitting particles or radiationAfter the changes in the nucleus, the element CAN transform into a new element entirelyBut doesnt alwaysThis process of changing the nucleus is called nuclear decayThe released energy and matter from nuclear decay is called nuclear radiationCAN cause damage to living tissue

  • Types of Nuclear RadiationThere are four types of nuclear radiationAlpha particlesBeta particlesGamma raysNeutron emissionAfter a radioactive atom decays, the nuclear radiation leaves the nucleus of the atomThis nuclear radiation then interacts with nearby matterThe result of this interaction depends on the nuclear radiation itself

  • Alpha Particles, Alpha particles are positively charged and more massive than any other type of nuclear radiationMade of 2 protons and 2 neutrons from the unstable nucleusIs effectively the nucleus of a helium atomAlpha particle dont travel far through other materialsWill barely pass through a sheet of paperThis is because its so massiveBecause they have a positive charge, they remove electrons (ionize) matter as they pass throughAs they ionize material they lose energy and slow down even more

  • Beta Particles, Beta particles are fast moving electronsTravels farther through matter than alpha particlesStill comes from the nucleusHow this worksA neutron (neutral) decays to form a proton and an electronThe electron is then ejected from the nucleusThe proton stays in the nucleus (making this atom a new element)Will penetrate paper, but can be stopped by 3mm of aluminum foil or 10 mm of wood.Like alpha particles, will also ionize matter they pass throughAnd as they ionize matter, they lose energy and slow down

  • Gamma Rays Discovered by Marie Curie in 1898Gamma rays are high energy electromagnetic radiation emitted by a nucleus during radioactive decay.Much more penetrating than beta particlesNot made of matter like alpha and beta particlesDoes not possess a electrical chargeBecause of no electrical charge, does not easily ionize matter it passes throughDamages materials due to high energy, not due to ionizationBecause they dont ionize matter, not easily stoppedCan penetrate up to 60 cm of aluminum or 7 cm of lead

  • Neutron Emission Neutron emission is the release of high-energy neutrons by some neutron-rich nuclei during radioactive decayScientists actually discovered the neutron by neutron emissionBecause neutrons have no charge, they do not ionize matter like alpha and beta particlesSince they dont ionize, they dont waste their energy ionizingSo neutrons will travel farther through matter than either alpha or beta particlesA block of lead about 15 cm thick is required to stop most fast moving neutrons during radioactive decay.

  • Nuclear DecayWhen an unstable nucleus emits and alpha or beta particle, the number of protons and neutrons changes.Example: radium-226 changes into radon-222 by emitting an alpha particle

  • Alpha DecayA nucleus will give up two protons and two neutrons during alpha decayWe can write the nuclear decay process like a chemical equationThe nucleus before the decay is like a reactantPlaced on the left side of the equationThe nucleus after the decay is like a productPlaced on the right side of the equationThe particle emitted also treated like a product

  • Example EquationRadium-226 Radon-222

    Hint on reading theseNumber on top is the mass numberNumber of protons + neutronsNumber on bottom is the atomic numberNumber of protonsHas mass been conserved here?Yes

  • Summary of Alpha DecayDuring alpha decayThe mass number (top #) goes down by 4The atomic number (bottom #) goes down by 2Will always have the alpha particle as part of the products.

  • Beta DecayDuring beta decay, a nucleus GAINS a proton and loses a neutronThis means that in total, the mass number (top #) stays the sameAtomic number (bottom #) increases by one

  • Gamma Ray DecayWhen a nucleus undergoes nuclear decay by gamma rays, there is no change to the atomic numberOnly the energy of the nucleus changes

  • Nuclear Decay Practice-Determine: A, Z, X and type of decay

  • Radioactive Decay RatesIt is impossible to predict the moment when any particular nucleus will decayBut it is possible to predict the time it takes for half of the nuclei in a given sample to decayThe time it takes for half of a sample of radioactive nuclei to decay is called the samples half-life.

  • Half-LifeHalf-life is just a certain amount of timeEach substance has a unique half-lifeAfter the first half life of a sample has passed, half of the sample will remain unchangedAfter the second half life of a sample has passed, half of the half decaysleaving only a of the original sample unchangedAfter 3rd half-life has passed, half of the remains (or 1/8) and so on and so forth

  • Half-Life is a Measure of How Quickly a Substance DecaysDifferent radioactive isotopes have different half-livesHalf-lives can be as small as nanoseconds to billions of years (like Uranium-238)The length of the half-life depends on the stability of the nucleusThe more stable the nucleus is, the longer the half-life

  • Use of Half-LifeIf you know how much of a particular radioactive isotope has present at the start, we can predict how old the object isGeologists know the half-life of long-lasting isotopes, like potassium-40They use the half-lives of these isotopes to calculate the age of rocksPotassium-40 decays into argon-40So the more argon-40 there is compared to the potassium-40, the older the rock is

  • Archaeologists us the half-life of carbon-14 to date more recent materialsRemains of animal or fibers from recent clothingCan only be used to date once-living thingsThe ratio of carbon-14 (radioactive) to carbon-12 (stable) decreases with timeSo the more carbon-14 to carbon-12 there is, the newer the once-living organism is

  • Nuclear ChangesSection 2 Nuclear Fission and Fusion

  • VocabularyStrong nuclear forceFissionNuclear chain reactionCritical massFusion

  • Nuclear ForcesProtons and neutrons are tightly packed into the tiny nucleus of an atomCertain nuclei are unstable, and undergo decayElements can have stable and unstable isotopesCarbon-12 is stable while carbon-14 is notThe stability of an nucleus depends on the nuclear forces holding the nucleus together.These forces act between the protons and the neutrons

  • Nuclei are held together by a special forceWe know that like charges repelTook scientists a while to determine how so many positively charged protons fit into an atomic nucleus without flying apartAnswer is the strong nuclear forceThe strong nuclear force is the interaction that binds protons and neutrons together in a nucleusThis attraction is MUCH stronger than the repulsion between the protonsBut force only occurs over very short distances (about the width of 3 protons)

  • Neutrons contribute to nuclear stabilityDue to the strong nuclear force, neutrons and protons in a nucleus attract other protons and neutronsBecause neutrons have no charge, they dont repel anythingWhereas the protons, having charge, repel other protonsIn a stable nuclei, the attractive forces are stronger than the repulsive forces

  • Too many neutrons or protonsWhile neutrons help hold a nucleus together, too many neutrons makes a nucleus unstableNuclei with more than 83 protons are always unstable, no matter how many neutrons they haveThese nuclei will always decayWill always release large amounts of energy and nuclear radiation

  • Nuclear FissionThe process of the production of lighter nuclei from heavier nuclei is called fissionDuring fission, a nucleus splits into two or more smaller nuclei, releasing neutrons and energy

    Note that this fission reaction was started by firing 1 neutron at the Uranium-235 nucleusAlso note we produced more neutrons, energy, and 2 different, lighter nuclei

  • Energy is released during nuclear fissionEach dividing nucleus releases about 3.2x10-11 J of energyCompare this with TNT, which releases only 4.8x10-18J per moleculeDuring fission reactions, a small amount of mass is lost after the reactionThat mass was converted into energyE = mc2

  • Neutrons released by fission can start a chain reactionA nucleus that splits when it is struck by a neutron forms smaller nucleiSmaller nuclei need fewer neutrons, so excess neutrons are emittedExcess neutrons can collide with another large nucleusTriggering another nuclear reactionWhich releases more neutrons, and so on and so on

  • Generally each nucleus split will generate about 3 neutronsSo each nucleus that fissions will cause 3 other nuclei to fissionThis generates a nuclear chain reactionA series of fission processes in which the neutrons emitted by a dividing nucleus cause the division of other nuclei.The more nuclei released, the bigger the chain reaction will be

  • Nuclear Fission of Uranium-235

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