chapter 39 the atomic nucleus and radioactivity

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Chapter 39 The Atomic Nucleus and Radioactivity. Conceptual Physics Hewitt, 1999 Bloom High School. 39.1 The Atomic Nucleus. Nucleons- particles in the nucleus of an atom Neutrons (n 0 ) & Protons (p + ) Almost equal masses Neutrons are “glue” in a nucleus - PowerPoint PPT Presentation

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Chapter 39: The Atomic Nucleus and Radioactivity

Chapter 39The Atomic Nucleus and RadioactivityConceptual PhysicsHewitt, 1999Bloom High School39.1 The Atomic NucleusNucleons- particles in the nucleus of an atomNeutrons (n0) & Protons (p+)Almost equal massesNeutrons are glue in a nucleusElectrical forces- like repels likep+ repel other p+ in the nucleusActs over a distanceInverse-square law

Neutrons & Nuclear Strong ForcesNuclear Strong forceActs between nucleonsOnly acts in close proximityNeutron- nucleon that is unstable when aloneMore neutrons are needed for more protons

39.2 Radioactive DecayAlpha (a) particle- helium nucleus2p+ with 2n0 ejected from nucleusPositively charged particleGamma (g) ray- electromagnetic radiation

Beta emissionsb- particle- electron (e-)With extra n0s, 1n0 is transformed into 1p+ + 1e-Conservation of charges(-) charged particleb+ particle- positron (e+)With extra p+s, 1p+ is transformed into 1n0 + 1e+Conservation of charges(+) charged particle

39.3 Radiation Penetrating Powerg ray- penetrate the mostNo charge or mass to slow them downNeed a very high density substance to block themb particle- penetrates slightlyLoses energy with a small number of collisionsThin sheets of metal can block thema particle- penetrates the leastRelatively slow and heavyPaper and skin can stop them

39.4 Radioactive IsotopesAtomic number- equal to the number of p+Carbon, 12p+Atomic mass number- equal to the number of nucleonsCarbon-24 (99% of all carbon)Hydrogen-1Deuterium-2 (1n0 & 1p+) stableTritium- 3 (2n0 & 1p+) radioactive

Ions vs. IsotopesIon- charged particleGain or loss of an e- through chemical reactionNeutral atom has equal numbers of p+ and e-

Isotope- gain or loss of n0 through nuclear reactionNumber of p+ must be constantIsotope number (upper number) is (n0 + p+)235U (U-235) is 92p+ + 143n0

39.5 Radioactive Half-LifeHalf-Life- amount of time for half of the substance to change or decayIf half-life is 1000 years, 50% remains at 1000 yearsAt 2000 years, of remains (25% or 2)At 4000 years, of of remains (12.5% or 3)39.6 Natural Transmutation of ElementsTransmutation- changing one element into anotherRadioactive Decay- can change to another elementa-decay- loss of 2p+Atomic number decreases by 2Atomic mass decreases by 4238U 234Th + 4Heb--decay- gain of 1p +Atomic number increases by 1Atomic mass unchanged234Th 234Pa + 0e-g-decay- no gain or loss of p +No change in atomic number or mass60Co 60Co + 0gCalculating a-decay Reactionsa-decay- loss of 2p+Atomic number decreases by 2Atomic mass decreases by 4The total mass and atomic number before must equal the total mass and atomic number after the reaction234Pa __ + 4HeNumber of nucleons must be equal234=x + 4Number of p+ must be equal91=Z + 2Missing element must be 230AcCalculating b--decay Reactionsb--decay- gain of 1p +Atomic number increases by 1Atomic mass unchangedThe total mass and atomic number before must equal the total mass and atomic number after the reaction216Po __ + 0e-Number of nucleons must be equal216=x + 0Number of p+ must be equal84=Z + -1Missing element must be 216At

Calculating g-decay Reactionsg-decay- no gain or loss of p +No change in atomic number or massThe total mass and atomic number before must equal the total mass and atomic number after the reaction60Co __ + 0gNumber of nucleons must be equal60=x + 0Number of p+ must be equal27=Z + 0Missing element must be 60Co

39.7 Artificial Transmutation of ElementsElements can be bombarded to change into other elements14N + 4He 17O + 1HTransuranic element- elements after UraniumHalf-lifes are relatively short, so they are not found in nature39.8 Carbon DatingC-12 very stable (99% of carbon is C-12)C-14 radioactiveFound in living tissue and is constantly replacedRatio of C-12 to C-14 fixed in living tissueIn dead tissue, C-14 is not replaced and decreases over timeNot found in non-living tissue (metals, rocks, etc.)39.9 Uranium DatingUranium- decays in a predictable patternLead is a decay product, so is found in all uranium samples

39.10 Radioactive TracersRadioactive tracer- radioactive isotope used to follow a pathUsed in agriculture to determine path of fertilizer and waterUsed in medicine to determine metabolic pathway of medicine or blood

39.11 Radiation and YouRadiation naturally occurs everywhereUnderground, its the cause of warmthRadiation more strong at high altitudes because there is less atmospheric shielding from cosmic radiation

Lab 97: Nuclear MarblesGroups of 410 marbles3 metersticksComputed diameterP=2N(R+r)/LP=H/TReplace (R+r) with d, combine formulae and solve for dProcedure Step 1200 trialsDONT AIM for the marbles, aim for 1cm, then 2cm, etc. and go back and forth until you have 200 trialsLab 98: Half-LifeGroups of 450 penniesProcedure Step 1Keep the heads pennies out of the box!Data Table ANumber of Pennies removed it the TOTAL removed, not just from that trialAnalysis 1: Percent error(computed-measured)/measured x 100 = %

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