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Chapter 39 The Atomic Nucleus and Radioactivity Conceptual Physics Hewitt, 1999 Bloom High School

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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 Electrical forces- like repels like – p + repel other p + in the nucleus – Acts over a distance – Inverse-square law

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Neutrons & Nuclear Strong Forces Nuclear Strong force – Acts between nucleons – Only acts in close proximity Neutron- nucleon that is unstable when alone – More neutrons are needed for more protons

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39.2 Radioactive Decay Alpha ( ) particle- helium nucleus – 2p + with 2n 0 ejected from nucleus – Positively charged particle Gamma ( ) ray- electromagnetic radiation

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Beta emissions particle- electron (e - ) – With extra n 0 ’s, 1n 0 is transformed into 1p + + 1e - Conservation of charges – (-) charged particle particle- positron (e + ) – With extra p + ’s, 1p + is transformed into 1n 0 + 1e + Conservation of charges – (+) charged particle

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39.3 Radiation Penetrating Power ray- penetrate the most – No charge or mass to slow them down – Need a very high density substance to block them particle- penetrates slightly – Loses energy with a small number of collisions – Thin sheets of metal can block them particle- penetrates the least – Relatively slow and heavy – Paper and skin can stop them

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39.4 Radioactive Isotopes Atomic number- equal to the number of p + – Carbon, 6p + Atomic mass number- equal to the number of nucleons – Carbon-12 (99% of all carbon) – Hydrogen-1 Deuterium-2 (1n 0 & 1p + ) stable Tritium- 3 (2n 0 & 1p + ) radioactive

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Ions vs. Isotopes Ion- charged particle – Gain or loss of an e - through chemical reaction – Neutral atom has equal numbers of p + and e - Isotope- gain or loss of n 0 through nuclear reaction – Number of p + must be constant – Isotope number (upper number) is (n 0 + p + ) 235 U (U-235) is 92p + + 143n 0

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39.5 Radioactive Half-Life Half-Life- amount of time for half of the substance to change or decay – If half-life is 1000 years, 50% remains at 1000 years – At 2000 years, ½ of ½ remains (25% or ½ 2 ) – At 3000 years, ½ of ½ of ½ remains (12.5% or ½ 3 )

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39.6 Natural Transmutation of Elements Transmutation- changing one element into another Radioactive Decay- can change to another element – -decay- loss of 2p + Atomic number decreases by 2 Atomic mass decreases by 4 238 U 234 Th + 4 He – - -decay- gain of 1p + Atomic number increases by 1 Atomic mass unchanged 234 Th 234 Pa + 0 e - – -decay- no gain or loss of p + No change in atomic number or mass 60 Co 60 Co + 0

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Calculating -decay Reactions -decay- loss of 2p + – Atomic number decreases by 2 – Atomic mass decreases by 4 – The total mass and atomic number before must equal the total mass and atomic number after the reaction 234 Pa __ + 4 He – Number of nucleons must be equal 234=x + 4 – Number of p + must be equal 91=Z + 2 – Missing element must be 230 Ac

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Calculating - -decay Reactions - -decay- gain of 1p + – Atomic number increases by 1 – Atomic mass unchanged – The total mass and atomic number before must equal the total mass and atomic number after the reaction 216 Po __ + 0 e - – Number of nucleons must be equal 216=x + 0 – Number of p + must be equal 84=Z + -1 – Missing element must be 216 At

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Calculating -decay Reactions -decay- no gain or loss of p + – No change in atomic number or mass – The total mass and atomic number before must equal the total mass and atomic number after the reaction 60 Co __ + 0 – Number of nucleons must be equal 60=x + 0 – Number of p + must be equal 27=Z + 0 – Missing element must be 60 Co

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39.7 Artificial Transmutation of Elements Elements can be bombarded to change into other elements – 14 N + 4 He 17 O + 1 H Transuranic element- elements after Uranium – Half-life’s are relatively short, so they are not found in nature

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39.8 Carbon Dating C-12 very stable (99% of carbon is C-12) C-14 radioactive – Found in living tissue and is constantly replaced – Ratio of C-12 to C-14 fixed in living tissue – In dead tissue, C-14 is not replaced and decreases over time – Not found in non-living tissue (metals, rocks, etc.)

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39.9 Uranium Dating Uranium- decays in a predictable pattern – Lead is a decay product, so is found in all uranium samples

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39.10 Radioactive Tracers Radioactive tracer- radioactive isotope used to follow a path Used in agriculture to determine path of fertilizer and water Used in medicine to determine metabolic pathway of medicine or blood

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39.11 Radiation and You Radiation naturally occurs everywhere – Underground, it’s the cause of warmth Radiation more strong at high altitudes because there is less atmospheric shielding from cosmic radiation

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Lab 97: Nuclear Marbles Groups of 4 – 10 marbles – 3 metersticks Computed diameter – P=2N(R+r)/L – P=H/T – Replace (R+r) with “d,” combine formulae and solve for d Procedure Step 1 – 200 trials – DON’T AIM for the marbles, aim for 1cm, then 2cm, etc. and go back and forth until you have 200 trials

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Lab 98: Half-Life Groups of 4 – 50 pennies Procedure Step 1 – Keep the “heads” pennies out of the box! Data Table A – Number of Pennies removed it the TOTAL removed, not just from that trial Analysis 4: Percent error – (Analysis#2-theory)/theory x 100 = %

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