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

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Presentation on theme: "Chapter 39 The Atomic Nucleus and Radioactivity Conceptual Physics Hewitt, 1999 Bloom High School."— Presentation transcript:

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

2 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

3 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

4 39.2 Radioactive Decay Alpha (  ) particle- helium nucleus – 2p + with 2n 0 ejected from nucleus – Positively charged particle Gamma (  ) ray- electromagnetic radiation

5 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

6 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

7 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

8 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 n 0

9 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 )

10 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 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 

11 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

12 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

13 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

14 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

15 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.)

16 39.9 Uranium Dating Uranium- decays in a predictable pattern – Lead is a decay product, so is found in all uranium samples

17 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

18 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

19 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

20 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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