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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 (n0) & 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**

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 (a) particle- helium nucleus**

2p+ with 2n0 ejected from nucleus Positively charged particle Gamma (g) ray- electromagnetic radiation

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**Beta emissions b- particle- electron (e-) b+ particle- positron (e+)**

With extra n0’s, 1n0 is transformed into 1p+ + 1e- Conservation of charges (-) charged particle b+ particle- positron (e+) With extra p+’s, 1p+ is transformed into 1n0 + 1e+ (+) charged particle

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**39.3 Radiation Penetrating Power**

g ray- penetrate the most No charge or mass to slow them down Need a very high density substance to block them b particle- penetrates slightly Loses energy with a small number of collisions Thin sheets of metal can block them a 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 (1n0 & 1p+) stable Tritium- 3 (2n0 & 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 n0 through nuclear reaction Number of p+ must be constant Isotope number (upper number) is (n0 + p+) 235U (U-235) is 92p n0

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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 ½2) At 3000 years, ½ of ½ of ½ remains (12.5% or ½3)

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**39.6 Natural Transmutation of Elements**

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

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**Calculating a-decay Reactions**

a-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 234Pa __ + 4He Number of nucleons must be equal 234=x + 4 Number of p+ must be equal 91=Z + 2 Missing element must be 230Ac

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**Calculating b--decay Reactions**

b--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 216Po __ + 0e- Number of nucleons must be equal 216=x + 0 Number of p+ must be equal 84=Z + -1 Missing element must be 216At

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**Calculating g-decay Reactions**

g-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 60Co __ + 0g Number of nucleons must be equal 60=x + 0 Number of p+ must be equal 27=Z + 0 Missing element must be 60Co

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**39.7 Artificial Transmutation of Elements**

Elements can be bombarded to change into other elements 14N + 4He 17O + 1H 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 Computed diameter Procedure Step 1**

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 Procedure Step 1 Data Table A**

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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