Chapter 39 The Atomic Nucleus and Radioactivity

1. Chapter 39The 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 (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

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

5. Beta emissions • b- particle- electron (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+ • Conservation of charges • (+) charged particle

6. 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

7. 39.4 Radioactive Isotopes • Atomic number- equal to the number of p+ • Carbon, 12p+ • Atomic mass number- equal to the number of nucleons • Carbon-24 (99% of all carbon) • Hydrogen-1 • Deuterium-2 (1n0 & 1p+) stable • Tritium- 3 (2n0 & 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 n0 through nuclear reaction • Number of p+ must be constant • Isotope number (upper number) is (n0+ p+) • 235U (U-235) is 92p+ + 143n0

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 ½2) • At 4000 years, ½ of ½ of ½ remains (12.5% or ½3)

10. 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

11. 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

12. 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

13. 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

14. 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

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 1: Percent error • (computed-measured)/measured x 100 = %