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 (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
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-) 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

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, 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

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 n0

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 3000 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 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

20. 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 = %