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, 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 is n0 + p+
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%)

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. 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

12. 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.)

13. 39.9 Uranium Dating Uranium- decays in a predictable pattern
Lead is a decay product, so is found in all uranium samples

14. 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

15. 39.11 Radiation and You Radiation naturally occurs everywhere
Radiation more strong at high altitudes because there is less atmospheric shielding from cosmic radiation