Chapter 21
Nuclear Reactions and Their Applications
Everyday chemical properties are governed by an atom’s number and arrangement of electrons.
Energies of nuclear reactions are millions of times greater than those of ordinary chemical reactions.
Isotopes & Nuclides
 atomic number, Z: # protons in nucleus
 mass number, A: # protons + neutrons in nucleus
 isotopes: atoms with the same atomic number but different mass number
 nuclide: a single type of nucleus
 nucleons: collective term for protons & neutrons
Radioactive Decay



is a first-order kinetic process
Most known nuclides undergo radioactive decay. (Relatively few are stable.)
Ratio of neutrons to protons determines a nuclide’s stability.

Stable nuclides stop at Z = 83 (bismuth). All elements with Z ≥ 84 are radioactive (whether
naturally occurring or synthetic).
Unstable nuclide will decay to eventually produce a stable nuclide….could take one step or
many.

Big Question 3
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Types of Radioactive Decay
1.
alpha () decay

2.
4.
5.
example:
Th 
10 e 
234
91
234
90
Pa
0
1
example:
38
19
K
10 e  38
18 Ar
+ particle = 10 e (a positive electron)
net effect: convert proton to neutron
+ particles likely to be produced by nuclides with low neutron-to-proton ratios
+ is “antiparticle” of -
electron capture
example: 80 Hg 
 an inner-orbital electron captured by nucleus
 generally slow
194
gamma () decay



6.
234
90
 particle = e (an electron)
energy released in decay process creates the – particle (not from an orbital)
net effect: convert neutron to proton
– particles likely to be produced by nuclides with high neutron-to-proton ratios
–
positron (+) decay




Th  2 00
U
42 He 
238
92
 particle = 42 He nucleus (i.e., 42 He 2+)
beta (–) decay




3.
example:
example:
60
27
0
1
0
e 
 194
79 Au  0 
Co 
 60
28 Ni 
0
1
e  2 00 
 = high energy photon (electromagnetic radiation)
frequently accompanies other decay types
a way of “draining off” excess energy (product may be in excited state)
0
0
 54 Xe  44 Ru  4
spontaneous fission
example: 98 Cf 
 generally slow
 “splitting” of heavy nuclide to lighter ones with similar mass numbers
Big Question 3
252
10
140
108
1
0
n
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Assessing Potential Biological Damage from Radiation
Radioactive nuclides are sources of high-energy particles and/or photons. This radiation can break
chemical bonds and ionize molecules. In order to assess the potential for biological damage from a
particular source of radiation, the following characteristics need to be considered.
1. Energy of radiation
 results from kinetic energy of particles and dosage
 measured in “rads” (“radiation absorbed dose”)
 1 rad = 1 × 10–2 J deposited per kg tissue
2. Penetrating ability
 results from particle charge
 uncharged particles (e.g., γ rays and neutrons) have greater penetrating ability than charged
particles (e.g., α, β– and β+)
 also affected by the kinetic energies of the particles
 low energy α stopped by dead layer of skin (external exposure)
 low energy β– and β+ penetrate about 1 cm (external exposure)
 γ rays and neutrons extremely penetrating
3. Ionizing ability
 results from particle mass
 α > neutron > β– ≈ β+ > γ
4. Chemical properties
 results from periodic trends
 damage from an ingested/inhaled radioactive nuclide depends on its residence time
e.g., strontium-90 versus krypton-85 (both β– emitters); strontium-90 would be expected to
have a much longer residence time in the body because it is chemically similar to calcium
(krypton-85 is an inert gas)
5. External vs. internal exposure
Half-life, t½
time required for half the sample to react
Recall: first-order half-life does not depend on the initial quantity.
ln N = –kt + ln No
Radioactive decay is a statistical process. ¡¡It’s impossible to determine the half-life from a very small
sample of radioactive atoms!!
The Dating Game
Carbon-14 has a half-life of 5730 years. How old is a sample of tree bark containing 67.9% of the 14C
activity of living bark?
Big Question 3
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