Great Ideas in Science:
Lecture 7 – Nuclear Reactions
Professor Robert Hazen
UNIV 301
Great Idea: Nuclear energy arises from
the conversion of mass into energy.
Nuclear Reactions
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Key Idea: Nuclear reactions result
from the rearrangement of an atom’s
protons and neutrons (i.e. the nucleus)
Key Words:
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Proton
Neutron
Nucleus
Isotope
Radioactivity
Nuclear Fission
Nuclear Fusion
The Building Blocks
of Matter
Of what is matter made?
• Atoms
• Nuclei and electrons
• Quarks
Key Words About Atoms
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Atom: Any object with a nucleus
and electrons
Element: An atom with a known
number of protons (the atomic
number)
Ion: An electronically-charged
atom with a different number of
protons (+) and electrons (-)
Isotope: An element with a
known number of neutrons
The Structure of the Atom
Electrons in shells (energy levels)
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Negatively charged
Shift during chemical reactions
The Structure of the Atom
Electrons in shells (energy levels)
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Negatively charged
Shift during chemical reactions
Central dense nucleus
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Composed of protons and neutrons
Positively charged
Nucleus - Stays put in chemical reactions
Isotopes: Hydrogen & Carbon
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H-1 – 1 proton
H-2 – 1 p & 1 neutron (Deuterium)
H-3 – 1 p & 2 n (Tritium)
C-12 – 6p & 6n
C-13 – 6p & 7n
C-14 – 6p & 8n (radioactive)
For any given element the number
of protons is fixed
Four Fates of Isotopes
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An isotope may be stable
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An isotope may be radioactive
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An isotope may be split apart
by fission
An isotope may combine with
another by fusion
Chart of the Isotopes (Z vs. N)
Stable Isotopes
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99.999+% of all the atoms
around us
Examples are carbon-12 and
carbon-13
Different isotopes don’t affect
chemical reactions.
Used in scientific research to
track chemical reactions (2 ways)
• As tracers
• Fractionation
Radioactivity or Radioactive
Decay (three kinds)
The spontaneous emission of an
energetic particle by a nucleus
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Alpha radiation
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Beta radiation
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Gamma radiation
Most Kinds of Isotopes
are Radioactive
STABLE
RADIOACTIVE
Alpha Radiation
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Atom spontaneously loses 2 protons and
2 neutrons (= a Helium-4 nucleus)
Alpha Radiation
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Atom spontaneously loses 2 protons and
2 neutrons (= a Helium-4 nucleus)
Uranium-238  Thorium-234 + 2n + 2p
Beta Radiation
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One neutron spontaneously becomes
a proton plus an electron
Thorium-234  Proactinium-234
Gamma Radiation
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Atom spontaneously emits a gamma
ray (electromagnetic radiation)
Uranium-238*  Uranium-238 + γ
Gamma Radiation
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Atom spontaneously emits a gamma
ray (electromagnetic radiation)
Uranium-238*  Thorium 234 + γ
SUMMARY: The Three Kinds
of Radioactive Decay
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Alpha Decay
• Release of α particle with 2
protons and 2 neutrons
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Beta Decay
• Neutron becomes a proton
• Emission of electron (β-ray)
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Gamma Radiation
• Electromagnetic radiation
Radioactivity and Health
Ionization
• Stripping off
electrons
Long-term effects
• Cancer
• Birth defects
Half-Life
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The average time for decay of ½
batch of radioactive isotopes
Wide range of half-lives
Radiometric Dating
1. Know half-life of isotope
2. Know how much was there
3. Measure what’s left
Carbon-14: Half-life = 5730 years
Radiometric Dating
Applications to geology
• Need longer half-lives
• Uranium, potassium
Radioactive Decay Chain (radon)
Radioactive Decay Chain (radon)
Radioactive Decay Chain (radon)
Radioactive Decay Chain (radon)
Radioactive Decay Chain (radon)
Radioactive Decay Chain (radon)
Radioactive Decay Chain (radon)
Radioactive Decay Chain (radon)
Radioactive Decay Chain (radon)
Radioactive Decay Chain (radon)
Radioactive Decay Chain (radon)
Radioactive Decay Chain (radon)
Radioactive Decay Chain (radon)
Four Fates of Isotopes
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An isotope may be stable
An isotope may be radioactive
An isotope may be split apart
by fission
An isotope may combine with
another by fusion
Nuclear Fission (Splitting)
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Fission = Splitting
of nucleus
A nuclear reactor
converts mass to
energy
Nuclear Fission (Splitting)
Nuclear Fission – The Atom Bomb
Hiroshima – August 6, 1945
Nuclear Fission – The Atom Bomb
Yucca Mountain, Nevada
(NIMBY)
Yucca Mountain, Nevada
(NIMBY)
Four Fates of Isotopes
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An isotope may be stable
An isotope may be radioactive
An isotope may be split apart
by fission
An isotope may combine with
another by fusion
Nuclear Fusion (Fusing)
• Hydrogen atoms
combine to form helium
• Some mass is converted
into energy
Nuclear Fusion – Hydrogen Bomb
Nuclear Fusion – Hydrogen Bomb
Stars are Giant Fusion Reactors
http://www.earth.northwestern.edu/people/seth/107/Solar/Image12.gif
Fates of Stars
Benefits of Isotopes
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Stable Isotopes
• Medical Research
• Environmental Tracers
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Radioactive Isotopes
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Medical diagnosis
Cancer treatments
Environmental tracers
Age Determination
Nuclear fission
• Power generation
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Nuclear Fusion
• The Sun