ch 19 radiactivity and nuclear chemistry · •the rate of change in the amount of radioactivity is...
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Ch 19 Radiactivity and Nuclear Chemistry
Modified by Dr. Cheng-Yu Lai
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© 2014 Pearson Education, Inc.
What Is Radioactivity?
• Radioactivity is the release of tiny, high-
energy particles or gamma rays from
an atom.
• Particles are ejected from the nucleus.
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© 2014 Pearson Education, Inc.
Nuclear Symbols
Element symbol
Mass number, A
(p+ + no)
Atomic number, Z
(number of p+)
U235
92
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Radioactive nuclei spontaneously decompose (decay) with the evolution of energy
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Nuclear Reactions vs. Chemical Reactions
• In a chemical reaction
– Only the outer electron configuration of atoms and molecules changes
– There is no change to the nucleus
• In a nuclear reaction
– Mass numbers may change
– Atomic numbers may change
• One element may be converted to another
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• In the nuclear equation, mass numbers
and atomic numbers are conserved.
• We can use this fact to determine the
identity of a daughter nuclide if we know
the parent and mode of decay.
Nuclear Equations
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Nuclear Equations
• We describe nuclear processes with nuclear
equations.
• Atomic numbers and mass numbers are
conserved. – The sum of the atomic numbers on both sides must
be equal.
– The sum of the mass numbers on both sides must
be equal.
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Balancing Nuclear Equations
235 1 142 91 1
92 0 56 36 03U n Ba Kr n
Areactants = Aproducts
Zreactants = Zproducts
235 + 1 = 142 + 91 + 3(1)
92 + 0 = 56 + 36 + 3(0)
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Balancing Nuclear Equations
226 4
88 2Ra
226 = 4 + ____ 222
222
88 = 2 + ___ 86
86
Atomic number 86 is radon, Rn
Rn
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Five Modes of Radioactive Decay
• five modes of radioactive decay
1. Alpha (α) particle emission
Mass number is 4, charge is +2, atomic number 2
Symbol is or
When a nucleus emits an alpha particle, its mass number decreases by 4 and its atomic number decreases by 2
He4
2 4
2
ThHeU 234
90
4
2
238
92
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2. Beta (β) particle emission
3. Gamma (γ) radiation emission
Five Modes of Radioactive Decay
PaTh 234
91
0
1
234
90 e
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Five Modes of Radioactive Decay 4. Positron emission
Positron emission: Positrons are the anti-particle of the electron
5. K-electron capture Electron capture: (inner-orbital electron is captured by the nucleus)
e0
1
Positron emission converts a proton to a neutron
NeeNa 22
10
0
1
22
11
Electron capture converts a proton to a neutron
0
0
201
79
0
1
201
80 AueHg
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Important Atomic Symbols
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e0
1 He4
2e0
1
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© 2014 Pearson Education, Inc.
• The rate of change in the amount of radioactivity is constant, and is different for each radioactive “isotope.” Change in radioactivity measured with Geiger counter
Counts per minute
• Each radionuclide had a particular length of time it required to lose half its radioactivity—a constant half-life. We know that processes with a constant half-life follow
first order kinetic rate laws.
• The rate of radioactive change was not affected by temperature. In other words, radioactivity is not a chemical reaction!
Rate of Radioactive Decay
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Decay Kinetics Decay occurs by first order kinetics (the rate of decay is proportional to the number of nuclides present)
0
lnN
ktN
N = number of nuclides remaining at time t
N0 = number of nuclides present initially
k = rate constant
t = elapsed time
Calculating Half-life
1/ 2
ln(2) 0.693t
k k
t1/2 = Half-life (units dependent on rate constant, k)
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Sample Half-Lives
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Decay Kinetics
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Nonradioactive Nuclear Changes
• Fission
– The large nucleus splits into two smaller nuclei.
• Fusion
– Small nuclei can be accelerated to smash together to make a larger nucleus.
• Both fission and fusion release enormous amounts of energy. Fusion releases more energy per
gram than fission.
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Energy and Mass Nuclear changes occur with small but measurable losses of mass. The lost mass is called the mass defect, and is converted to energy according to Einstein’s equation:
DE = Dmc2
Dm = mass defect
DE = change in energy
c = speed of light
Because c2 is so large, even small amounts of mass are converted to enormous amount of energy.
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Nuclear Fusion and Stars
The Sun generates its energy by nuclear fusion of hydrogen nuclei into helium.
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Nuclear Fission and Fusion
Fusion: Combining two light nuclei to form a heavier, more stable nucleus.
Fission: Splitting a heavy nucleus into two nuclei with smaller mass numbers.
3 1 4 0
2 1 2 1He H He e
1 235 142 91 1
0 92 56 36 03n Ba Kr n U
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Fission Bomb Design
Fat Man
Little Boy
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© 2014 Pearson Education, Inc. Chemistry: A Molecular Approach, 3rd Edition
Nivaldo J. Tro
Example 19.4 Radioactive Decay Kinetics
You are given the initial mass of Pu-236 in a sample and asked to find the mass after 5.00 years.
Sort
Plutonium-236 is an alpha emitter with a half-life of 2.86 years. If a sample initially contains 1.35 mg of
Pu-236, what mass of Pu-236 is present after 5.00 years?
Given: mPu-236(initial) = 1.35 mg;
t = 5.00 yr; t1/2 = 2.86 yr
Find: mPu-236 (final)