chapter 21 nuclear chemistry
TRANSCRIPT
Nuclear ChemistryChapter 21
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Radioactivity
• _________________—unstable atomic nuclei spontaneously emit particles, electromagnetic radiation (EMR), or both
• ________________________—results from bombarding nuclei with neutrons, protons, or other nuclei
XAZ
Mass Number
Atomic NumberElement Symbol
______________ (Z) = number of protons in nucleus
______________ (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
A
Z
1p11H1or
proton1n0
neutron0e-1
0-1or
electron0e+1
0+1or
positron4He2
42or
particle
21.1
What is the difference between and
21.1p.673
0e+10+1
0e+1
0+1
represents an electron in or from an atomic orbital
represents an electron that is physically identical to an electron in or from an atomic orbital, but this electron comes from the decay of a neutron to a proton and an electron—it is also called a beta particle or ray
21.1
Comparison of Chemical Reactions and Nuclear Reactions
Balancing Nuclear Equations
1. Conserve mass number (A).
The sum of protons plus neutrons in the products must equal the sum of protons plus neutrons in the reactants.
1n0U23592 + Cs138
55 Rb9637
1n0+ + 2
235 + 1 = 138 + 96 + 2x1
2. Conserve atomic number (Z) or nuclear charge.
The sum of nuclear charges in the products must equal the sum of nuclear charges in the reactants.
1n0U23592 + Cs138
55 Rb9637
1n0+ + 2
92 + 0 = 55 + 37 + 2x021.1
212Po decays by alpha emission. Write the balanced nuclear equation for the decay of 212Po.
4He242oralpha particle -
212Po 4He + AX84 2 Z
212 = 4 + A A = 208
84 = 2 + Z Z = 82
212Po 4He + 208Pb84 2 82
21.1Ex 21.1, p.673
Nuclear Stability and Radioactive Decay
Beta decay
14C 14N + 0 + 6 7 -1
40K 40Ca + 0 + 19 20 -1
1n 1p + 0 + 0 -1
________ # of ________ by 1
________ # of ________ by 1
Positron decay
11C 11B + 0 + 6 5 +1
38K 38Ar + 0 + 19 18 +1
1p 1n + 0 + 1 0 +1
________ # of ________ by 1
________ # of ________ by 1
and have A = 0 and Z = 021.2
1
Electron capture decay
________ # of ________ by 1
________ # of ________ by 1
Nuclear Stability and Radioactive Decay
37Ar + 0e 37Cl + 18 17-1
55Fe + 0e 55Mn + 26 25-1
1p + 0e 1n + 1 0-1
Alpha decay
________ # of ________ by 2
________ # of ________ by 2212Po 4He + 208Pb84 2 82
Spontaneous fission
252Cf 2125In + 21n98 49 021.2
Nuclear Stability
• Certain numbers of neutrons and protons are extra stable
• n or p = 2, 8, 20, 50, 82 and 126
• Like extra stable numbers of electrons in noble gases (e = 2, 10, 18, 36, 54 and 86)
• Nuclei with even numbers of both protons and neutrons are more stable than those with odd numbers of neutron and protons
• All isotopes of the elements with atomic numbers higher than 83 are radioactive
• All isotopes of Tc and Pm are radioactive
21.2
Number of Stable Isotopes with Even and Odd Numbers of Protons and Neutrons
n/p too large
beta decay
X
n/p too small
positron decay or electron capture
Y
21.2p.675
______________________ is the energy required to break up a nucleus into its component protons and neutrons.
BE is an indication of the stability of a nucleus.
In order to compare nuclei of two different isotopes/elements, we must take into account the fact that they have different numbers of _________.
For this reason, nuclear binding energy per nucleon is more useful.
21.2p.676
Mass Defect
The difference betweenthe ________________ of an atom
andthe ________________ of the masses of
protons, neutrons, and electrons
What does the mass defect tell us? How much mass was changed to energy in the formation of the atom.
p.676
The Law of Charges tells us…?
So how can all those positively charged protons be crammed into the tiny space of the nucleus?
We call it the “strong nuclear force” or just the “strong force.”
Some of the mass of the nucleons is converted to energy and lost.
This is the general idea behind fusion:
Build new, larger nuclei and release great amounts of energy!
Nuclear binding energy per nucleon vs Mass number
nuclear binding energynucleon nuclear stability
21.2p.678
note
Which element has the greatest net attractive forces among its nucleons? (graph)
Radioactivity: unstable nuclei spontaneously emit particles, electromagnetic radiation (EMR), or both
Main types of radioactivity: particles (He2+) particles (e-) rays (short-wavelength emr)
•positron emission
•electron capture
Often it involves a multi-step sequence, a series…. and all obey first-order kinetics.
Kinetics of Radioactive Decay
N daughter
rate = -Nt
rate = N
Nt
= N-
N = N0exp(-t) lnN = lnN0 - t
N = the number of atoms at time t
N0 = the number of atoms at time t = 0
is the decay or rate constant
ln2=
t½
21.3
The Uranium Decay Series
Kinetics of Radioactive Decay
[N] = [N]0exp(-t) ln[N] = ln[N]0 - t
[N]
ln [
N]
21.3
is the first order rate constant and N is the number of radioactive nuclei present at time t
p.679f
Radiometric Assumptions
The method measures the parent/daughter ratio of the elements.
1. The system must initially contain none of ____________________________________.
2. The decay rate must _______________________.
3. The amounts of the parent element and the daughter products must be affected by ___________________________________.
Radiocarbon Dating
14N + 1n 14C + 1H7 160
14C 14N + 0 + 6 7 -1t½ = 5730 years
Uranium-238 Dating
238U 206Pb + 4 + 6 092 -182 2
t½ = 4.51 x 109 years
21.3
p.681f
Potassium-40 Dating
t½ = 1.2 x 109 years
40K + 0e 40Ar 19 18-1
Nuclear Transmutation
Cyclotron Particle Accelerator
14N + 4 17O + 1p7 2 8 1
27Al + 4 30P + 1n13 2 15 0
14N + 1p 11C + 47 1 6 2
21.4p.683
Nuclear Transmutation
21.4
The Transuranium Elements
Nuclear Fission
21.5
235U + 1n 90Sr + 143Xe + 31n + Energy92 54380 0
Energy = [mass 235U + mass n – (mass 90Sr + mass 143Xe + 3 x mass n )] x c2
Energy = 3.3 x 10-11J per 235U
= 2.0 x 1013 J per mole 235U
Combustion of 1 ton of coal = 5 x 107 J
p.685f
Nuclear Fission
21.5
235U + 1n 90Sr + 143Xe + 31n + Energy92 54380 0
Representative fission reaction
p.686
Nuclear Fission
21.5
A ________________________ is a self-sustaining sequence of nuclear fission reactions.
The minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction is the ________________________.
Non-critical
Critical
Nuclear Fission
21.5
Schematic diagram of a
nuclear fission reactor
Annual Waste Production
21.5
35,000 tons SO2
4.5 x 106 tons CO2
1,000 MW coal-firedpower plant
3.5 x 106
ft3 ash
1,000 MW nuclearpower plant
70 ft3 vitrified waste
Nuclear Fission
21.5
Nuclear Fission
Hazards of the radioactivities in spent
fuel compared to uranium ore
From “Science, Society and America’s Nuclear Waste,” DOE/RW-0361 TG
21.6
Nuclear Fusion
2H + 2H 3H + 1H1 1 1 1
Fusion Reaction Energy Released
2H + 3H 4He + 1n1 1 2 0
6Li + 2H 2 4He3 1 2
6.3 x 10-13 J
2.8 x 10-12 J
3.6 x 10-12 J
Tokamak magnetic plasma
confinement
21.6
Radioisotopes in Medicine• 1 out of every 3 hospital patients will undergo a nuclear
medicine procedure
• 24Na, t½ = 14.8 hr, emitter, _______________________
• 131I, t½ = 14.8 hr, emitter, ________________________
• 123I, t½ = 13.3 hr, ray emitter, _____________________
• 18F, t½ = 1.8 hr, emitter, ________________________
• 99mTc, t½ = 6 hr, ray emitter, _____________________
Brain images with 123I-labeled compound
21.6
Biological Effects of RadiationRadiation absorbed dose (rad)
1 rad = 1 x 10-5 J/g of material
Roentgen equivalent for man (rem)
1 rem = 1 rad x Q Quality Factor-ray = 1
= 1 = 20
Average Yearly Radiation Doses for Americans
Biological Effects of RadiationFormation of _______________ and/or ________________ that attack membranes, enzymes, or DNA.
Damage can be ________________ or _______________.
p.695f