no slide title · atoms are composed of protons neutrons electrons + positively charged mass =...
TRANSCRIPT
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Nuclear Chemistry
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Atoms are composed of
PROTONS
NEUTRONS
ELECTRONS
positively charged mass = 1.6726 x 10–27 kg+
neutral mass = 1.6750 x 10–27 kg
negatively charged–• mass = 9.1096 x 10–31 kg
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the Nucleus
+
+
made up of protons and neutrons
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Nuclear Force
+
+
the neutrons within the nucleus act as sort of glue countering the electrostatic repulsion between the protons
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when an unstable nuclei increases its stability by altering its number of neutrons and protons
Radioactivity
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Types of Nuclear Decay
Alpha emission ( α)42
0 -1Beta emission ( β )
Electron capture ( e) 0 -1
Positron emission ( β ) 0 +1
Gamma emission ( γ)00
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Alpha emission ( α)42
the nucleus emits a helium nuclei ( 2 protons and 2 neutrons
α42 = 4
2 He
Po Pb +210 20684 82
42 He
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Beta emission ( β )the nucleus changes a neutron into a proton by emitting an electron
0 -1
β 0 -1
10
n 1 1
p+
C +146
147 Nβ 0
-1
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Positron emission ( β )the nucleus changes a proton into a neutron by emitting a positron
0 +1
β 0 +1
11
p 1 0
n+
B +85
84 Beβ 0
+1
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Electron capture ( e )the nucleus captures an electron and changes a proton into a neutron
0
β 0 -1
11
p 1 0
n+
-1
C116
11
5 Bβ 0 -1 +
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electromagnetic radiation emitted during nuclear decay
Gamma emission ( γ )00
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Po Pb +210 20684 82 X
Cs Ba +137 13755 56
Na Ba +20 2011 10
X
X
balance the following nuclear equations
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Po Pb +210 20684 82
42 He
Cs Ba +137 13755 56
Na Ba +20 2011 10
X
X
balance the following nuclear equations
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Po Pb +210 20684 82
42 He
Cs Ba +137 13755 56
Na Ne +20 2011 10 X
0-1β
balance the following nuclear equations
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Po Pb +210 20684 82
42 He
Cs Ba +137 13755 56
Na Ne +20 2011 10
0-1β
0+1β
balance the following nuclear equations
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Nuclear Stability
the principle factor in determining whether a nucleus is stable is the neutron-to-proton ratio
as the mass number increases, the neutron-to-proton ratios become greater than one
for elements of low atomic number the value is close to one
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nuclei that contain 2, 8, 20, 50, 82, and 126 protons are generally more stable (magic numbers)
nuclei with even numbers of both protons and neutrons are generally more stable than odd numbers
Nuclear Stability
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If an isotope’s mass number is greater than its atomic weight, beta emission is expected
If an isotope’s mass number is less than its atomic weight, positron emission or electron capture is expected
All elements having an atomic number greater than 83 are radioactive. Alpha particles are emitted by most of these isotopes.
Nuclear Stability
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a decay series
when a radioactive nucleus disintegrates, the products formed may also be unstable and under go further disintegration’s until a stable product is formed
U Pb238 20692 82
involves 14 steps
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U238
92Pb
206
82
+U23892 Th
23490
42 He
+Th23490 Pa
23491
0-1 β
+Pa23491 U
23492
0-1 β
+U23492 Th
23090
42 He
+Th23090 Ra
22688
42 He
+Ra22688 Rn
22286
42 He
+Rn22286 Po
21884
42 He
+Rn22286 Po
21884
42 He
+Po21884 Pb
21482
42 He
+Pb21482 Bi
21483
0-1 β
+Bi21483 Po
21484
0-1 β
+Po21484 Pb
21082
42 He
+Pb21082 Bi
21083
0-1 β
+Bi21083 Po
21084
0-1 β
+Po21084 Pb
20682
42 He
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all radioactive decays obey first-order kinetics
Kinetics of Radioactive Decay
Rate of decay at time t
= Nk
k = rate constant
the number of radioactive nuclei present at time t
N =
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Time ( s )The plot shows the decay of uranium-238 to thorium-234
First-order rate plot
U23892 U Th +
238 23492 90
42 He
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First-Order rate law Integrated
the integrated form of the rate law is:
t1/2 = k
0.693= kt
N0
Nt
ln
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Integrated rate law
is an equation for a straight line
Plot ln Nt versus t
ln Nt = -kt + ln N0
y = mx + b
Slope = -k
y intercept is ln N0
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Half-life
the time for the concentration of a reactant to decrease to one-half of its initial concentration
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Time ( s )0
U23892
t 1/2 = 4.51 x 107yr
U Th +238 23492 90
42 He
Half-life
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Radiocarbon Dating
Willard Libby (Nobel Prize, 1960)
Carbon-14
Natural abundance: 1 part in 1012
β − emitter
Half-life = 5730 yearsused to date archeological artifacts younger than 30,000 years
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- +
14 C614 N7
0 e-1 +
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Example
The C-14 decay rate of wood obtained from a live tree is 0.260 disintegration per second per gram of sample A sample of wood from an archaeological site has C-14 decay rate of 0.186 disintegration per second per gram. How old is the sample?
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The C-14 decay rate of wood obtained from a live tree is 0.260 disintegration per second per gram of sample A sample of wood from an archaeological site has C-14 decay rate of 0.186 disintegration per second per gram. How old is the sample?
t 1/2 for 14C is known to be 5730 years
t1/2 = k
0.693
Therefore,k = (0.693/5730 yr)= 1.21 x 10-4 yr-1
ln[A]0
[A]= kt ln
260
186= (1.21 x 10-4 yr-1 ) t
t =2770 years
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Some representative half-lives
Tc-99 6 hours
C-14 5730 years
Sr-90 28.8 years
Mo-99 67 hours
K-40 1,300,000 years
U-238 45, 000,000 years