Transcript CH2ch19_1

Chapter 19 Nuclear Chemistry
I.
Properties of the Nucleus
A.
Chemist’s View:
1) Seat of positive charge and mass in atoms and molecules
2) Not very important to chemical reactivity; valence electrons are key
B.
Nuclear Characteristics
1) Very small size: about 1 x 10-13 cm (Whole atom = 1 x 10-8 cm)
2) Very high density: 1.6 x 1014 g/cm3
3) Very high energy processes (106 time greater than typical chemical reactions)
4) Components = “Nucleons”
a) Protons = +1 charge, 1 mass unit (Atomic Number = Z = # of protons)
b) Neutrons = 0 charge, 1 mass unit
c) Mass Number = A = sum of neutrons + protons
d) Isotopes = same atomic number but different mass numbers (#’s of neutrons)
e) Nuclide = a particular isotope
A
Z
X  126 C, 136C,
14
6
C
II. Nuclear Stability and Radioactive Decay
A.
B.
Thermodynamic Stability = potential energy of the nucleus compared to separate parts
Kinetic Stability = Probability that the nucleus will undergo Radioactive Decay
14
0
1) Example: 14
6
2)
3)
Both A and Z must be conserved (must be the same on both sides of equation)
Zone of Stability
a) All nuclides with Z > 84 unstable
b) (A-Z):Z ratio = 1 stable if light
12
6
c)
C
(A-Z):Z ratio > 1 stable if heavy
202
80
d)
Calcium-40
is“Doubly
Magic”
C 7 N  -1 e
Hg
Magic Numbers:
i. Z = even, (A-Z) = even stable
ii. Z = odd, (A-Z) = odd unstable
iii. Proton or Neutron numbers of
2, 8, 20, 28, 50, 82, 126 very stable
C. Types of Radioactive Decay
1) Decay involving the change in mass number of the nucleus
a) a-particle production: loss of a helium nucleus; very common
238
92
U 
Th 
230
90
b)
226
88
Ra  42 He
Spontaneous Fission: splitting of a heavy nuclide into about equal parts; rare
254
98
2)
Th  42 He
234
90
Cf  lighter nuclides  neutrons
Decay when mass number stays the same
a) b-particle production: loss of an electron
i. Fairly common for nuclides where Neutrons:Protons > 1.0
ii. Nucleus doesn’t contain electrons; loss of energy that becomes electron
iii. Net effect: changes a neutron to a proton (Z increases by +1)
Th 
234
90
131
53
I 
Pa 
234
91
Xe 
131
54
0
-1
0
-1
e
e
b)
g-ray production: loss of a high energy photon
i. Can accompany other decay types
ii. Way for nucleus in an excited state to return to ground state
238
92
c)
U 
Th  42 He  2 00g
234
90
Positron production: loss of mass of an electron, but positive charge
i. Occurs for nuclides with Neutron:Proton ratio < 1.0
ii. Net effect is change of a proton to a neutron (Z changes by -1)
22
11
Na 
22
10
Ne  01e
iii. Positron is the Antiparticle of an Electron; collision with an electron
leads to annihilation
0
1
d)
e 
0
-1
e  2 00g
Electron capture: an inner orbital electron is captured by the nucleus
i. Always produces g-rays as well
ii. The ideal reaction for an alchemist, but too slow to be useful
201
80
Hg 
0
-1
e 
Au  00g
201
79
Examples
214
III. The Kinetics of Radioactive Decay
A.
Rate of Decay = - change in number of nuclides per unit time
1) Radioactive nuclides decay at a rate proportional to the size of the sample
N
Rate   N  kN
t
2)
3)
This is the same as a first order rate law
Integrated first order rate law and half life equation work too!
 N
  kt
ln 
 N0 
4)
0.693
k
Example: Technicium-99 is used for medical imaging. k = 0.116/h. t1/2 =?
t1/2 
5)
t1/2 
0.693
0.693

 5.97 h
k
0.116/h
Example: t1/2 of Molybdenum-99 is 67.0 h. How much of a 1.000 mg sample is
left after 335 h?
0.693
0.693 0.693
t1/2 
k

 0.0103/h
k
t1/2
67.0h
 N
N
  kt 
ln 
 e kt  N  (N 0 )e kt  (1.000mg )e ( 0.0103/ h )(335h )  0.032mg
N0
 N0 
B. Carbon Dating
1) Archeological technique to determine the age of artifacts
2) Willard Libby received the Nobel Prize in Chemistry for this work
3) Based on the radioactive decay of carbon-14
14
6
4)
14
7
N 
0
-1
e
Carbon-14 is continuously produced in the atmosphere by neutrons from space
14
7
5)
C 
N  01n 
14
6
C  11H
a) These processes have reached equilibrium: no net change in [carbon-14]
b) Plants take up the carbon as CO2 while alive, but stop when they die
c) Ratio of 14C to 12C begins to get smaller as soon as the plant dies
d) t1/2 = 5730 years for the decay of 14C
Example: 14C decay is 3.1/min. Fresh wood is 13.6/min. t1/2 = 5730 y.
t1/2 
0.693
0.693 0.693
k

 1.21 x 10  4 /y
k
t1/2
5730y
 N
 3.1 

ln 
ln 

N
 N
 13.6   12,000 y
  kt  t   0  
ln 
4
N

k

1.21
x
10
/y
 0
IV. Applications of Nuclear Reactions
A.
Nuclear Transformations
1) Particle accelerators: device to propel particles at high speed
a) Linear accelerator uses changing electric fields
b) Cyclotron uses oscillating voltage to accelerate; magnets cause circular path
2) Bombarding Nuclides with other nuclides or particles can lead to new Nuclides
3) Most of the “trans-Uranium” elements were synthesized this way (Z = 93-112)
a) Neutron Bombardment
238
92
b)
239
93
Np 
0
-1
e
Positive-Ion Bombardment
239
94
B.
U  01n 
U  42 He 
242
96
Cm  01n
Medical Uses
1) Radiotracers = radioactive nuclides introduced to an organism to follow pathway
a) Iodine-131 is used to diagnose thyroid gland problems
b) Thallium-201 and Technetium-99 diagnose heart damage
2) PET scan = Positron Emission Tomography
Targeted Imaging: PET
DRUG
radiopharm
C.
Energy Production
1) Fission = splitting a heavy nuclide into 2 lighter, more stable ones (H = -)
a) Uranium fission provides electrical power
235
92
U  01n 
Ba 
141
56
92
36
Kr  3 01n
b)
c)
3.5 x 10-11 J/nuclide = 2.1 x 1013 J/mol of energy is given off by loss of mass
E = mc2 is used to calculate the amount of energy from the mass loss
d)
Chain reaction: neutrons produced can cause more reactions
i) Subcritical: < 1 neutron/reaction causes another fission (rxn dies out)
ii) Critical: = 1 neutron/reaction causes another fission (rxn sustained)
iii) Supercritical: > 1 neutron/reaction causes another fission (explosion)
e)
Nuclear Reactor: Fission heats water, runs turbine, make electricity
i) Reactor core: enriched uranium (3% U-235) sustains the reaction
ii) Control rods absorb neutrons to regulate the reaction
f)
Breeder Reactor: produces its own fissionable Pu-239 from U-238
Pu-239 is toxic and flames in air, so U.S. doesn’t use, France does
2)
Fusion = combining 2 light nuclides to form a heavier, more stable one (H = -)
a) Stars produce their heat through this process
1
1
H  11H  21H  01e
3
2
He  23 He  42 He  2 11H
1
1
H  21H  23 He
3
2
He  11H  42 He  01e
b)
D.
Would be great energy source on Earth
i. Lots of small nuclei to use as fuel
ii. But, only takes place at high temperatures (40,000,000 Kelvins)
iii. High temperature overcomes strong nuclear repulsion (+/+)
iv. E = mc2 (4.03298 amu in; 4.00260 amu out)
Effects of Radiation
1) Damage to organisms
a) Somatic damage = damage to the organisms itself (sickness or death)
b) Genetic damage = damage to genetic material (offspring are effected)
2) Factors controlling radiation effects
a) Energy of the radiation: higher energy = more damage (1 Rad = 0.01 J/kg)
b) Penetrating ability: g-ray > b-particle (1cm) > a-particle (stopped by skin)
c) Ionizing ability: removing electrons; a-particle >> g-ray
d) Chemical properties: Kr-85 inert, excrete quickly; Sr-90 replaces Ca, stays
3)
REM
4)
REM = Roentgen Equivalent for Man = normalizes radiation effects for different
types of radiation exposure
a. Short term effects of radiation exposure
b. There are natural and man-made sources of radiation exposure
Models for radiation exposure damage
a. Linear model: any exposure is bad, minimize all exposures
b. Threshold model: no damage unless a certain amount of exposure occurs
c. Better safe than sorry: we don’t know which model is correct, follow linear