Transcript 1 - Solon City Schools

NUCLEAR CHEMISTRY
Atomic Structure
Recall:
Atoms – consist of a positively charged nucleus, which has protons and
neutrons.
Isotope – atoms of the same chemical element that have a different
number of neutrons. Each isotope of a given element is designated by
the total number of its protons plus its neutrons.
Isotope
Mo-101
H-2
C-14
U-238
Bi-210
He-4
Symbol
101
42
2
1
14
6
238
92
210
83
4
2
Mo
H
C
U
Bi
He
# protons
# neutrons
Atomic Mass
42
59
101
1
1
2
6
8
14
92
146
238
83
127
210
2
2
4
Nuclear Forces
Mass Defect
• Difference between the mass of an
atom and the mass of its individual
particles.
4.00260 amu
4.03298 amu
Nuclear Binding Energy
• Energy released when a nucleus is
formed from nucleons.
• High binding energy = stable nucleus.
E =
2
mc
E: energy (J)
m: mass defect (kg)
c: speed of light
(3.00×108 m/s)
Nuclear Binding Energy
The seven most widely recognized magic numbers as of
2007 are 2,
8, 20, 28, 50, 82, 126
Unstable nuclides are radioactive and
undergo radioactive decay.
Types of Radiation
• Alpha particle ()
– helium nucleus
• Beta particle (-)
– electron
• Positron (+)
– positron
• Gamma ()
– high-energy photon
4
2
Charge Shielding
He
2+
0
-1
1-
e
0
1
0
0
e

paper
lead
1+
0
concrete
Penetrating Ability of Radiation
Nuclear Decay
•Transmutation-One element becomes another.
238
92
U
I
131
54
K
38
18
131
53
38
19
106
47
Th  He
234
90
Ag 
0
-1
4
2
Xe 
0
-1
Ar 
0
1
e
106
46
e
e
Pd
More than 83 protons means that the nuclei is unstable (radioactive)
Alpha Emission
occurs when the nucleus has
too many protons which cause
excessive repulsion.
238
92
parent
nuclide
U
Th  He
234
90
daughter
nuclide
4
2
alpha
particle
Numbers must balance!!
Beta Emission
occurs when the neutron to proton ratio is too great.
131
53
I
131
54
Xe  e
0
-1
electron
Neutron to Proton
Beta Emission
Ex. Polonium-210 undergoes beta decay to produce
this daughter nuclide
210
84
A
Z
Po
A
Z
X
=
210
85
X
At
0
+
-1
e
Positron Emission
Occurs when the neutron to proton ratio is too small.
38
19
K  Ar  e
38
18
0
1
positron
Positron Emission
Ex. Polonium-210 undergoes positron emission to produce
this daughter nuclide
210
84
A
Z
Po
A
Z
X
=
210
83
X
Bi
0
+
1
e
Electron Capture
occurs when the neutron to proton ratio in the nucleus is too small.
106
47
Ag  e 
0
-1
106
46
Pd
electron
Electron Capture
Ex. Polonium-210 captures an electron to produce this
daughter nuclide
210
84
Po
0
+
-1
A
Z
A
Z
e
X
=
210
83
Bi
X
Gamma Emission
occurs when the nucleus is at too high an energy.
– Emission of high energy electromagnetic
wave.
Gamma Emission
Ex. Polonium-210 undergoes gamma decay to produce
this daughter nuclide
210
84
A
Z
Po
A
Z
X
=
210
84
X
Po
+
0
0

Types of Radiation
Nuclear Fission – splitting of
heavier nuclei into lighter
nuclei.
235
92
U
1
+
0
n
137
56
Ba
84
+
36
Xe
1
+15 0 + energy
How much energy? E=mc2
_______=
Energy
_______
mass x (_____
speed of
_ _____)
light 2
n
c=3.0x108
mass defect
E=mc2 explains _____
_____ (total mass of
nucleus is less than sum of individual particles)
Nuclear Fusion
- Energy released when
two light nuclei combine or fuse
•However, a large amount of energy is
required to start a fusion reaction:
repulsion forces
o Need this energy to overcome ________
of protons.
o Extremely high temperatures can provide start-up
energy.
More energy in fusing hydrogen that fission of uranium
Nuclear Fusion
A radioactive nucleus reaches a stable state
by a series of steps
A
Decay
Series
Copyright© by
Houghton Mifflin
27
Half-life Concept
Decay Kinetics
Decay occurs by first order kinetics (the
rate of decay is proportional to the number
of nuclides present)
N0 = number of nuclides
present initially
 N 
   kt
ln 
 N0 
N = number of nuclides
remaining at time t
k = rate constant
t = elapsed time
Calculating Half-life
ln( 2) 0.693
t1/ 2 

k
k
t1/2 = Half-life (units dependent on rate
constant, k)
Sample Half-Lives
Half-life
mf  m ( )
1 n
i 2
mf: final mass
mi: initial mass
n: # of halflives