Transcript lecture slides of chap23

Nuclear Chemistry
Chapter 23
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Some nuclei are unstable; they emit particles and/or
electromagnetic radiation spontaneously. This phenomenon
is radioactivity. All isotopes of the elements with atomic
numbers higher than 83 are radioactive.
Mass Number
Atomic Number
A
ZX
Element Symbol
proton
1p
1H
1 or 1
neutron
1n
0
electron
0
0
-1e or -1b
positron
0
0
+1e or +1 b
a particle
4He
or 42a
2
A
1
1
0
0
4
Z
1
0
-1
+1
2
23.1
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.
235
92 U
+ 10n
138
55 Cs
+
96
37 Rb
+ 2 10n
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.
235
92 U
+ 10n
138
55 Cs
+
96
37 Rb
92 + 0 = 55 + 37 + 2x0
+ 2 10n
23.1
212Po
decays by alpha emission. Write the balanced
nuclear equation for the decay of 212Po.
4
alpha particle - 42He or 2a
212Po
84
4He
2
+ AZX
212 = 4 + A
A = 208
84 = 2 + Z
Z = 82
212Po
84
4He
2
+ 208
82Pb
23.1
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 (nuclei are unstable and dissipate excess
energy by spontaneously emitting radiation in the form of alpha, beta,
and gamma rays)
•
All isotopes of Tc (technetium, Z=43) and Pm
(promethium, Z=61) are radioactive
23.2
The principle factor
that determines
whether a nucleus is
stable is the
neutron-to-proton
ratio (n/P)
n/p too large
beta decay
X
Y
n/p too small
positron decay or
Electron capture
23.2
Nuclear Stability and Radioactive Decay
Beta decay (n/P too large; above the belt of stability)
+-10b + n
14C
6
14N
7
40K
19
40Ca
20
Decrease # of neutrons by 1
+ -10b + n
1n
0
Increase # of protons by 1
1p
1
+ -10b + n
Positron decay (n/P too small)
++10b + n
Increase # of neutrons by 1
++10b + n
Decrease # of protons by 1
11C
6
11B
5
38
19K
38Ar
18
1p
1
1n
0
++10b + n
23.2
Nuclear Stability and Radioactive Decay
Electron capture decay(same effect of positron decay)
+n
37Ar
18
+ -10e
37Cl
17
55Fe
26
+ -10e
55Mn
25
1p
1
Increase # of neutrons by 1
+n
Decrease # of protons by 1
+ -10e
1n
0
+n
Alpha decay
212Po
84
4He
2
+ 208
82Pb
Decrease # of neutrons by 2
Decrease # of protons by 2
23.2
Nuclear binding energy (BE) is the energy required to break up a
nucleus into its component protons and neutrons---a quantitative
measure of nuclear stability.
Masses of nuclei are always less than the sum of the masses of
Nucleons---a general term for the protons and neutrons in a nucleus.
Nuclear binding energy (BE) is the energy required to break up a
nucleus into its component protons and neutrons---a quantitative
measure of nuclear stability.
BE + 199F
911p + 1010n
ΔE = (Δm)c2
E = mc2
Δm = 9 x (p mass) + 10 x (n mass) – 19F mass
Δm(amu) = 9 x 1.007825 + 10 x 1.008665 – 18.9984=0.1587amu
Mass defect
ΔE = (Δm)c2
= 0.1587amu x(3.00x108 m/s)2=1.43 x 1016 amu m2/s2
1 amu = 1.66 x 10-27 kg 1J= 1kg m2/s2
BE = ΔE =2.37 x 10-11J
binding energy
binding energy per nucleon =
number of nucleons
2.37 x 10-11 J
= 1.25 x 10-12 J 23.2
=
19 nucleons
Natural radioactivity
Nuclei outside the belt stability, as well as nuclei with more than
83 protons, tend to have spontaneous emission, called
radioactivity.
The main types of radiation:
α particles
β particles
Ɣ rays
Electron capture
Positron decay
Kinetics of Radioactive Decay
DN
rate = Dt
rate = lN
DN
= lN
Dt
N = N0exp(-lt)
lnN = lnN0 - lt
N = the number of atoms at time t
N0 = the number of atoms at time t = 0
l is the decay constant
ln2
l =
t½
t½ half-life
23.3
The half lives have been used as “atomic clocks” to determine
the ages of certain objects.
Radiocarbon Dating
14N
7
+ 01n
14C
6
14C
6
14N
7
+ 11H
+ -10b + n
t½ = 5730 years
Uranium-238 Dating
238U
92
206Pb
82
+ 8 24a + 6-10b
t½ = 4.51 x 109 years
It is possible to estimate the ages of the
rocks from the mass ratio between
Nuclear Transmutation
14N
7
+ 24a
Transmutation is brought about by
the collision of two particles.
14N(a,
7
27Al
13
14N
7
+ 24a
+ 11p
17O
8
+ 11p
p)7O
8
30P
15
11C
6
+ 01n
+ 42a
23.4
Worked Example 23.3
Nuclear Transmutation
23.4
Nuclear Fission is the process in which a heavy nucleus
(mass number >200) divides to form smaller nuclei of
intermediate mass and one or more neutrons.
Nuclear Fusion is the combining of small nuclei into large
ones
Nuclear Fission
235U
92
+ 01n
90Sr
38
1n + Energy
+ 143
Xe
+
3
0
54
Energy = [(mass 90Sr + mass 143Xe + 3 x mass n )-( mass 235U + mass n )] x c2
Energy = 3.3 x 10-11J per 235U
= 2.0 x 1013 J per mole 235U
23.5
Nuclear Fission
Nuclear chain reaction is a self-sustaining sequence of
nuclear fission reactions. The neutron generated in the initial
stage of fission can induce fission in other uranium-235 nuclei,
which in turn produce more neutrons.
The minimum mass of fissionable material required to
generate a self-sustaining nuclear chain reaction is the
critical mass.
Non-critical
Critical
23.5
Atomic bomb
The TNT was set off first. The explosion
forces the section of fissionable material
together to form an amount considerably
larger than the critical mass.
Nuclear Fusion
Combining of small nuclei into large ones
Fusion Reaction
2
2
3
1
1 H + 1H
1 H + 1H
2H
1
+ 13H
4He
2
+ 10n
6Li
3
+ 12H
2 42He
Energy Released
6.3 x 10-13 J
2.8 x 10-12 J
3.6 x 10-12 J