Nuclear Chemistry
Chapter 23
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The symbols for various chemical elements and numbers of
electrons, protons and neutrons in every species.
• Atomic number (Z) = number of protons in nucleus
•
Mass number (A) = number of protons + number of neutrons
•
= atomic number (Z) + number of neutrons
Mass Number
Atomic Number
A
ZX
Element Symbol
a particle
4He
4a
or
2
2
proton
1p
1H
1 or 1
neutron
1n
0
electron
0
0
-1e or -1b
positron
0
0
+1e or +1 b
A
1
1
0
0
4
Z
1
0
-1
+1
2
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
Worked Example 23.1
Worked Example 23.3
Nuclear Stability and Radioactive Decay
Beta decay
+-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
++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
n and n have A = 0 and Z = 0
23.2
Electron capture 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
Spontaneous fission
252Cf
98
1n
2125
In
+
2
49
0
23.2
23.3 natural radioactivity
The radioactive decay series
Is a sequence of nuclear reaction that
ultimately result in the formation of a
stable isotope.
The decay series of
naturally
occurring uranium-238 to thorium234 involves 14 steps. The decay
scheme, also shows the half – lives
of all the products.
The beginning radioactive isotope in
the series is called the parent and the
product, the daughter.
238U
92
206Pb
82
Parent
daughter
23.3
Kinetics of Radioactive Decay
All radioactive decays obey first –order kinetics.
DN
DN
[N]t
rate = rate = lN
ln
= lN
= - lt
Dt
Dt
[N]0
N = the number of atoms at time t
N0 = the number of atoms at time t = 0
l is the decay constant
ln[N] = ln[N]0 - lt
ln [N]
[N] = [N]0exp(-lt)
[N]
ln2
l =
t½
23.3
Problems
1 – 2 – 3 – 4 – 5 – 6 – 7 – 23 – 25 – 28 – 29 –
31 – 33 – 34 – 35 – 36 – 55 – 57 – 59