Nuclear Chemistry

1
Nuclear Chemistry

• Chapter 20

2
Radioactive Decay

• Radioactive decay is the process in which a
  nucleus spontaneously disintegrates, giving off
  radiation.

3
Number of Stable IsotopesEven vs. Odd
Number of Stable Isotopes
157 52 50 5
Protons Even Even Odd Odd Neutrons Even Odd
Even Odd
4
The Five Types of Radioactive Decay

• Alpha emission
• Beta emission
• Positron emission
• Electron capture
• Gamma emission

mass number
238
U
92
atomic number
5
Alpha Emission

• Emission of an He nucleus, or alpha particle,
  from an unstable nucleus.

222 86
4 2
226 88
Rn He
Ra
alpha particle
6
Beta Emission

• Emission of a high-speed electron from an
  unstable nucleus (equivalent to conversion of a
  neutron to a proton).

14 7
0 -1
14 6
N e
C
? particle
7
Positron Emission

• Emission of a positron from an unstable nucleus
  (equivalent to an electron with positive charge).

95 42
0 1
95 43
Mo e
Tc
positron
8
Electron Capture

• Electron capture is the decay of an unstable
  nucleus by capturing an electron from an inner
  orbital of an atom.

40 18
40 19
0 -1
Ar
K e
9
Gamma Emission

• Gamma emission is emission from an excited
  nucleus of a gamma photon, corresponding to
  radiation with a wavelength of 10-12 m.

10
Radioactivity

• Predict the type of radioactive decay process
  that is likely for each of the following nuclides

228 92
U
8 5
B
68 29
Cu
11
Radioactivity

• Predict the type of radioactive decay process
  that is likely for each of the following nuclides

224 90
4 2
?-emission is most likely for nuclei with Z gt 83
228 92
Th He
U
8 5
B
68 29
Cu
12
Radioactivity

• Predict the type of radioactive decay process
  that is likely for each of the following nuclides

224 90
4 2
?-emission is most likely for nuclei with Z gt 83
228 92
Th He
U
positron emission is most likely with Z lt 20
electron capture is also possible.
8 4
0 1
8 5
Be e
B
68 29
Cu
13
Radioactivity

• Predict the type of radioactive decay process
  that is likely for each of the following nuclides

224 90
4 2
?-emission is most likely for nuclei with Z gt 83
228 92
Th He
U
positron emission is most likely with Z lt 20
electron capture is also possible.
8 4
0 1
8 5
Be e
B
68 29
68 30
0 -1
?-emission is possible with Z gt He 4
Cu
Zn e
14
Rate of Radioactive Decay

• Radioactive decay is a first order process with a
  half-life described by

15
Rate of Radioactive Decay

• Radioactive decay is a first order process with a
  half-life described by

t1/2 0.693/k
16
Radioactive Dating

• Carbon from the Dead Sea Scrolls gave 12.1
  disintegrations of 14C per minute per gram of
  carbon. How old are the manuscripts? (Carbon in
  living materials gives 15.3.)

17
Radioactive Dating

• Carbon from the Dead Sea Scrolls gave 12.1
  disintegrations of 14C per minute per gram of
  carbon. How old are the manuscripts? (Carbon in
  living materials gives 15.3.)

0.693 t 2.303 t1/2
log (N0/Nt)
so,
2.303 t1/2 log (N0/Nt) 0.693
t
18
Radioactive Dating

• Carbon from the Dead Sea Scrolls gave 12.1
  disintegrations of 14C per minute per gram of
  carbon. How old are the manuscripts? (Carbon in
  living materials gives 15.3.)

2.303 t1/2 log (N0/Nt) 0.693
t
The ratio N0/Nt is found from the ratio of the
decay rates.
N0 Nt
rate0 ratet
15.3 12.1


1.26
19
Radioactive Dating

• Carbon from the Dead Sea Scrolls gave 12.1
  disintegrations of 14C per minute per gram of
  carbon. How old are the manuscripts? (Carbon in
  living materials gives 15.3.)

2.303 t1/2 log (N0/Nt) 0.693
t
2.303 (5730 yrs.) log (1.26) 0.693
1.94 x 103 yrs.
20
Mass-Energy Equivalence

• The change in mass gives the change in internal
  energy for the system according to

E (?m)c2