Transcript Chapter 19 Nuclear Chemistry
19.1 Nuclear Stability and Radioactive Decay
19.2 The Kinetics of Radioactive Decay
19.4 Detection and Uses of Radioactivity
19.5 Thermodynamic Stability of the Nucleus
19.6 Nuclear Fission and Nuclear Fusion
Review
• Atomic Number (
Z
) – number of protons • Mass Number (
A
) – sum of protons and neutrons
Z A
X
Radioactive Decay • Nucleus undergoes decomposition (or decay) to form a different nucleus.
Radioactive Stability • Nuclides with 84 or more protons are unstable.
• Light nuclides are stable when
Z
equals
A
–
Z
(neutron/proton ratio is 1).
• For heavier elements the neutron/proton ratio required for stability is greater than 1 and increases with
Z
.
Radioactive Stability • Certain combinations of protons and neutrons seem to confer special stability.
Even numbers of protons and neutrons are more often stable than those with odd numbers.
Radioactive Stability • Certain specific numbers of protons or neutrons produce especially stable nuclides.
2, 8, 20, 28, 50, 82, and 126
The Band of Stability
Types of Radioactive Decay • Alpha production ( ): • Beta production ( ):
Types of Radioactive Decay • Gamma ray production ( ): • Positron production :
Types of Radioactive Decay • Electron capture : Inner-orbital electron
Decay Series (Series of Alpha and Beta Decays)
Concept Check Which of the following produces a particle?
31 29 0 1 30 1 62 28 electron capture positron c) 212 Fr 87 2 d) 129 51 Sb 1 208 At 85 129 52 Te alpha particle beta particle
Rate of Decay Rate =
kN
• The rate of decay is proportional to the number of nuclides. This represents a first order process.
Half-Life
• Time required for the number of nuclides to reach half the original value.
t
1/ 2 =
k
= 0.693
k
Nuclear Particles
Half-Life of Nuclear Decay
Exercise A first order reaction is 35% complete at the end of 55 minutes. What is the value of
k
?
k = 7.8 x 10 -3 min -1
Nuclear Transformation • The change of one element into another.
27 13 4 Al + He 2 30 15 1 P + n 0 249 98 Cf + 18 8 O 263 106 1 Sg + 4 n 0
A Schematic Diagram of a Cyclotron
A Schematic Diagram of a Linear Accelerator
Measuring Radioactivity Levels • Geiger counter • Scintillation counter
Geiger Counter
Carbon–14 Dating • Used to date wood and cloth artifacts.
• Based on carbon–14 to carbon–12 ratio.
Radiotracers • Radioactive nuclides that are introduced into organisms in food or drugs and whose pathways can be traced by monitoring their radioactivity.
Radiotracers
Energy and Mass • When a system gains or loses energy it also gains or loses a quantity of mass.
E
=
mc
2
m
= mass defect
E
= change in energy • If
E
is negative (exothermic), mass is lost from the system.
Mass Defect (
m
) • Calculating the mass defect for 4 2 He : Since atomic masses include the masses of the electrons, we must account for the electron mass.
4 2 4 2
m
1 1 1 1
m
• 4 2 He nucleus is “synthesized” from 2 protons and two neutrons.
m
m
e
m
= 0.0304 amu
Binding Energy • The energy required to decompose the nucleus into its components.
• Iron-56 is the most stable nucleus and has a binding energy of 8.97 MeV.
Binding Energy per Nucleon vs. Mass Number
Nuclear Fission and Fusion • Fusion – Combining two light nuclei to form a heavier, more stable nucleus.
• Fission – Splitting a heavy nucleus into two nuclei with smaller mass numbers.
0 1 n + 235 U 92 142 Ba + 56 91 36 1 Kr + 3 n 0
Nuclear Fission
Fission Processes • A self-sustaining fission process is called a chain reaction.
Event
subcritical critical supercritical
Neutrons Causing Fission Event
< 1 = 1 > 1
Result
reaction stops sustained reaction violent explosion
Schematic Diagram of a Nuclear Power Plant
Schematic Diagram of a Reactor Core
Nuclear Fusion
Biological Effects of Radiation Depend on: 1. Energy of the radiation 2. Penetrating ability of the radiation 3. Ionizing ability of the radiation 4. Chemical properties of the radiation source
rem (roentgen equivalent for man) • The energy dose of the radiation and its effectiveness in causing biologic damage must be taken into account.
Number of rems = (number of rads) × RBE rads = radiation absorbed dose RBE = relative effectiveness of the radiation in causing biologic damage
Effects of Short-Term Exposures to Radiation