1. Nuclear Chemistry Chapter 18 by Christopher G. Hamaker
Illinois State University
© 2014 Pearson Education, Inc. 1

2. Introduction
As Earth’s supply of fossil fuels is diminishing, nations are increasingly turning to nuclear power.
Currently, approximately 15% of the world’s electricity needs are met using nuclear power. In the US, about 20% of the electricity is from nuclear power.

3. Natural Radioactivity
There are three types of radioactivity:
Alpha particles (a) are identical to helium nuclei, containing two protons and two neutrons.
Beta particles (b) are identical to electrons.
Gamma rays (g) are high-energy photons.

4. Charges of Radiation Types
Alpha particles have a +2 charge, and beta particles have a –1 charge. Both are deflected by an electric field.
Gamma rays are electromagnetic radiation and have no charge, so they are not deflected.

5. Behavior of Radiation
Since alpha particles have the largest mass, they are the slowest moving type of radiation.
Gamma rays move at the speed of light since they are electromagnetic radiation.

6. Penetrating Power of Radiation Types
Alpha particles can be stopped by the skin.
Beta particles can penetrate to a depth of about 1 cm into the body.
Gamma radiation can easily pass through the human body.

7. Atomic Notation
Recall that we learned atomic notation in Chapter 4. Sy
symbol of the element
mass number (p+ and n0)
atomic number (p+) A Z
A nuclide is the nucleus of a specific atom.
The radioactive nuclide strontium-90 has 90 protons and neutrons. The atomic number is 38. Sr 90 38

8. Nuclear Reactions
A nuclear reaction involves a high-energy change in an atomic nucleus.
For example, a uranium-238 nucleus changes into a thorium-231 nucleus by releasing a helium-4 particle and a large amount of energy. U
235 92 He 4 2 Th
231 90 +
In a balanced nuclear reaction, the atomic numbers and masses for the reactants must equal those of the products.

9. Balancing Nuclear Reactions
The sum of the atomic numbers (subscripts) on the left side of the equation must equal the sum of the atomic numbers on the right side.
The sum of the mass numbers (superscripts) on the left side of the equation must equal the sum of the mass numbers on the right side.
After completing the equation by writing all the nuclear particles in atomic notation, a coefficient may be necessary to balance the reaction.

10. Common Nuclear Particles
Below is a listing of the common nuclear particles used to balance nuclear reactions.

11. Alpha Particle Emission
Radium-226 decays by alpha emission. Ra
226 88 He 4 2 X A Z +
First, balance the number of protons:
88 = Z + 2, so Z = 86 (Rn)
Balance the number of protons plus neutrons:
226 = A + 4, so A = 222 Ra
226 88 He 4 2 Rn
222 86 +

12. Beta Particle Emission
Some radioactive nuclides decay by beta emission.
Radium-228 loses a beta particle to yield actinium-228. Ra
228 88 e –1 Ac 89 +

13. Gamma Ray Emission
For example, uranium-233 decays by releasing both alpha particles and gamma rays.
Note that a gamma ray has a mass and a charge of zero, so it has no net effect on the nuclear reaction. U
233 92 He 4 2 Th
229 90 + g

14. Positron Particle Emission
A positron (b+) has the mass of an electron, but a +1 charge.
During positron emission, a proton decays into a neutron and a positron.
Sodium-22 decays by positron emission to neon-22. Na 22 11 e +1 + Ne 10

15. Electron Capture
A few large, unstable nuclides decay by electron capture. A heavy, positively charged nucleus attracts an electron.
The electron combines with a proton to produce a neutron.
Lead-205 decays by electron capture. Pb
205 82 e –1 Tl 81 +

16. Decay Series
Some heavy nuclides must go through a series of decay steps to reach a nuclide that is stable.
This stepwise disintegration of a radioactive nuclide until a stable nucleus is reached is called a radioactive decay series.
For example, uranium-235 requires 11 decay steps until it reaches the stable nuclide lead-207.

17. Uranium-238 Decay Series
Uranium-238 undergoes 14 decay steps before it ends as stable lead-206.
The decay series for uranium-238 is shown to the right.
The series includes eight alpha decays and six beta decays.

18. Parent and Daughter Nuclides
The term parent–daughter nuclides describes a parent nucleus decaying into a resulting daughter nucleus.
For example, the first step in the decay series for U-238 is: U
238 92 He 4 2 Th
224 90 +
U-238 is the parent nuclide and Th-234 is the daughter nuclide.

19. Activity
The number of nuclei that disintegrate in a given period of time is called the activity of the sample.
A Geiger counter is used to count the activity of radioactive samples.
Radiation ionizes gas in a tube, which allows electrical conduction.
This causes a clicking to be heard and the number of disintegrations to be counted.

20. Half-Life Concept
The level of radioactivity for all radioactive samples decreases over time.
Radioactive decay shows a systematic progression.
If we start with a sample that has an activity of 1000 disintegrations per minute (dpm), the level will drop to 500 dpm after a given amount of time. After the same amount of time, the activity will drop to 250 dpm.
The amount of time for the activity to decrease by half is called the half-life, t½.

21. Half-Life Concept, Continued
After each half-life, the activity of a radioactive sample drops to half its previous level.
The decay curve on the right shows the activity of a radioactive sample over time.

22. Radioactive Waste
A sample of plutonium-239 waste from a nuclear reactor has an activity of 20,000 dpm. How many years will it take for the activity to decrease to 625 dpm?
The half-live for Pu-239 is 24,000 years.
It takes five half-lives for the activity to drop to 625 dpm.
5 t½ x
= 120,000 y
1 t½
24,000 y

23. Half-Life Calculation
Iodine-131 is used to measure the activity of the thyroid gland. If 88 mg of I-131 are ingested, how much remains after 24 days (t½ = 8 days)?
First, find out how many half-lives have passed.
24 days x
= 3t½
1 t½
8 days
Next, calculate how much I-131 is left.
88 mg I-131 x
= 11 mg I-131 1 2 x

24. Radiocarbon Dating
A nuclide that is unstable is called a radionuclide.
Carbon-14 decays by beta emission with a half-life of 5730 years. C 14 6 e -1 + N 7
The amount of carbon-14 in living organisms stays constant with an activity of about 15.3 dpm. After the plant or animal dies, the amount of C-14 decreases.

25. Radiocarbon Dating, Continued
The age of objects can therefore be determined by measuring the C-14 activity. This is called radiocarbon dating.
The method is considered reliable for items up to 50,000 years old.

26. Uranium–Lead Dating
Uranium-238 decays in 14 steps to lead-206. The half-life for the process is 4.5 billion years. e –1
+ 6 U
238 92 He 4 2 Pb
206 82
+ 8
The age of samples can be determined by measuring the U-238/Pb-206 ratio.
A ratio of 1:1 corresponds to an age of about 4.5 billion years.

27. Agricultural Applications of Radionuclides
Gamma radiation is used to sterilize male insects instead of killing them with pesticides.
Cobalt-60 emits gamma rays when it decays and is often used in agriculture.
Gamma-irradiation of food is used to kill microorganisms.

28. Critical Thinking: Nuclear Medicine
The term nuclear medicine refers to the use of radionuclides for medical purposes.
Iodine-131 is used to measure thyroid activity.
The gas xenon-133 is used to diagnose respiratory problems.
Iron-59 is used to diagnose anemia.
Breast cancer can be treated using the isotope iridium-192.

29. Induced Radioactivity
A nuclide can be converted into another element by bombarding it with an atomic particle.
This process is called transmutation.
The elements beyond uranium on the periodic table do not occur naturally and have been made by transmutation.
For example, rutherfordium can be prepared from californium. n 1
+ 4 Cf
249 98 12 C 6 Rf
257
104 +

30. Nuclear Fission
Nuclear fission is the process where a heavy nucleus splits into lighter nuclei.
Some nuclides are so unstable, they undergo spontaneous nuclear fission.
n + energy 1
+ 4 Cf
252 98 Ba
142 56 + Mo
106 42
A few nuclides can be induced to undergo nuclear fission by a slow-moving neutron. n 1
+ 3 U
235 92 Ba
141 56 + Kr 36
+ energy

31. Nuclear Chain Reaction
Notice that one neutron produces three neutrons. These neutrons can induce additional fission reactions and produce additional neutrons.
If the process becomes self-sustaining, it is a chain reaction.

32. Nuclear Chain Reaction, Continued
The mass of material required for a chain reaction is the critical mass.

33. Chemistry Connection: Nuclear Power Plant
Nuclear energy is an attractive source because of its potential.
A nuclear power plant produces energy from a nuclear chain reaction.
Fuel rods are separated by control rods to regulate the rate of fission.
A liquid coolant is circulated to absorb heat.

34. Nuclear Fusion
Nuclear fusion is the combining of two lighter nuclei into a heavier nucleus.
It is more difficult to start a fusion reaction than a fission reaction, but it releases more energy.
Nuclear fusion is a cleaner process than fission because very little radioactive waste is produced.
The Sun is a giant fusion reactor, operating at temperatures of millions of degrees Celsius.

35. Fusion in the Sun (and Other Stars)
The Sun is about 73% hydrogen, 26% helium, and 1% all other elements.
Three common fusion reactions that occur in the Sun are as follows: + H 1
+ energy e +1 2 + H 2 1
+ energy He 3 + He 3 2
+ energy e +1 H 1 4

36. Fusion Energy
There are two fusion reactions being investigated for use in commercial power generation.
The first reaction, as noted below, uses deuterium as a fuel: + H 2 1
+ energy He 4
2. The second, as noted below, involves deuterium and tritium as fuels: + H 3 1
+ energy n 2 He 4

37. Fusion Reactor
A fusion reactor is a large “magnetic bottle” that contains the nuclei that have been heated to a temperature of millions of degrees for fusion.

38. Chapter Summary There are three types of natural radiation:
Alpha particles
Beta particles
Gamma rays
Alpha particles are helium nuclei, and beta particles are electrons.
Gamma rays are electromagnetic radiation.

39. Chapter Summary, Continued
Radioactive nuclides decay by four processes:
Alpha emission
Beta emission
Positron emission
Electron capture
The parent nuclide decays to yield the daughter nuclide.
If a nuclide decays through the emission of radiation in more than one step, the overall process is called a radioactive decay series.

40. Chapter Summary, Continued
The time required for 50% of the radioactive nuclei in a sample to decay is constant and is called the half-life. After each half-life, only 50% of the radioactive nuclei remain.
Artificial nuclides are produced by transmutation.
The splitting of a heavy nucleus into two lighter nuclei is nuclear fission.
The combining of two lighter nuclei into one nucleus is nuclear fusion.