Nuclear Chemistry Notes



Nuclear Chemistry Notes

I. Radioactive Elements

A. Unstable configurations will break into other elements over a period of time.

B. Process where unstable elements emit rays or particles is called radioactivity.

C. Some stable elements have isotopes that are radioactive (cobalt, iodine). These isotopes that are unstable and will undergo radioactive decay are called radioisotopes.

D. Radiation is the particles and rays given off as a radioactive material breaks down. There are three main components of radiation.

1. Alpha particles are composed of two protons and two neutrons (helium nucleus). They are large but have little penetrating power. Alpha particles can be stopped with a piece of paper.

2. Beta particles are electrons. Electrons are much smaller than alpha particles but have more penetrating power. Aluminum foil is needed to stop a beta particle.

3. Gamma rays are high-energy electromagnetic waves much like X-rays. They have a great deal of penetrating power. Gamma rays can penetrate through inches of lead and feet of concrete.

E. Most nuclei are not stable, only about 20% of nuclei do not decay over time to form a more stable nuclei. The stability of a nucleus depends on its neutron to proton ratio. For small elements with an atomic number less than 20, a neutron-proton ratio of 1 is stable. In other words, carbon 12 has 6 neutrons and 6 protons and is stable. Carbon 14 has 8 neutrons and 6 protons (ratio 1.3) and is not stable. For elements with an atomic number greater than 20, a stable nucleus has more neutrons than protons. A neutron proton ratio up to 1.5 is stable. For elements that have too many neutrons, they undergo beta emission. They break a neutron into a proton and emit a beta particle (electron) in the process. Elements with too many protons undergo positron emission. In positron emission a proton is converted to a neutron emitting a positron. A positron has the mass of an electron with a positive charge. All elements with an atomic number greater than 83 are radioactive. They have too many neutrons and protons to be stable. They undergo alpha emission which reduces both their number of neutrons and protons.

F. A half-life is the time it takes for one half of a radioactive sample to decay. The half-life of a radioactive element can vary from less than a second to billions of years.

Example – Mercury 195 has a half-life of 31 hours. If you start with 5.00 g., how many grams of Mercury 195 will be left after 93 hours?

5.00 g. X .5 X .5 X .5 = .625g

G. Radioactive dating is a technique to determine the age of a fossil or artifact.

The most used radioisotope to date objects is carbon 14. Carbon 14 decays to produce nitrogen 14 and beta particles. Carbon 14 is formed by cosmic rays colliding with nitrogen 14 atoms. Carbon 14 is produced at approximately the same rate as the existing carbon 14 atoms decay, so the concentration of carbon 14 in the environment remains constant. The half-life of carbon 14 is 5730 years.

A portion of the carbon 14 formed combines with oxygen to form radioactive carbon dioxide. The ratio of radioactive carbon dioxide to normal carbon dioxide is constant. Some of the radioactive carbon 14 is fixed into sugars by photosynthesis and thus ends up in plants and animals. The incorporation of radioactive carbon 14 ends at death and the accumulated radioactive carbon will break down according to its half-life. Since beta particles are given off in the decay of carbon 14, the amount of carbon 14 can be determined by the beta radiation. After 5730 years one half of the beta radiation has been emitted. Carbon 14 dating can be used on fossils up to 30,000 years old.

Any radioactive element or isotope can be used to radioactive dating. Uranium is used to determine the age of landmasses and the earth. Radioactive potassium has been used to date objects that contain volcanic ash.

Example – Carbon 14 has a half-life of 5,730 years and is found in all living things. If a bone originally contained one gram of carbon 14, how old is the bone if .125 g. of the compound is left?

1 > .5 > .25 > .125 The carbon 14 went through 3 half-lives.

5,730 years X 3 = 17,200 years old.

• Read Pages 799-806

II. Measuring Radiation

A. The Curie is equal to the number of nuclear disintegrations per second from one gram of radium. The Curie does not take into account the type of radiation so is not very useful in determining the danger to living things.

B. The rem (roetgen equivalent for man) includes the amount of energy transferred and the sensitivity of the body to that type of radiation. Doses above 1000 rem are always fatal; doses below 150 rem are not fatal but can cause tissue damage.

C. The dosimeter is used to measure the total amount of radiation a person has received. Photographic film covered with plastic and worn as a badge is used in many facilities where radiation may be present.

D. Radiation causes damage to living cells. Radiation follows a straight path through the cell acting like tiny bullets destroying whatever is in its path. It can strip electrons from atoms creating ions that can further damage the cell.

1. Damage to the somatic cells (body cells) will cause cells to die or be damaged. Damaged cells may form cancer cells over time.

2. Damage to germ cells (reproductive cells) can kill the reproductive cell or damage the DNA of the cell. These damaged cells can produce mutations in the next generation.

E. A Geiger counter is a hollow cylinder with a wire in the center used to measure radiation. The wire is given a positive charge and the cylinder a negative charge. Both ends of the cylinder are blocked but one end has a very thin window through which radiation can easily pass. Radiation ionizes the gas in the cylinder. Positive ions are attracted to the negatively charged cylinder and negative ions are attracted to the positively charged wire. When ions strike the cylinder or wire they create an electric pulse which is amplified to produce an audible click.

III. Uses of Radiation

A. Radiotracers can be used to follow the movement of materials.

1. Iodine 131 is used to follow the uptake of iodine by the thyroid gland.

2. Barium 140 is used to follow the flow of pollutants in rivers.

3. Cobalt 58 is used in the body to follow the uptake of vitamin B-12.

B. Cancer Treatment

1. Radiation acts as tiny bullets killing what it hits. Radiation is directed at cancer cells to kill them. Unfortunately, it is almost impossible to hit only cancer cells in the body, many normal cells are also killed or we do not kill all the cancer cells to save the normal cells.

2. Today radiation is being used in conjunction with monoclonal antibodies that target the cancer cells.

C. Food Preservation

Radiation can be used to kill microorganisms that are responsible for the spoiling of food. The FDA has recently approved its usage.

D. Neutron Activation Analysis

1. A procedure used to detect the trace amounts od elements in a sample.

2. The sample is bombarded by electrons from a radioactive source. The half-life and type of radiation emitted are then recorded. The information is processed by a computer. Half-lives and the type of radiation emitted are specific for each radioisotope, from the information scientists can determine the elements originally present.

IV. Radiation as a Power Source

A. Natural radioactive decay is not a practical energy source. Natural decay can not be controlled (sped up or slowed down) and the output is not consistent.

B. Nuclear Fission

1. In nuclear fission neutrons are used to break apart unstable isotopes.

2. Based on Einstein’s equation E = mc2 . E is energy, mass is mass, and c is the speed of light. Since c is a constant, changes in E are a result of changes in m. This equation states that mass can be changed directly into energy.

3. The fission reaction most often used is uranium 235 being broken into Barium 141 and krypton 92. In this reaction a small amount of matter is converted into energy and the nuclear force is broken resulting in additional energy release.

-------- Kr91

Neutron ------ U235 ------ U236 ------- Energy -------- 3 neutrons

Unstable -------- Ba142

4. In a nuclear bomb, the uranium 235 reaction is used and the neutrons released split more and more uranium 235 atoms resulting in an uncontrolled nuclear reaction.

5. Nuclear reactors have controlled reactions and can not explode like a bomb. However, they can take place too fast and cause a meltdown. In a meltdown, the nuclear core becomes so hot it burns through the containment vessel releasing radiation to the environment.

6. The following adaptations prevent nuclear power plants from exploding.

a) The nuclear core contains too little uranium 235 for an uncontrolled chain reaction to take place.

b) Control rods fit into the core and absorb neutrons, allowing workers to determine the rate of the reaction.

7. Problems with nuclear power.

a) What do we do with the barium, krypton and other radioactive wastes that will give off radiation for thousands of years? Most of the spent fuel rods containing nuclear waste are stored in large pools of water at the nuclear power plant.

b) Cooling system failures can lead to meltdowns or radiation release like at Three Mile Island and Chernobyl.

C. Nuclear Fusion

1. Two small nuclei combine together to form a larger nucleus.

2. By Einstein’s equation, mass is converted to energy.

3. All the stars in the universe are powered by fusion reactions.

4. Fusion reactions produce more energy per gram of reactant than fission reactions. There also is no harmful waste product in fusion reactions.

H2 + H3 ------ He4 + Neutron + Energy

5. Nuclear fusion requires extremely high temperatures to strip off electrons and speed up the nuclei so they can combine. Temperatures of at least 40 million degrees Celsius are needed.

6. We have trouble attaining these temperatures on earth and containing the reaction if we do. No material can withstand temperatures of millions of degree Celsius. An electromagnetic field will work because the plasma is charged and will be repelled by the electromagnetic field.

* Read Pages 810-819

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