NUCLEAR CHEMISTRY
Nuclear Chemistry is the study of reactions involving changes in atomic nuclei. This branch of chemistry began with the discovery of natural radioactivity by Antoine-Henri Becquerel and grew as a result of subsequent investigations by Pierre and Marie Curie and many others. Nuclear chemistry is very much in the news today. In addition to applications in the manufacture of atomic bombs, hydrogen bombs, and neutron bombs, even the peaceful use of nuclear energy has become controversial, in part because of safety concerns about nuclear power plants and also because of problems with radioactive waste disposal. In this chapter we will study the stability of atomic nucleus, radioactivity, and the effects of radiation on biological systems.
Nuclear Stability
Why are some nuclei stable, while others are unstable?
Studies made on stable nuclides and radionuclides showed that there is a relationship between neutron-proton ratio and stability of nuclides.
Figure 1.1 shows that for a low number of protons, the stable nuclei follow n=p or n/p=1. As the atomic number increases, the stable nuclei tend to have n/p ratio greater than unity which is approximately 1.6. Generally, nuclides lying below or above the stability belt (shaded area) are radioactive.
Another factor in the stability of nuclei is the concept of complete nuclear shell or closed shell configurations for neutrons and protons. This is analogous to the shell theory of electronic structure. Just as complete electronic shells (octet rule) confer stability, so do completed nuclear shells. The closed shell configurations or “magic numbers” are as follows: 2, 8, 20, 50 82, and 126. For heavier elements, closed nuclear shells are predicted for protons (p) = 114 and the neutron number (n) = 184. This would make the doubly closed shell nucleus
and its immediate neighbors (“superheavy” elements) especially stable.Table 1.1
Number of Protons (p) | Number of Neutrons (n) |
2 | 2 |
8 | 8 |
20 | 20 |
28 | 28 |
50 | 50 |
82 | 82 |
114 | 126 |
184 |
The odd-even number of the nucleus also plays an important role in the stability of a nuclide. Nuclei with even numbers of neutrons and protons are most stable; those with odd numbers of both protons and neutrons are least stable. Nuclei for which either proton or neutron number is even are intermediate in stability. The probable explanation for the odd-even term is the so-called spin effect.
Table 1.2
ODD-EVEN NUMBER AND NUCLEAR STABILITY
Atomic mass | even | even | odd | odd |
p | even | odd | even | odd |
n | even | Odd | odd | even |
Number of stable isotopes | 164 | 4 | 55 | 53 |
Examples:
n=126 p=82
This contains magic numbers 82 and 126 and is an even-even combination. Figure 1.1 shows that the nuclide lies within the stability belt. Pb-208 is a stable nuclide.
2. 
n=20 p=20
This contains the magic number 20 and is an even-even combination. The n/p=1 and lies within the stability belt. Ca-40 is a stable nuclide.
3.
3. 
n= 43 p=33
This is an odd-odd combination and the nuclide is outside the stability belt. As-76 is a radioactive nuclide.
When alpha, beta, or positrons are emitted from the nuclei of a radioactive atom, it changes into a nucleus of another element. Scientists refer to this as transformation. Emission of gamma rays results only in a release of energy, not in transformation.
Alpha particles
An alpha particle is simply a helium nuclei (He) which is ejected with high energy from an unstable nucleus. This particle, which consists of two protons and two neutrons, has a net positive charge. Although emitted with high energy, alpha particles lose energy quickly as they pass through matter of air and therefore, do not travel long distances. They can even be stopped by a piece of paper or the outer layers of human skin. These slow moving particles are generally the product of heavier elements.
Example : 23892U ----> 42He + 23490Th
Beta particles
Beta particles are identical to electrons and thus have a charge of (-1). This type of decay process leaves the mass number of the nuclei unchanged. A beta particle is minute in comparison to that of an alpha particle and has about one hundred times the penetrating ability. Where an alpha particle can be stopped by a piece of paper a beta particle can pass right through. It takes aluminum foil or even wood to stop a beta particle. The electron that is released was not present before the decay occured, but was actually created in the decay process itself.
Example : 3215P ----> 0-1e + 3216S
Note that the mass number is unchanged and a new element is formed. So what was the effect of this Beta particle production? It actually changed a neutron into a proton. Notice that this new element will be down and to the right on the zone of stability plot.
Positron
This type of particle production is just the opposite of Beta particle decay.
Example : Na ----> 0 1e + Ne
Notice that is still has the same zero mass as an electron but an opposite charge. This is what is known as an antiparticle of the electron.
What happens when a positron collides with an electron? Annihilation!!
This can be shown by the following reaction:
This can be shown by the following reaction:
Example : 0-1e + 01e ----> 2
Gamma Rays
As the name implies, these are not particles but high energy photons and can be found on the electromagnetic spectrum. They are very similar to x-rays but have a shorter wavelength and therefore more energy. The penetrating ability of gamma rays is much greater than that of alpha or beta particles. They can only be stopped by several centimeters of lead or more than a meter of concrete. In fact, gamma rays can pass right through the human body. Gamma rays often accompany other processes of decay such as alpha or beta. An example of this was our previous representation of an alpha particle process.
23892U ----> 23490Th + 200 + 42He
A ramification of alpha or beta particle production is that the newly formed nucleus is left in a state of excess energy. A way for the nucleus to release this excess energy is by emitting gamma rays. Since gamma rays have no mass, and are waves rather than particles, the elements atomic number does not change after emission.
Fill in the blanks :
12553I ----> 125Xe + 0-1e + 200
22688Ra ---->
+ 42He + 200
+ 42He + 200 Natural Radioactivity
Radioactivity is the spontaneous disintegration of an unstable atomic nucleus and the emission of particles or electromagnetic radiation. All naturally occurring elements with atomic numbers greater than 83, as well as some isotopes of lighter elements, are radioactive. Three different types of radiation are identified.
Alpha particles (a) are helium nuclei, containing two protons and two neutrons. They are deflected slightly in an electric of magnetic field. Their penetrating power is very low, being stoppable by a thin sheet of aluminum or paper.
Beta particles (b) are electrons capable of travelling at speeds approaching the speed of light. Their low mass allows them to be deflected greatly in an electric or magnetic field, in the opposite direction as the deflection of alpha particles. Their high speed gives them greater penetrating power than alpha particles. Some beta particles can penetrate several centimetres of aluminum. (Some refer to beta particles as "beta negative particles", to distinguish them from beta positive particles -- positrons.) Alpha particle emissions and beta particle emissions change the composition of the nucleus.
Gamma rays (g) are high energy electromagnetic radiation with short wavelengths. Gamma rays, unlike alpha and beta particles, do not change the composition of the nuclide. They have the highest penetrating power, being able to penetrate at least 30 centimetres of lead.
Background radiation comes from a variety of radioactive sources. Cosmic rays penetrating the Earth's atmosphere from outer space usually account for less than 25% of background radiation (but this depends on altitude). Minute quantities of naturally occurring radioactive sources in the surroundings (e.g., soil, air, drinking water, building materials, food, etc.) also contribute to background radiation.
Alpha particles (a) are helium nuclei, containing two protons and two neutrons. They are deflected slightly in an electric of magnetic field. Their penetrating power is very low, being stoppable by a thin sheet of aluminum or paper.
Beta particles (b) are electrons capable of travelling at speeds approaching the speed of light. Their low mass allows them to be deflected greatly in an electric or magnetic field, in the opposite direction as the deflection of alpha particles. Their high speed gives them greater penetrating power than alpha particles. Some beta particles can penetrate several centimetres of aluminum. (Some refer to beta particles as "beta negative particles", to distinguish them from beta positive particles -- positrons.) Alpha particle emissions and beta particle emissions change the composition of the nucleus.
Gamma rays (g) are high energy electromagnetic radiation with short wavelengths. Gamma rays, unlike alpha and beta particles, do not change the composition of the nuclide. They have the highest penetrating power, being able to penetrate at least 30 centimetres of lead.
Background radiation comes from a variety of radioactive sources. Cosmic rays penetrating the Earth's atmosphere from outer space usually account for less than 25% of background radiation (but this depends on altitude). Minute quantities of naturally occurring radioactive sources in the surroundings (e.g., soil, air, drinking water, building materials, food, etc.) also contribute to background radiation.
Radioactivity is found in naturally occurring sources and in artificially produced ones. People are constantly being exposed to radiation from a variety of natural and human-created sources. Exposure should be minimized, but it can never be reduced to zero. Dosimetry is the measurement of radiation and the study of its effects on living organisms.
There are several different units used to measure radiation.
The absorbed dose describes the amount of energy deposited per kilogram of exposure time, measured in the gray (Gy).
1 Gy = 1 J / kg = 100 rads (rads are non-SI, but in general use.)
The biological damage produced on a given organism is called the dose equivalent, measured in sieverts (Sv).
1 Sv = 100 rem = 105 mrem
Becquerel (Bq) is the activity of a source produced when one disintegration per second occurs from a radioactive source.
1 Bq = 1 disintegration per second
kBq and MBq are often used to express the radioactivity of a source.
1 curie (Ci) = 3.7 × 1010 Bq
The absorbed dose describes the amount of energy deposited per kilogram of exposure time, measured in the gray (Gy).
1 Gy = 1 J / kg = 100 rads (rads are non-SI, but in general use.)
The biological damage produced on a given organism is called the dose equivalent, measured in sieverts (Sv).
1 Sv = 100 rem = 105 mrem
Becquerel (Bq) is the activity of a source produced when one disintegration per second occurs from a radioactive source.
1 Bq = 1 disintegration per second
kBq and MBq are often used to express the radioactivity of a source.
1 curie (Ci) = 3.7 × 1010 Bq
Biological Effects of Radiation
~The losing of hair quickly and in clumps occurs with radiation exposure at 200 rems or higher.
~Since brain cells do not reproduce, they won't be damaged directly unless the exposure is 5,000 rems or greater. Like the heart, radiation kills nerve cells and small blood vessels, and can cause seizures and immediate death.
~The certain body parts are more specifically affected by exposure to different types of radiation sources. The thyroid gland is susceptible to radioactive iodine. In sufficient amounts, radioactive iodine can destroy all or part of the thyroid. By taking potassium iodide, one can reduce the effects of exposure.
~When a person is exposed to around 100 rems, the blood's lymphocyte cell count will be reduced, leaving the victim more susceptible to infection. This is often refered to as mild radiation sickness. Early symptoms of radiation sickness mimic those of flu and may go unnoticed unless a blood count is done.According to data from Hiroshima and Nagaski, show that symptoms may persist for up to 10 years and may also have an increased long-term risk for leukemia and lymphoma.
~Intense exposure to radioactive material at 1,000 to 5,000 rems would do immediate damage to small blood vessels and probably cause heart failure and death directly.
~Radiation damage to the intestinal tract lining will cause nausea, bloody vomiting and diarrhea. This is occurs when the victim's exposure is 200 rems or more. The radiation will begin to destroy the cells in the body that divide rapidly. These including blood, GI tract, reproductive and hair cells, and harms their DNA and RNA of surviving cells.
~Because reproductive tract cells divide rapidly, these areas of the body can be damaged at rem levels as low as 200. Long-term, some radiation sickness victims will become sterile.
Major Radiation Exposure in Real Life Events
Hiroshima and Nagasaki
Many people at Hiroshima and Nagasaki died not directly from the actual explosion, but from the radiation released as a result of the explosion. For example, a fourteen-year-old boy was admitted to a Hiroshima hospital two days after the explosion, suffering from a high fever and nausea. Nine days later his hair began to fall out. His supply of white blood cells dropped lower and lower. On the seventeenth day he began to bleed from his nose, and on the twenty-first day he died.At Hiroshima and Nagasaki, the few surviving doctors observed symptoms of radiation sickness for the first time. In his book Nagasaki 1945, Dr. Tatsuichiro Akizuki wrote of the puzzling, unknown disease, of symptoms that "suddenly appeared in certain patients with no apparent injuries." Several days after the bombs exploded, doctors learned that they were treating the effects of radiation exposure. "We were now able to label our unknown adversary 'atomic disease' or 'radioactive contamination' among other names. But they were only labels: we knew nothing about its cause or cure... Within seven to ten days after the A-bomb explosion, people began to die in swift succession. They died of the burns that covered their bodies and of acute atomic disease. Innumerable people who had been burnt turned a mulberry color, like worms, and died... The disease," wrote Dr. Akizuki, "destroyed them little by little. As a doctor, I was forced to face the slow and certain deaths of my patients."
Doctors and nurses had no idea of how their own bodies had been affected by radioactivity. Dr. Akizuki wrote, "All of us suffered from diarrhea and a discharge of blood from the gums, but we kept this to ourselves. Each of us thought: tomorrow it might be me... We became stricken with fear of the future." Dr. Akizuki survived, as did several hundred thousand others in or near Hiroshima and Nagasaki. In fact, at least ten people who had fled from Hiroshima to Nagasaki survived both bombs.
The survivors have suffered physically from cataracts, leukemia and other cancers, malformed offspring, and premature aging, and also emotionally, from social discrimination. Within a few months of the nuclear explosions, leukemia began to appear among the survivors at an abnormally high rate. Some leukemia victims were fetuses within their mothers' wombs when exposed to radiation. One child who was born two days after the Hiroshima explosion eventually died of acute leukemia at the age of eighteen. The number of leukemia cases has declined with time, but the incidence of lung cancer, thyroid cancer, breast cancer, and cancers of other organs has increased among the survivors.
Chernobyl
A far more serious accident occured at Chernobyl, in what was then still the Soviet Union. At the time of the accident, the Chernobyl nuclear power station consisted of four operating 1,000 megawatt power reactors. Without question, the accident at Chernobyl was the result of a fatal combination of ignorance and complacency. "As members of a select scientific panel convened immediately after the... accident," writes Nobel laureate Hans Bethe, "my colleagues and I established that the Chernobyl disaster tells us about the deficiencies of the Soviet political and administrative system rather than about problems with nuclear power."Although the problem at Chernobyl was relatively complex, it can basically be summarized as a mismanaged electrical engineering experiment which resulted in the reactor exploding. The explosion was chemical, driven by gases and steam generated by the core runaway, not by nuclear reactions. Flames, sparks, and chunks of burning material were flying into the air above the unit. These were red-hot pieces of nuclear fuel and graphite. About 50 tons of nuclear fuel evaporated and were released by the explosion into the atmosphere. In addition, about 70 tons were ejected sideways from the periphery of the core. Some 50 tons of nuclear fuel and 800 tons of reactor graphite remained in the reactor vault, where it formed a pit reminiscent of a volcanic crater as the graphite still in the reactor had turned up completely in a few days after the explosion.
The resulting radioactive release was equivalent to ten Hiroshimas. In fact, since the Hiroshima bomb was air-burst--no part of the fireball touched the ground--the Chernobyl release polluted the countryside much more than ten Hiroshimas would have done. Many people died from the explosion and even more from the effects of the radiation later. Still today, people are dying from the radiation caused by the Chernobyl accident. The estimated total number of deaths will be 16,000.
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