|Problem Set Activity: Nuclear Chemsitry
Problem Set: Nuclear Chemistry
Objective: The purpose of this exercise is to familiarize you the properties of nuclear processes, to write nuclear equations.
Discussion: Chemical reactions occur when valence electrons of atoms interact with each other. In nuclear processes however, the nucleus of an atom is responsible for the process. Henri Becquerel is given credit for the discovery of radiation when he noted that a photographic plate was exposed from a piece of uranium rock. Soon after Becquerel's discovery, Marie Sklodowska Curie began studying radioactivity and completed much of the pioneering work on nuclear changes. Curie found that radiation was proportional to the amount of radioactive element present, and she proposed that radiation was a property of atoms (as opposed to a chemical property of a compound). Marie Curie was the first woman to win a Nobel Prize and the first person to win two (the first, shared with her husband Pierre and Becquerel for discovering radioactivity; the second for discovering the radioactive elements radium and polonium).
In 1902, Frederick Soddy proposed the theory that "radioactivity is the result of a natural change of an isotope of one element into an isotope of a different element." Nuclear reactions involve changes in particles in an atom's nucleus and thus cause a change in the atom itself. All elements heavier than bismuth (Bi) (and some lighter) exhibit natural radioactivity and thus can "decay" into lighter elements. Unlike normal chemical reactions that form molecules, nuclear reactions result in the transmutation of one element into a different isotope or a different element altogether (remember that the number of protons in an atom defines the element, so a change in protons results in a change in the atom). There are three common types of radiation and nuclear changes:
1. Alpha Radiation (α) is the emission of an alpha particle from an atom's nucleus. An α particle contains two protons and two neutrons (and is similar to a 4He nucleus). When an atom emits an a particle, the atom's atomic mass will decrease by four units (because two protons and two neutrons are lost) and the atomic number (z) will decrease by two units. The element is said to "transmute" into another element that is two z units smaller. An example of an a transmutation takes place when uranium decays into the element thorium (Th) by emitting an alpha particle, as depicted in the following equation:
(Note: in nuclear chemistry, element symbols are traditionally preceded by their atomic weight (upper left) and atomic number (lower left).
2. Beta Radiation (β) is the transmutation of a neutron into a proton and a electron (followed by the emission of the electron from the atom's nucleus: 0-1 e). When an atom emits a β particle, the atom's mass will not change (since there is no change in the total number of nuclear particles), however the atomic number will increase by one (because the neutron transmutated into an additional proton). An example of this is the decay of the isotope of carbon named carbon-14 into the element nitrogen:
3. Gamma Radiation (γ) involves the emission of electromagnetic energy (similar to light energy) from an atom's nucleus. No particles are emitted during gamma radiation, and thus gamma radiation does not itself cause the transmutation of atoms, however γ radiation is often emitted during, and simultaneous to, α or β radioactive decay. X-rays, emitted during the beta decay of cobalt-60, are a common example of gamma radiation.
Radionuclides sometimes go through a series of emission (Radioactive series) before becoming a stable nuclei. The radium series starts from one isotope of uranium, the actinium series from another isotope of uranium, and the thorium series from thorium. The final product of each series, after ten or twelve successive emissions of alpha and beta particles, is a stable isotope of lead. Nuclear disintegration series for U-238 under goes a-emission (blue arrows) and b-emission (red arrows) until it forms stable Pb-206. .
As mentioned previously, radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation. As a radioisotope atom decays to a more stable atom, it emits radiation only once. To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step. However, once the atom reaches a stable configuration, no more radiation is given off. For this reason, radioactive sources become weaker with time. As more and more unstable atoms become stable atoms, less radiation is produced and eventually the material will become non-radioactive. The decay of radioactive elements occurs at a fixed rate. The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material.
For example, a source will have an intensity of 100% when new. At one half-life, its intensity will be cut to 50% of the original intensity. At two half-lives, it will have an intensity of 25% of a new source. After ten half-lives, less than one-thousandth of the original activity will remain. Although the half-life pattern is the same for every radioisotope, the length of a half-life is different. For example, Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days.
The table below summarizes these radiation processes and the properties of the subatomic particles.
Ionizing radiation comes from natural and artificial sources. The energy absorbed from exposure to radiation is called a dose. Absorption of a dose changes the state of a device, and the changes provide measures of the dosages received. Such devices are called dosimeters. Physical, chemical, and biological changes are used as the bases for dosimeters. Radiation effects depend on the type of radiation, and various units are used for dosages.
Radioactive sources emit alpha, beta, or gamma rays. Each type has a unique effect on health of living beings. Strengths of sources are measured in the SI unit Bq (Becquerel), which is the number of disintegration per second, disintegration or decay rate. However, the cgs unit curie (=3.700 x 1010 Bq), is still used in medical and technical practices. For convenience, modifiers have been used for the unit Ci. Decay rates say nothing about energies or type of particles emitted. When neutron and other particles are the sources, the intensity is either expressed as the total number of particles per unit time or the number of particles per unit time per unit area. However, these numbers do not contain information on energy of the beam. For electromagnetic radiation such as laser, the rate of energy emission (watt) of the beam is often specified. No particular unit is used for intensity of X-rays, but the rate of photon emission is similar to the rate of gamma ray emission.
The radiation effect depends on the amount of energy and the type of radiation a person is exposed to. The amount of energy a subject exposed to differ from that absorbed. However, to tell them apart is very difficult. In practice, the reading from a dosimeter represents the amount of energy exposure and absorption. The amount of radiation energy exposed to or absorbed by a subject is called a dose. A roentgen (R) is the dose of X- or g that produce 1 esu (negative and positive each) charge in 1.0 L (at standard temperature and pressure, STP, 298 K and 1 atmosphere) of air. This dose is equivalent to 0.12 erg absorbed by 0.00123 g of mass, or approximately 100 erg in 1.0 g.
For other particles, absorption of 100 erg per gram is called a rad. Rad and roentgen are equivalent, (1 R = 1 rad). The SI dose unit is gray (Gy), which is the absorption of 1.0 J per kg of mass. Thus, 1 Gy = 100 R or rad.
Usually, high-dosage exposures cause symptoms to develop immediately, and low dose exposures have delayed effects.
High-dose Radiation Exposures: From the experiences in industrial and laboratory accidents, atomic bomb explosion in Hiroshima and Nagasaki, atomic and thermonuclear testing grounds, and miscalculated and accidental medical exposures of patients, we have learned the consequences of high-dose radiation exposures.
Injuries due to radiation in the past led the medical profession to divide the radiation clinical cases into four categories. From these categories, we learn to appreciate the level of danger when a whole-body is exposed to various doses.
Low dosage - less than 1 sv (100 rem) Patients under radiological treatments with a one-time whole-body exposure of 14-100 rem showed no particular radiation syndromes and they all recovered well. Few cases showed nausea and vomit. Symptoms and harmful effects vary due to different health conditions of individuals. Data for delayed effects are not reliable..
Medium low dosage - 1-2 sv (100 - 200 rem) Victims receiving 100-200 rem showed nausea and occasional vomiting on the day of exposure or the day after. Itching and burning were felt in the skin after exposure, and these sensations subsided in few days. Two weeks later, however, dermatitis (skin inflammation), itching, burning and pain were severe. More serious ones showed epilation (loss of hair), erythema (abnormal redness due to inflammation), necrosis (tissue death), wet desquamation (peel off), followed by weeping, crusting and ulceration (open sore). Some cases recovered if infections were prevented by medical treatment.
Medium high dosage exposure 2-5 sv (200-450 rem) All victims receiving 200-450 rem showed anorexia (loss of appetite), fatigue, nausea and vomiting, some had diarrhea. These symptoms might persist for months, but some may show signs of recovery. However, the patients in this group were susceptible to infection. Hemorrhage (discharge of blood) in various tissues may happen, and chances of recovery are limited to only a few.
High dosage exposure more than 5 sv (500 rem and more) The human lethal dose (LD50) is generally believed to be 400 to 500 rem, lower if the dose is received in a short time period. Clinically, survival for victims who had received more than 500 rem was impossible. Higher doses had resulted in quick death. Victims would go through stages of disorientation and shock due to injury to central nervous (CN) and cardiovascular systems. On the other hand, some victims overcome infections and they survived after bone marrow transplant.