Radioactive Elements
Radioactive elements are substances characterized by the instability of their atomic nuclei, leading to spontaneous decay and the emission of radiation. This decay process can produce various types of emissions, including alpha particles, beta particles, and gamma rays. Most elements have known radioactive isotopes, though only a few naturally occurring ones are radioactive; the majority of radioactive elements are man-made, synthesized through nuclear reactions. The stability of atomic nuclei is influenced by the balance of neutrons and protons, with a significant increase in neutrons relative to protons occurring in heavier elements beyond calcium.
Radioactive decay follows predictable patterns, often described by the concept of half-life, which indicates the time required for half of a radioactive substance to decay. Commonly recognized radioactive elements include uranium, thorium, and carbon-14, with applications ranging from diagnostic medicine to scientific research. Understanding radioactive elements involves not only their properties but also the complex decay chains they can undergo, leading to various isotopes over time. In summary, radioactive elements play a critical role in both natural processes and human technology, making their study essential for advancements in multiple fields.
Radioactive Elements
FIELDS OF STUDY: Inorganic Chemistry; Geochemistry; Metallurgy
ABSTRACT
The basic properties of radioactive elements are described in the context of nuclear composition. Radioisotopes are typically described by their particular half-life, since all radioactive isotopes decompose according to the same rate law.
The Nature of Radioactive Elements
Essentially all elements have known radioactive isotopes. Only a small number of naturally occurring isotopes are radioactive, however. The majority of radioactive elements are man-made elements that have been produced and identified by nuclear synthesis reactions in high-energy particle collision experiments. Some of these are valuable tools in diagnostic medicine and other scientific applications, but most are of value only as empirical data for academic study and theoretical models of atomic behavior. Radioactive elements are characterized by the spontaneous radioactive decay of their unstable atomic nuclei. Radioactive decay can emit a number of different kinds of radiation, but the conversion of one radioactive element into an element having a lower atomic number always requires the emission of protons.
Nuclear Structure of Radioactive Elements
All atoms, except the simplest isotope of hydrogen (protium), have both neutrons and protons in their nuclei. For the first twenty elements, the number of both neutrons and protons are almost exactly equal; for the remaining elements, however, the number of neutrons increases rapidly relative to the number of protons. Each proton bears a single positive electrical charge. This charge is stable in hydrogen atoms but not in any other nucleus. The force of electrostatic repulsion that exists between similar electrical charges would drive the protons apart from each other and destroy the nucleus. The presence of neutrons counteracts this effect or, at the very least, provides a stabilizing factor.
Neutrons themselves are electrically neutral, but advanced studies in particle physics has suggested that the neutron has an internal structure that includes a negative charge. It is theorized that the internal structure of the neutron rearranges in such a way as to interpose its internal negative charge between the positively charged protons in a nucleus. Since the supposed internal structure of the neutron would not be allowed to change in composition, there must then be a point at which the one-to-one ratio of neutrons to protons is not able to provide the necessary stabilization. It appears to be at twenty protons that additional stabilization becomes necessary. Thus, all elements higher than calcium have increasingly more neutrons than protons. The stable element with the highest atomic number is uranium, which occurs naturally as uranium-238. Only a small percentage of naturally occurring uranium has a different atomic weight, that which comprises uranium-235. All known elements having an atomic number higher than polonium (84) are unstable and radioactive. The implication is that there is a limit to the amount of nuclear stabilization that neutrons can provide neutrons; at some point the number of neutrons becomes unstable. (In an ironic reversal, the neutrons that function to stabilize protons above atomic number 20 can themselves apparently no longer be stabilized by the protons when there are more than 125 of them.)
Radioactive Decay Processes
Unstable atomic nuclei decay at an exponential rate described by the mathematical equation
where [A]0 is the amount of a specific material present at some starting point in time, [A]t is the amount of that material remaining after a certain amount of time has passed, t is the amount of time that has passed, and k is the specific rate constant for that material. For radioactive nuclei, the decay rate can be measured directly using a Geiger or scintillation counter. Counting the number of radioactive emissions given off by a specific amount of material over a specific period of time gives a direct measurement of the rate constant for that material. The most generally recognized description of radioactive elements is by the half-life, sometimes called the "lifetime," of that specific isotope. The half-life of the isotope thorium-226, for example, has been determined to be just 30.9 minutes, while the half-life of uranium-235 is 7.1 x 108 years and that of uranium-238 is even longer, at 4.51 x 109 years. The half-life is calculated by defining the ratio of [A]t as exactly half that of [A]0, or
Radioactive nuclei decay by different mechanisms. The type of decay and the observed products of decay offer some indication that the elementary particles (protons, neutrons, and electrons) are themselves composed of even smaller particles, such as quarks and bosons. For several decades, these were just hypothetical constructs proposed in order to fit the mathematics of quantum mechanics. However, in the late twentieth century and early twenty-first century, high-energy experiments have demonstrated that quarks and bosons have physical reality. Their existence makes understandable the conversion of a neutron into a proton accompanied by the emission of an electron or a positron (the positively charged "antimatter" version of the electron).
Nuclei decompose, or decay, by the emission of different kinds of subnuclear particles and energy. Decay of one nucleus to another of smaller atomic mass, such as the conversion of uranium-238 to thorium-234, occurs with the emission of an alpha (α) particle. The α particle consists of two protons and two neutrons and bears two positive charges as a result. Once emitted, the α particle quickly acquires two electrons to become an atom of helium.
The nucleus can also decay with the emission of a beta (β) particle. A β particle is an electron, but it may also be a positron. These are differentiated as β− and β+, respectively. A β particle is typically emitted when a neutron decomposes into a proton, which increases the atomic number of the nucleus but not the atomic mass. This process occurs in the decay of 90thorium-234, and produces 91protactinium-234. A third radioactive emission is the gamma (γ) ray. Gamma rays are high-frequency, or short-wavelength, x-rays. They are emitted from the nucleus rather than from the electron shells surrounding the nucleus.
Radioactive decay processes pose complex problems with regard to the amounts of materials. It is often the case that a short-lived radioisotope decays into an isotope with a much longer half-life. Therefore, a sequential series by which one radioactive isotope decays to a stable nonradioactive isotope of a different element is neither short nor consistent. For example, an isotope having a half-life of just several hours may decay initially to another radioactive isotope having a half-life measured in tens of thousands of years. That isotope, in turn, may decay to yet another radioactive isotope having a shorter or even longer half-life, and so on until a final decay step produces a nonradioactive nucleus. This is similar to considerations of the rate-determining step in a series of chemical reactions.
Occurrence of Radioactive Elements
The list of commonly known radioactive elements is short: uranium, plutonium, carbon-14, and the few that are used in medical treatment and research. A few others occur naturally, including polonium, astatine, radon, thorium, uranium, and protactinium. Uranium, polonium, radium, radon, and similar radioactive isotopes are typically obtained from the ore pitchblende. Uranium can be purified by formation of the volatile compound uranium hexafluoride (UF6), a solid that sublimes into the gas phase at a temperature of just 56 degrees Celsius (133 degrees Fahrenheit). Thorium is mined commercially as the mineral monazite, which is from 3 to 9 percent thorium oxide (ThO2) by weight. Metallic thorium, useful as a fuel for nuclear reactors and a number of other commercial applications, can be produced from thorium oxide by reduction with calcium metal in a thermite-type reaction. Radon, the heaviest of the inert gases, is produced by the radioactive decay of radium. Francium, the heaviest of the alkali metals, is also the most reactive. It is estimated that no more than fifteen grams of francium exist in the entire crust of the planet at any one time, and no bulk amount of the element has ever been available in sufficient quantities to carry out even the simplest chemical tests.
Carbon-14 occurs naturally in the atmosphere as a result of the interaction of atmospheric carbon dioxide with cosmic rays. The incoming cosmic ray particles strike normal carbon-12 nuclei and fuse to produce carbon-14. The normal abundance of carbon-14 is low but constant. Thus, living organisms that consume the glucose and other carbohydrates produced from atmospheric carbon dioxide incorporate a similarly constant amount of carbon-14 into their tissues. When the organism dies, it ceases to maintain the constant carbon-14 level in its tissues, and the radioactive isotope begins to decrease in quantity. The half-life of carbon-14 is fairly precisely known to be 5,730 years, and the amount remaining in an object of organic origin, relative to the amount that would be present in the living organic source, can be used to determine the amount of time that has passed since the organism ceased to take in carbon-14 from the atmosphere.
Essentially all of the elements higher than uranium in the actinide series are synthetic, having been prepared by high-energy physics experiments. Some are known only by the traces left in detection systems from the formation of a mere handful of atoms. All of the hundreds of known isotopes of the lower elements have also been formed by synthetic nuclear reactions in which stable nuclei have been bombarded by beams of high-energy particles in particle accelerators and nuclear reactors.
PRINCIPAL TERMS
- half-life: the length of time required for one-half of a given amount of material to decompose or be consumed through a continuous decay process.
- isotope: an atom of a specific element that contains the usual number of protons in its nucleus but a different number of neutrons.
- man-made element: an element or isotope that does not occur naturally but is synthesized in high-energy particle accelerators by bombarding other elements with streams of nuclear particles.
- radioactive decay: the loss of particles from the nucleus of an unstable atom in the form of ionizing radiation.
- unstable: describes a chemical species with a structure or composition that is prone to spontaneous decomposition.
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