Isotopes
Isotopes are variants of chemical elements that have the same number of protons but differing numbers of neutrons in their atomic nuclei. This variation in neutron count leads to differences in atomic mass and can result in the isotopes being stable or unstable (radioactive). For instance, carbon has stable isotopes like carbon-12, which has six neutrons, and a radioactive isotope, carbon-14, which contains eight neutrons. The radioactive isotopes decay over time at a predictable rate, characterized by their half-lives, which can vary widely from fractions of a second to billions of years.
A common application of isotopic decay is carbon dating, a method used to determine the age of organic materials by measuring the remaining carbon-14. Isotopes also play important roles in various fields, such as medicine, where they can be used as tracers to study biological processes or as targeted treatments in radiation therapy. Understanding isotopes enhances our grasp of atomic structure and the behavior of elements, contributing to fields ranging from geology to medicine.
Isotopes
FIELDS OF STUDY: Nuclear Chemistry; Geochemistry
ABSTRACT
The basic characteristics of isotopes are defined in the context of modern atomic theory. Isotopes are characterized by different numbers of neutrons in the nuclei of atoms with the same number of protons. All radioactive isotopes decay exponentially, which can be used to determine the age of materials.
The Nature of Isotopes
An element is any material that consists of one and only one type of atom, identified by the number of protons that are in the nucleus of each atom. This definition does not depend in any way on the number of neutrons that are in the nucleus; indeed, for every element, there are atoms with the same number of protons but different numbers of neutrons. Such atoms are called isotopes. The essential carbon atom, for example, is carbon-12 (12C), which has six protons and six neutrons in the nucleus. Another kind of carbon atom occurs naturally: carbon-14 (14C), in which the nucleus contains eight neutrons, instead of just six, along with the six protons. Due to the extra neutrons, the atomic mass of carbon-14 is 14 atomic mass units (u) instead of 12. The extra neutrons also render the carbon nucleus unstable, causing it to undergo spontaneous fission. In other words, the carbon-14 atom is radioactive. Radioactive isotopes are generally referred to as radioisotopes. The presence of different isotopes that occur naturally in the elements is responsible for the fractional values of atomic masses in the periodic table. These are the atomic weights, or weighted averages of the different atomic masses, according to the naturally occurring proportions of each isotope.
Atomic Structure of the Isotopes
Atoms are defined as having a very small, dense nucleus composed of a fixed number of protons and a variable number of neutrons. These constitute essentially all of the mass of the atom. Each proton bears a single positive charge, and electrostatic repulsion between like charges would drive the protons apart if not for the presence of the neutrons, which bind the protons together in the nucleus and make it stable. Some combinations of neutrons and protons are not stable, however, and such nuclei can undergo spontaneous nuclear fission, with half-lives ranging from millionths of a second (e.g., polonium-212) to several billion years (e.g., uranium-238).
The simplest example of this phenomenon is hydrogen, which is known in three different isotopes, all of which exhibit the normal chemistry associated with hydrogen. The simplest and most abundant, protium, consists of one proton and one electron, with no neutrons in the nucleus. Deuterium, the isotope used to prepare heavy water (D2O), also occurs naturally in about one of every 6,500 hydrogen atoms. Deuterium has one proton and one neutron in its nucleus. The third isotope is tritium, which is synthesized by nuclear reactions, either in the upper atmosphere or in reactors. Tritium has one proton and two neutrons in its nucleus. This unstable arrangement breaks down spontaneously, with one neutron becoming a proton and a beta particle, or high-energy electron, that is then ejected. The result is a stable helium atom.
Half-Lives of Isotopes
All radioactive isotopes decay according to the same exponential rate law. The half-life of an isotope is defined as the time required for half of an amount of the material to decay or be consumed in a process. Mathematically, this is stated as
where k is the disintegration constant of a particular isotope, determined by counting the number of disintegrations that occur in a certain period of time. Carbon dating is perhaps the best-known application of this principle. The isotope carbon-14 is incorporated into living tissues according to its natural abundance; this process ceases when life ceases, and the amount of carbon-14 present in an organic material then decreases at a known rate. Comparing the amount of carbon-14 that remains with the amount that would have been present in the living material determines the number of half-lives that have passed since the tissue was alive, thereby giving its age. A similar method uses the proportions of uranium-238 and the specific isotope of lead that it produces to determine the age of rocks.
Analytical Applications
Isotopes are also used as tracers in various analytical methods. Incorporating a radioactive element into the molecular structure of a bioactive compound allows the analyst to deduce the pathways by which materials are synthesized or metabolized biologically. A radioactive element can also be incorporated into a pharmaceutical compound in order to deliver radiation treatment to specific diseased tissues, as opposed to general radiation.
PRINCIPAL TERMS
- atomic mass: the total mass of the protons, neutrons, and electrons in an individual atom.
- carbon dating: a method of dating that uses the proportion of radioactive carbon-14 atoms remaining in organic material to determine how much time has elapsed since it was part of a living organism.
- deuterium: an isotope of hydrogen that contains one neutron and one proton; occurs naturally in about 1 in 6,500 hydrogen atoms.
- neutron: a fundamental subatomic particle in the atomic nucleus that is electrically neutral and about equal in mass to the mass of one proton.
- protium: the essential form of hydrogen, containing one proton and no neutron; the most common form of matter in the known universe.
- radioisotope: any radioactive isotope of an element that undergoes spontaneous nuclear fission until a stable, nonradioactive isotope is formed.
- tritium: an unstable radioisotope of hydrogen that contains one proton and two neutrons.
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