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17.1: Radioactive Isotopes

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    Recall that an atom is the smallest component of an element that retains all of the chemical properties of that element (see Matter). As previously discussed, atoms contain uncharged neutrons and positively charged protons in the nucleus. Negatively charged electrons surround the nucleus. The atomic mass of an atom is determined by the number of protons and neutrons because the mass of electrons is negligible. Each proton or neutron weighs 1 atomic mass unit (AMU). The atomic mass values displayed in the periodic table of elements are not whole numbers because they represent the average atomic mass for atoms of that element (figure \(\PageIndex{a}\)). Atoms of the same element do not necessarily have the same mass because they can differ in neutron number. 

    The cells representing hydrogen and uranium from the periodic table of elements.
    Figure \(\PageIndex{a}\): The whole number in each cell of periodic table is the atomic number, or number of protons (1 for hydrogen and 92  for uranium). The atomic mass number is also shown, which is the average number of protons and neutrons (1.01 for hydrogen and 238.03 for uranium). Images cropped and labeled from National Center for Biotechnology Information (public domain).

    Isotopes are different forms of the same element that have the same number of protons, but a different number of neutrons. Some elements, such as carbon, potassium, and uranium, have naturally occurring isotopes. Carbon-12, the most common isotope of carbon, contains six protons and six neutrons. Therefore, it has a mass number of 12 (six protons and six neutrons) and an atomic number of 6 (which makes it carbon). Carbon-14 contains six protons and eight neutrons. Therefore, it has a mass number of 14 (six protons and eight neutrons) and an atomic number of 6, meaning it is still the element carbon. These two alternate forms of carbon are isotopes. Some isotopes are unstable and emit radiation in the form of particles and energy to form more stable elements. Some forms of radiation are dangerous. These are called radioactive isotopes or radioisotopes (figure \(\PageIndex{b}\)). During radioactive decay, one type of atom can change into another type of atom in this way (figure \(\PageIndex{c}\)).

    Models of protium, deuterium, and tritium, which are all isotopes of hydrogen
    Figure \(\PageIndex{b}\): Isotopes of hydrogen. All of these atoms have one proton (pink circle labeled "p+"), but protium has no neutrons, deuterium has one neutron (orange circle labeled "n"), and tritium has two neutrons. The proton and neutron(s) are located in the center of the atom (the nucleus). An electron (blue circle labeled "e-") rotates around each atom nucleus. Image from National Isotope Development Center/U.S. Department of Energy Isotope Program (public domain).
    A carbon-14 atom decays to nitrogen-14, releasing radiation
    Figure \(\PageIndex{c}\): A radioactive isotope of carbon (carbon-14) has six protons and eight neutrons. It decays to a stable isotope of nitrogen (nitrogen-14), which has seven protons and seven neutrons. Radioactive decay releases radiation. (The particular type of radiation that occurs in this example is called beta minus decay, β-.) Carbon-14 decays at a predictable rate, with half of it decaying every 5730 years. Because carbon is abundant in organisms, this predictable decay rate is commonly used for dating fossils. Image from CDC (public domain).


    The half-life is the amount of time that it takes for half of the original radioactive isotope to decay (figure \(\PageIndex{d}\)). For example, the half-life of uranium-238 is about 4.5 billion years. After 4.5 billion years, only half (50%) of the original amount of uranium-238 will remain. The rest will have decayed to thorium-234 (which is also radioactive and quickly decays to a series of radioactive isotopes, until it ultimately becomes lead-206, which is stable; figure \(\PageIndex{e-f}\)). After two half-lives (9 billion years), only half of the 50% would remain (25% of the original). After three half-lives, only 12.5% of the original uranium-238 would remain.

    Graph of the fraction of an original sample that remains after each half-life.
    Figure \(\PageIndex{d}\): After each half life, 50% of a radioactive isotope decays. After one half-life, 50% (1/2; 0.5) of a radioactive isotope remains. After two half-lives, only 25% of the original radioactive isotope remains. After three half-lives, it is 12.5%; four half-lives = 6.25%; five half-lives = 3.125%. Image by Fraknoi, Morrison, and Wolff/OpenStax (CC-BY). Access for free at
    The nucleus of U-238, consisting of many protons and neutrons decays to Th-235 by releasing two protons and two neutrons.
    Figure \(\PageIndex{e}\): When unstable uranium-238 decays, it emits an alpha (α) particle, which is two protons and two neutrons. This changes it into a new element (thorium-234). Image by OpenStax (CC-BY). Download for free at
    The decay chain of uranium-238 is represented by a series of isotopes connected with arrows
    Figure \(\PageIndex{f}\): The decay chain of uranium-238. Each isotopes mass, atomic number, and half-live is listed in order, to the right of the atomic symbol. Each arrow is labeled with the type of radiation released: alpha (α) radiation, which is two protons and two neutrons, or beta (β) radiation, which is a high-energy electron. Decay  continues from uranium until ending at a stable (non-radioactive) isotope, lead (Pb-206). Image by Tosaka (CC-BY).

    Evolution in Action: Carbon Dating

    Carbon-14 (14C) is a naturally occurring radioisotope that is created in the atmosphere by cosmic rays. This is a continuous process, so more 14C is always being created. As a living organism develops, the relative level of 14C in its body is equal to the concentration of 14C in the atmosphere. When an organism dies, it is no longer ingesting 14C, so the ratio will decline. 14C decays to 14N by a process called beta decay; it gives off energy in this slow process (figure \(\PageIndex{c}\)). After approximately 5,730 years, only one-half of the starting concentration of 14C will have been converted to 14N. The time it takes for half of the original concentration of an isotope to decay to its more stable form is called its half-life.

    Because the half-life of 14C is long, it is used to age formerly living objects, such as fossils. Using the ratio of the 14C concentration found in an object to the amount of 14C detected in the atmosphere, the amount of the isotope that has not yet decayed can be determined. Based on this amount, the age of the fossil can be calculated to about 50,000 years (figure \(\PageIndex{g}\) below). Isotopes with longer half-lives, such as potassium-40, are used to calculate the ages of older fossils. Through the use of carbon dating, scientists can reconstruct the ecology and biogeography of organisms living within the past 50,000 years.

    Two men uncover fossil, which appears like ribs buried in the earth.
    Figure \(\PageIndex{g}\): The age of remains that contain carbon and are less than about 50,000 years old, such as this pygmy mammoth, can be determined using carbon dating. (credit: Bill Faulkner/ NPS)

    Nuclear Fission Reactions

    Nuclear fission reactions are those that involve splitting the nucleus of an atom (figure \(\PageIndex{h}\)). They can be induced by blasting radioactive elements with neutrons. As with natural radioactive decay, induced nuclear fission reactions release energy. The heat energy released when nuclear fission can be used to generate electricity. This is the basis of nuclear power. Currently, uranium-235 (235U; an isotope of uranium with an atomic mass of 235) is currently used as fuel for nuclear fission reactions (figure \(\PageIndex{h}\)).

    Nuclear fission shows an atom nucleus splitting. Nuclear fusion shows two smaller nuclei combining.
    Figure \(\PageIndex{h}\): Nuclear fission and fusion are physical processes that produce energy from atoms. Nuclear fission involves splitting the nucleus of an atom, but nuclear fusion involves combining smaller nuclei into a larger one. Image by Sarah Harmann/U.S. Department of Energy (public domain).
    A cluster of red and blue spheres represent a U-235 nucleus, which is hit with a neutron (red sphere) and split
    Figure \(\PageIndex{i}\): Fission of uranium-235 (235U) can be induced by bombarding it with a neutron. This produces U-236, which is unstable and splits into fission fragments and additional neutrons. Energy is released as a result. Red spheres are neutrons, and blue spheres are protons. Image by BC Open Textbooks (CC-BY).


    Modified by Melissa Ha from Matter from Environmental Biology by Matthew R. Fisher (licensed under CC-BY)

    This page titled 17.1: Radioactive Isotopes is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Melissa Ha and Rachel Schleiger (ASCCC Open Educational Resources Initiative) .