In nature, most elements are comprised of a mixture of isotopes. More than 3,000 additional radioactive isotopes have been artificially created. The approximately ninety naturally occurring elements are estimated to occur as 339 different isotopes, of which roughly 250 are stable and 35 are unstable (radioactive) with extremely long half-lives of millions of years. Twenty-six elements only have one stable element, while tin has the most stable isotopes with ten. Over time, Sr-82 decays to rubidium-82 with a half-life of 25 days, making it suitable for use in generators to provide rubidium-82, the most convenient positron emission tomography agent for heart imaging. Strontium, for example, has four stable isotopes: Sr-84, Sr-86, Sr-87, and Sr-88 and one radioactive isotope, Sr-82. Each radioisotope has a definite half-life, ranging from a fraction of a second (e.g., 215At at 0.1 milliseconds) to billions of years (e.g., 238U at 4.468 billion years).Įlements can have both stable and radioactive isotopes. The rate at which a radioisotope decays is measured by the atom's half-life (t H), which represents the time it takes for half of the material to decay. Often, a sequence of daughter decays called a "decay chain" must complete before a stable nucleus is achieved. During radioactive decay, a "parent" isotope transforms into a "daughter" isotope (possibly of a different element). An isotope is "radioactive" if its nucleus has a probability of spontaneously changing (i.e., radioactively decaying) over time. Most naturally occurring isotopes are stable. Some are referred to as “stable”, meaning their nuclear makeup does not change with time. Isotopes of the same element have nearly identical chemical and physical properties, but their nuclear properties vary, making some invaluable for mankind, while others have no practical value (at least, for the time being). However, since the atomic number "Z" can always be determined by the chemical symbol, it is common practice to only state the mass number (e.g., 14C) and pronounce it as "carbon fourteen." If the letter "m" immediately follows the mass number (e.g., technetium-99m), it indicates a metastable state of the atomic nucleus (i.e., a stable state other than the isotope's lowest-energy ground state). The mass number "A" is indicated with a superscript to the left of the chemical symbol "E" while the atomic number "Z" is indicated with a subscript. Isotopes can also be defined in standard, or "AZE", notation where A is the mass number, Z is the atomic number, and E is the element symbol. ![]() ![]() Most commonly, they are specified by the name or symbol of the particular element, immediately following by a hyphen and the mass number (e.g., carbon-14 or C-14). While most (82.58%) strontium atoms have 50 neutrons, others have anywhere from 44 to 52 neutrons, yet they are still strontium because they have 38 protons. Hydrogen has four additional isotopes ( 4H through 7H) that are manmade and highly unstable.Īnother example of an element with numerous isotopes is strontium. ![]() Hydrogen-2 and Hydrogen-3 have one and two neutrons, respectively, and are often man-made. Hydrogen-1, or protium, is the most prevalent hydrogen isotope, accounting for 99.98% hydrogen atoms, and has no neutrons. It has three naturally occurring isotopes- 1H, 2H, and 3H. Isotopes are atoms of the same element that have the same number of protons (i.e., atomic number, "Z") but a different number of neutrons, meaning that their mass number, "A", varies. There is an equal number of electrons surrounding the nucleus to keep the atom electrically neutral, and these electrons determine the chemical properties of the element-enabling molecules like strontium chloride, SrCl 2, to form bonds from individual strontium and chlorine atoms. The number of protons(i.e., atomic number, "Z") determines the element for example, a strontium nucleus always has 38 protons, and a rubidium nucleus always has 37. Atoms are composed of a cloud of electrons surrounding a dense nucleus that is 100,000 times smaller and comprised of protons and neutrons.
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