Potassium-40 (K) is a long lived and the main naturally occurring radioactive isotope of potassium, with a half-life of 1.248 billion years. It makes up about 117 ppm of natural potassium, making that mixture very weakly radioactive; the short life meant this was significantly larger earlier in Earth's history.
Potassium-40 undergoes four different paths of radioactive decay, including all three main types of beta decay:
Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abundant as water vapor (which averages about 4000 ppmv, but varies greatly), 23 times as abundant as carbon dioxide (400 ppmv), and more than 500 times as abundant as neon (18 ppmv). Argon is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust.
Nearly all argon in Earth's atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in Earth's crust. In the universe, argon-36 is by far the most common argon isotope, as it is the most easily produced by stellar nucleosynthesis in supernovas.
Potassium is a chemical element; it has symbol K (from Neo-Latin kalium) and atomic number 19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to form flaky white potassium peroxide in only seconds of exposure. It was first isolated from potash, the ashes of plants, from which its name derives. In the periodic table, potassium is one of the alkali metals, all of which have a single valence electron in the outer electron shell, which is easily removed to create an ion with a positive charge (which combines with anions to form salts). In nature, potassium occurs only in ionic salts. Elemental potassium reacts vigorously with water, generating sufficient heat to ignite hydrogen emitted in the reaction, and burning with a lilac-colored flame. It is found dissolved in seawater (which is 0.04% potassium by weight), and occurs in many minerals such as orthoclase, a common constituent of granites and other igneous rocks.
Potassium is chemically very similar to sodium, the previous element in group 1 of the periodic table. They have a similar first ionization energy, which allows for each atom to give up its sole outer electron. It was first suggested in 1702 that they were distinct elements that combine with the same anions to make similar salts, which was demonstrated in 1807 when elemental potassium was first isolated via electrolysis. Naturally occurring potassium is composed of three isotopes, of which
K is radioactive. Traces of
K are found in all potassium, and it is the most common radioisotope in the human body.
A fermionic condensate (or Fermi–Dirac condensate) is a superfluid phase formed by fermionic particles at low temperatures. It is closely related to the Bose–Einstein condensate, a superfluid phase formed by bosonic atoms under similar conditions. Examples of fermionic condensates include superconductors and the superfluid phase of helium-3. The first fermionic condensate in dilute atomic gases was created by a team led by Deborah S. Jin using potassium-40 atoms at the University of Colorado Boulder in 2003.
Argon (18Ar) has 26 known isotopes, from Ar to Ar, of which three are stable (Ar, Ar, and Ar). On Earth, Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are Ar with a half-life of 302 years, Ar with a half-life of 32.9 years, and Ar with a half-life of 35.01 days. All other isotopes have half-lives of less than two hours, and most less than one minute. Isotopes lighter than Ar decay to chlorine or lighter elements, while heavier ones beta decay to potassium.
The naturally occurring K, with a half-life of 1.248×10 years, decays to stable Ar by electron capture (10.72%) and by positron emission (0.001%), and also to stable Ca via beta decay (89.28%). These properties and ratios are used to determine the age of rocks through potassium–argon dating.
KREEP, an acronym built from the letters K (the atomic symbol for potassium), REE (rare-earth elements) and P (for phosphorus), is a geochemical component of some lunar impact breccia and basaltic rocks. Its most significant feature is somewhat enhanced concentration of a majority of so-called "incompatible" elements (those that are concentrated in the liquid phase during magma crystallization) and the heat-producing elements, namely radioactive uranium, thorium, and potassium (due to presence of the radioactive K).
Isobars are atoms (nuclides) of different chemical elements that have the same number of nucleons. Correspondingly, isobars differ in atomic number (or number of protons) but have the same mass number. An example of a series of isobars is S, Cl, Ar, K, and Ca. While the nuclei of these nuclides all contain 40 nucleons, they contain varying numbers of protons and neutrons.
The term "isobars" (originally "isobares") for nuclides was suggested by British chemist Alfred Walter Stewart in 1918. It is derived from Greek ἴσος (isos) 'equal' and βάρος (baros) 'weight'.