Noble gas

Noble gas is translated from the German noun , first used in 1900 by Hugo Erdmann to indicate their extremely low level of reactivity. The name makes an analogy to the term "noble metals", which also have low reactivity. The noble gases have also been referred to as inert gases, but this label is deprecated as many noble gas compounds are now known. Rare gases is another term that was used, but this is also inaccurate because argon forms a fairly considerable part (0.94% by volume, 1.3% by mass) of the Earth's atmosphere due to decay of radioactive potassium-40. s, Helium was discovered to exist on the Sun before it was discovered on Earth.|alt=A line spectrum chart of the visible spectrum showing sharp lines on top. Pierre Janssen and Joseph Norman Lockyer had discovered a new element on 18 August 1868 while looking at the chromosphere of the Sun, and named it helium after the Greek word for the Sun, (). No chemical analysis was possible at the time, but helium was later found to be a noble gas. Before them, in 1784, the English chemist and physicist Henry Cavendish had discovered that air contains a small proportion of a substance less reactive than nitrogen. A century later, in 1895, Lord Rayleigh discovered that samples of nitrogen from the air were of a different density than nitrogen resulting from chemical reactions. Along with Scottish scientist William Ramsay at University College, London, Lord Rayleigh theorized that the nitrogen extracted from air was mixed with another gas, leading to an experiment that successfully isolated a new element, argon, from the Greek word (, "idle" or "lazy"). Ramsay continued his search for these gases using the method of fractional distillation to separate liquid air into several components. In 1898, he discovered the elements krypton, neon, and xenon, and named them after the Greek words (, "hidden"), (, "new"), and (, "stranger"), respectively. Radon was first identified in 1898 by Friedrich Ernst Dorn, and was named radium emanation, but was not considered a noble gas until 1904 when its characteristics were found to be similar to those of other noble gases. Rayleigh and Ramsay received the 1904 Nobel Prizes in Physics and in Chemistry, respectively, for their discovery of the noble gases; in the words of J. E. Cederblom, then president of the Royal Swedish Academy of Sciences, "the discovery of an entirely new group of elements, of which no single representative had been known with any certainty, is something utterly unique in the history of chemistry, being intrinsically an advance in science of peculiar significance". In 1962, Neil Bartlett discovered the first chemical compound of a noble gas, xenon hexafluoroplatinate. Compounds of other noble gases were discovered soon after: in 1962 for radon, radon difluoride (), which was identified by radiotracer techniques and in 1963 for krypton, krypton difluoride (). The first stable compound of argon was reported in 2000 when argon fluorohydride (HArF) was formed at a temperature of . In October 2006, scientists from the Joint Institute for Nuclear Research and Lawrence Livermore National Laboratory successfully created synthetically oganesson, the seventh element in group 18, by bombarding californium with calcium. ==Physical and atomic properties==

The noble gases have weak interatomic force, and consequently have very low melting and boiling points. They are all monatomic gases under standard conditions, including the elements with larger atomic masses than many normally solid elements. – a pressure of must be applied at a temperature of to convert it to a solid while a pressure of about is required at room temperature. The noble gases up to xenon have multiple stable isotopes; krypton and xenon also have naturally occurring radioisotopes, namely 78Kr, 124Xe, and 136Xe, all have very long lives (> 1021 years) and can undergo double electron capture or double beta decay. Radon has no stable isotopes; its longest-lived isotope, 222Rn, has a half-life of 3.8 days and decays to form helium and polonium, which ultimately decays to lead. The macroscopic physical properties of the noble gases are dominated by the weak van der Waals forces between the atoms. The attractive force increases with the size of the atom as a result of the increase in polarizability and the decrease in ionization potential. This results in systematic group trends: as one goes down group 18, the atomic radius increases, and with it the interatomic forces increase, resulting in an increasing melting point, boiling point, enthalpy of vaporization, and solubility. The increase in density is due to the increase in atomic mass. The noble gases are nearly ideal gases under standard conditions, but their deviations from the ideal gas law provided important clues for the study of intermolecular interactions. The Lennard-Jones potential, often used to model intermolecular interactions, was deduced in 1924 by John Lennard-Jones from experimental data on argon before the development of quantum mechanics provided the tools for understanding intermolecular forces from first principles. The theoretical analysis of these interactions became tractable because the noble gases are monatomic and the atoms spherical, which means that the interaction between the atoms is independent of direction, or isotropic. ==Chemical properties==

. Noble gases have eight electrons in their outermost shell, except in the case of helium, which has two.|alt=An atomic shell diagram with neon core, 2 electrons in the inner shell and 8 in the outer shell. The noble gases are colorless, odorless, tasteless, and nonflammable under standard conditions. They were once labeled group 0 in the periodic table because it was believed they had a valence of zero, meaning their atoms cannot combine with those of other elements to form compounds. However, it was later discovered some do indeed form compounds, causing this label to fall into disuse. However, heavier noble gases such as radon are held less firmly together by electromagnetic force than lighter noble gases such as helium, making it easier to remove outer electrons from heavy noble gases. As a result of a full shell, the noble gases can be used in conjunction with the electron configuration notation to form the noble gas notation. To do this, the nearest noble gas that precedes the element in question is written first, and then the electron configuration is continued from that point forward. For example, the electron notation of phosphorus is , while the noble gas notation is . This more compact notation makes it easier to identify elements, and is shorter than writing out the full notation of atomic orbitals. The noble gases cross the boundary between blocks—helium is an s-element whereas the rest of members are p-elements—which is unusual among the IUPAC groups. All other IUPAC groups contain elements from one block each. This causes some inconsistencies in trends across the table, and on those grounds some chemists have proposed that helium should be moved to group 2 to be with other s2 elements, but this change has not generally been adopted. Compounds (), one of the first noble gas compounds to be discovered|alt=A model of planar chemical molecule with a blue center atom (Xe) symmetrically bonded to four peripheral atoms (fluorine). The noble gases show extremely low chemical reactivity; consequently, only a few hundred noble gas compounds have been formed. Neutral compounds in which helium and neon are involved in chemical bonds have not been formed (although some helium-containing ions exist and there is some theoretical evidence for a few neutral helium-containing ones), while xenon, krypton, and argon have shown only minor reactivity. In 1933, Linus Pauling predicted that the heavier noble gases could form compounds with fluorine and oxygen. He predicted the existence of krypton hexafluoride () and xenon hexafluoride () and speculated that xenon octafluoride () might exist as an unstable compound, and suggested that xenic acid could form perxenate salts. These predictions were shown to be generally accurate, except that is now thought to be both thermodynamically and kinetically unstable. Xenon compounds are the most numerous of the noble gas compounds that have been formed. Most of them have the xenon atom in the oxidation state of +2, +4, +6, or +8 bonded to highly electronegative atoms such as fluorine or oxygen, as in xenon difluoride (), xenon tetrafluoride (), xenon hexafluoride (), xenon tetroxide (), and sodium perxenate (). Xenon reacts with fluorine to form numerous xenon fluorides according to the following equations: ::Xe + F2 → XeF2 ::Xe + 2F2 → XeF4 ::Xe + 3F2 → XeF6 Some of these compounds have found use in chemical synthesis as oxidizing agents; , in particular, is commercially available and can be used as a fluorinating agent. As of 2007, about five hundred compounds of xenon bonded to other elements have been identified, including organoxenon compounds (containing xenon bonded to carbon), and xenon bonded to nitrogen, chlorine, gold, mercury, and xenon itself. Compounds of xenon bound to boron, hydrogen, bromine, iodine, beryllium, sulfur, titanium, copper, and silver have also been observed but only at low temperatures in noble gas matrices, or in supersonic noble gas jets. Radon goes further towards metallic behavior than xenon; the difluoride RnF2 is highly ionic, and cationic Rn2+ is formed in halogen fluoride solutions. For this reason, kinetic hindrance makes it difficult to oxidize radon beyond the +2 state. Only tracer experiments appear to have succeeded in doing so, probably forming RnF4, RnF6, and RnO3. Krypton is less reactive than xenon, but several compounds have been reported with krypton in the oxidation state of +2. but are only stable below and respectively. Oganesson is expected to be rather like silicon or tin in group 14: a reactive element with a common +4 and a less common +2 state, which at room temperature and pressure is not a gas but rather a solid semiconductor. Empirical / experimental testing will be required to validate these predictions. (On the other hand, flerovium, despite being in group 14, is predicted to be unusually volatile, which suggests noble gas-like properties.) The noble gases—including helium—can form stable molecular ions in the gas phase. The simplest is the helium hydride molecular ion, HeH+, discovered in 1925. Because it is composed of the two most abundant elements in the universe, hydrogen and helium, it was believed to occur naturally in the interstellar medium, and it was finally detected in April 2019 using the airborne SOFIA telescope. In addition to these ions, there are many known neutral excimers of the noble gases. These are compounds such as ArF and KrF that are stable only when in an excited electronic state; some of them find application in excimer lasers. In addition to the compounds where a noble gas atom is involved in a covalent bond, noble gases also form non-covalent compounds. The clathrates, first described in 1949, consist of a noble gas atom trapped within cavities of crystal lattices of certain organic and inorganic substances. The essential condition for their formation is that the guest (noble gas) atoms must be of appropriate size to fit in the cavities of the host crystal lattice. For instance, argon, krypton, and xenon form clathrates with hydroquinone, but helium and neon do not because they are too small or insufficiently polarizable to be retained. Neon, argon, krypton, and xenon also form clathrate hydrates, where the noble gas is trapped in ice. Noble gases can form endohedral fullerene compounds, in which the noble gas atom is trapped inside a fullerene molecule. In 1993, it was discovered that when , a spherical molecule consisting of 60 carbon atoms, is exposed to noble gases at high pressure, complexes such as can be formed (the @ notation indicates He is contained inside but not covalently bound to it). As of 2008, endohedral complexes with helium, neon, argon, krypton, and xenon have been created. These compounds have found use in the study of the structure and reactivity of fullerenes by means of the nuclear magnetic resonance of the noble gas atom. Noble gas compounds such as xenon difluoride () are considered to be hypervalent because they violate the octet rule. Bonding in such compounds can be explained using a three-center four-electron bond model. This model, first proposed in 1951, considers bonding of three collinear atoms. For example, bonding in is described by a set of three molecular orbitals (MOs) derived from p-orbitals on each atom. Bonding results from the combination of a filled p-orbital from Xe with one half-filled p-orbital from each F atom, resulting in a filled bonding orbital, a filled non-bonding orbital, and an empty antibonding orbital. The highest occupied molecular orbital is localized on the two terminal atoms. This represents a localization of charge that is facilitated by the high electronegativity of fluorine. The chemistry of the heavier noble gases, krypton and xenon, are well established. The chemistry of the lighter ones, argon and helium, is still at an early stage, while a neon compound is yet to be identified. ==Occurrence==

The abundances of the noble gases in the universe decrease as their atomic numbers increase. Helium is the most common element in the universe after hydrogen, with a mass fraction of about 24%. Most of the helium in the universe was formed during Big Bang nucleosynthesis, but the amount of helium is steadily increasing due to the fusion of hydrogen in stellar nucleosynthesis (and, to a very slight degree, the alpha decay of heavy elements). Abundances on Earth follow different trends; for example, helium is only the third most abundant noble gas in the atmosphere. The reason is that there is no primordial helium in the atmosphere; due to the small mass of the atom, helium cannot be retained by the Earth's gravitational field. Helium on Earth comes from the alpha decay of heavy elements such as uranium and thorium found in the Earth's crust, and tends to accumulate in natural gas deposits. Xenon has an unexpectedly low abundance in the atmosphere, in what has been called the missing xenon problem; one theory is that the missing xenon may be trapped in minerals inside the Earth's crust. Radon is formed in the lithosphere by the alpha decay of radium. It can seep into buildings through cracks in their foundation and accumulate in areas that are not well ventilated. Due to its high radioactivity, radon presents a significant health hazard; it is implicated in an estimated 21,000 lung cancer deaths per year in the United States alone. Oganesson does not occur in nature and is instead created manually by scientists. For large-scale use, helium is extracted by fractional distillation from natural gas, which can contain up to 7% helium. == Extraction ==

Neon, argon, krypton, and xenon are obtained from air using the methods of liquefaction of gases, to convert elements to a liquid state, and fractional distillation, to separate mixtures into component parts. Helium is typically produced by separating it from natural gas, and radon is isolated from the radioactive decay of radium compounds. The prices of the noble gases are influenced by their natural abundance, with argon being the cheapest and xenon the most expensive. As an example, the adjacent table lists the 2004 prices in the United States for laboratory quantities of each gas. ==Biological chemistry==

None of the elements in this group has any biological importance. ==Applications==

Noble gases have very low boiling and melting points, which makes them useful as cryogenic refrigerants. In particular, liquid helium, which boils at , is used for superconducting magnets, such as those needed in nuclear magnetic resonance imaging and nuclear magnetic resonance. Liquid neon, although it does not reach temperatures as low as liquid helium, also finds use in cryogenics because it has over 40 times more refrigerating capacity than liquid helium and over three times more than liquid hydrogen. Due to its reduced solubility, little helium is taken into cell membranes, and when helium is used to replace part of the breathing mixtures, such as in trimix or heliox, a decrease in the narcotic effect of the gas at depth is obtained. Helium's reduced solubility offers further advantages for the condition known as decompression sickness, or the bends. The reduced amount of dissolved gas in the body means that fewer gas bubbles form during the decrease in pressure of the ascent. Another noble gas, argon, is considered the best option for use as a drysuit inflation gas for scuba diving. Helium is also used as filling gas in nuclear fuel rods for nuclear reactors. Since the Hindenburg disaster in 1937, helium has replaced hydrogen as a lifting gas in blimps and balloons: despite an 8.6% Helium is used as the carrier medium in gas chromatography, as a filler gas for thermometers, and in devices for measuring radiation, such as the Geiger counter and the bubble chamber. Krypton is used in high-performance light bulbs, which have higher color temperatures and greater efficiency, because it reduces the rate of evaporation of the filament more than argon; halogen lamps, in particular, use krypton mixed with small amounts of compounds of iodine or bromine. Some noble gases have direct application in medicine. Helium is sometimes used to improve the ease of breathing of people with asthma. Xenon finds application in medical imaging of the lungs through hyperpolarized MRI. Radon, which is highly radioactive and is only available in minute amounts, is used in radiotherapy. They can unravel the Earth's degassing history and its effects to the surrounding environment (i.e., atmosphere composition). Due to their inert nature and low abundances, change in the noble gas concentration and their isotopic ratios can be used to resolve and quantify the processes influencing their current signatures across geological settings. Helium Helium has two abundant isotopes: helium-3, which is primordial with high abundance in earth's core and mantle, and helium-4, which originates from decay of radionuclides (232Th, 235,238U) abundant in the earth's crust. Isotopic ratios of helium are represented by RA value, a value relative to air measurement (3He/4He = 1.39*10−6). Volatiles that originate from the earth's crust have a 0.02-0.05 RA, which indicate an enrichment of helium-4. Volatiles that originate from deeper sources such as subcontinental lithospheric mantle (SCLM), have a 6.1± 0.9 RA and mid-oceanic ridge basalts (MORB) display higher values (8 ± 1 RA). Mantle plume samples have even higher values than > 8 RA. Solar wind, which represent an unmodified primordial signature is reported to have ~ 330 RA. Neon Neon has three main stable isotopes:20Ne, 21Ne and 22Ne, with 20Ne produced by cosmic nucleogenic reactions, causing high abundance in the atmosphere. 21Ne and 22Ne are produced in the earth's crust as a result of interactions between alpha and neutron particles with light elements; 18O, 19F and 24,25Mg. The neon ratios (20Ne/22Ne and 21Ne/22Ne) are systematically used to discern the heterogeneity in the Earth's mantle and volatile sources. Complimenting He isotope data, neon isotope data additionally provide insight to thermal evolution of Earth's systems. Argon Argon has three stable isotopes: 36Ar, 38Ar and 40Ar. 36Ar and 38Ar are primordial, with their inventory on the earth's crust dependent on the equilibration of meteoric water with the crustal fluids. 40Ar is a product of radiogenic decay of 40K. Different endmembers values for 40Ar/36Ar have been reported; Air = 295.5, MORB = 40,000, Krypton isotopes have been used to decipher the mechanism of volatiles delivery to earth's system, which had great implication to evolution of earth (nitrogen, oxygen, and oxygen) and emergence of life. This is largely due to a clear distinction of krypton isotope signature from various sources such as chondritic material, solar wind and cometary. Xenon Xenon has nine isotopes, most of which are produced by the radiogenic decay. Krypton and xenon noble gases requires pristine, robust geochemical sampling protocol to avoid atmospheric contamination. Furthermore, sophisticated instrumentation is required to resolve mass peaks among many isotopes with narrow mass difference during analysis. Sampling of noble gases Noble gas measurements can be obtained from sources like volcanic vents, springs, and geothermal wells following specific sampling protocols. The classic specific sampling protocol include the following. • Copper tubes - These are standard refrigeration copper tubes, cut to ~10 cm3 with a 3/8" outer diameter, and are used for sampling volatile discharges by connecting an inverted funnel to the tube via TygonⓇ tubing, ensuring one-way inflow and preventing air contamination. Their malleability allows for cold welding or pinching off to seal the ends after sufficient flushing with the sample. • Giggenbach bottles – Giggenbach bottles are evacuated glass flasks with a Teflon stopcock, used for sampling and storing gases. They require pre-evacuation before sampling, as noble gases accumulate in the headspace. These bottles were first invented and distributed by a Werner F. Giggenbach, a German chemist. Analysis of noble gases Noble gases have numerous isotopes and subtle mass variation that requires high-precision detection systems. Originally, scientists used magnetic sector mass spectrometry, which is time-consuming and has low sensitivity due to "peak jumping mode". Multiple-collector mass spectrometers, like Quadrupole mass spectrometers (QMS), enable simultaneous detection of isotopes, improving sensitivity and throughput. Cryogenic traps are used for sequential analysis without peak interference by stepwise temperature raise. Research labs have successfully developed miniaturized field-based mass spectrometers, which can analyze noble gases with an analytical uncertainty of 1–3% using low-cost vacuum systems and quadrupole mass analyzers. ==See also==