Noble gases

Download tubes containing noble gases, electrically excited, showing the light emitted

The noble gases are a group of chemical elements with very similar properties: for example, under normal conditions, they are odorless, colorless monatomic gases and have very low chemical reactivity. They are located in group 18 (VIIIA) of the periodic table (previously called group 0). The six gases are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn)

The properties of the noble gases can be explained by modern theories of atomic structure: their electron shell of valent electrons is considered complete, giving them little tendency to participate in chemical reactions, so only a few noble gas compounds have been prepared up to 2008. Xenon reacts spontaneously with fluorine (due to its high electronegativity), and others have been reached from the resulting compounds. Some compounds with krypton have also been isolated. The melting and boiling points of each noble gas are very close, differing by less than 10 °C; consequently, they are only liquid in a very small range of temperatures.

Neon, argon, krypton, and xenon are obtained from air using liquefaction and fractional distillation methods. Helium is typically separated from natural gas and radon is typically isolated from the radioactive decay of dissolved radium compounds. The noble gases have many important applications in industries such as lighting, welding, and space exploration. The combination helium-oxygen-nitrogen (trimix) is used for breathing in deep dives to prevent divers from suffering from the narcotic effect of nitrogen. After seeing the risks caused by the flammability of hydrogen, it was replaced by helium in airships and hot air balloons.

History

Noble Gas is a translation of the German name Edelgas, first used in 1898 by Hugo Erdmann, to indicate its extremely low level of reactivity. The name makes an analogy with the term "noble metals", such as gold, associated with wealth and nobility, and which also has a low reactivity. The noble gases have also been given the name inert gases, but this label has been deprecated as the noble gases have become more widely known. Rare gases is another This term has been used, but is also incorrect because argon makes up a fairly large part (0.94% by volume, 1.3% by mass) of the Earth's atmosphere.

The helium was first detected in the Sun due to its spectral lines.

Pierre Janssen and Joseph Norman Lockyer were the first to discover a noble gas on August 18, 1868 while examining the Sun's chromosphere, and named it helium from the Greek word for Sun, ἥλιος (helios). Formerly, in 1784, the English chemist and physicist Henry Cavendish had discovered that air contained a small proportion of a less reactive substance than nitrogen. A century later, in 1895, Lord Rayleigh discovered that samples of nitrogen in air are of different density than nitrogen as a consequence of chemical reactions. In collaboration with William Ramsay, a scientist at University College London, Lord Rayleigh postulated that the nitrogen extracted from the air was mixed with another gas and carried out an experiment that successfully isolated a new element: argon, a word derived from the Greek ἀργός (argós), "inactive". From this discovery, they noticed a missing class full range of gases on the periodic table. During his search for argon, Ramsay also succeeded in isolating helium for the first time, by heating cleveite, a mineral. In 1902, after accepting the evidence for the existence of the elements helium and argon, Dmitri Mendeleev included these noble gases as Group 0 in his classification of elements, which would later become the periodic table.

Ramsay continued his search for these gases by using the method of fractional distillation to separate liquid air into various components. In 1898, he discovered krypton, neon, and xenon, named after the Greek κρυπτός (kryptós, "hidden"), νέος (néos, "new"), and ξένος (xénos, & #34;strange"), respectively. For its part, radon was identified for the first time in 1898 by Friedrich Ernst Dorn, and was called radium emanation, but it was not considered a noble gas until 1904, when it was determined that its characteristics were similar to those of the other noble gases. That same year, Rayleigh and Ramsay received the Nobel Prize in Physics and Chemistry, respectively, for the discovery of the noble gases.

The discovery of noble gases helped to understand the atomic structure. In 1895, the French chemist Heri Moissan tried unsuccessfully to produce a reaction between fluorine, the most electronegative element, and argon, one of the noble gases, in order to isolate from the atmosphere those gases characterized by their extraordinary chemical inertness, beginning by the one in greater relative abundance, and to create new elements or compounds. Scientists were unable to produce compounds of argon until the late 20th century, but their attempts helped develop new theories of atomic structure. Based on these experiments, the Danish physicist Niels Bohr proposed in 1913 that the electrons in atoms were arranged in electron shells around the nucleus and that in the case of noble gases, except for helium, the outer shell always contained eight electrons. In 1916, Gilbert N. Lewis formulated the octet rule, which concluded that the most stable configuration for any atom is to have eight electrons in the outer shell; this configuration produces elements that do not react with others, since they do not need more electrons to complete their outer shell.

In 1962 Neil Bartlett discovered the first noble gas chemical compound, xenon hexafluoroplatinate. Other noble gas compounds were discovered soon after: radon fluoride in 1962, and krypton difluoride in 1963 (KrF₂). The first stable argon compound was reported in 2000 when argon fluorohydride was formed at a temperature of 40 K (−233.2 °C; −387.7 °F).

In December 1998, scientists at the Joint Institute for Nuclear Research working in Dubna, Russia, bombarded plutonium (Pu) with calcium (Ca) to produce a single atom of element 114, under the name Flerovium (Fl). Preliminary chemical experiments indicate that this element may be the first transuranium element to display abnormal, noble gas-like properties, even though it is a member of group 14 on the periodic table. In October 2006, scientists at the Joint Institute for Nuclear Research and the Lawrence Livermore National Laboratory successfully synthesized oganesson (Og), the seventh element in Group 18, by bombarding californium (Cf) with calcium (Ca). These gases led to the discovery of superconductivity by the Dutch physicist Heike Kamerlingh Onnes.

Physical and Atomic Properties

Noble gases have very weak intermolecular forces and therefore have very low boiling and melting points. They are all monatomic gases under standard conditions, including those that have atomic masses greater than some elements that are normally found in the solid state. Helium has several unique properties compared to other elements: both its boiling and melting points are lower than those of any other known substance; it is the only element known to exhibit superfluidity; likewise it cannot be solidified by cooling under standard conditions, but becomes a solid under a pressure of 25 atm (2,500 kPa; 370 psi) and 0.95 K (−272.20 °C; −457,960 °F).). The noble gases up to xenon have multiple stable isotopes. Radon does not have stable isotopes; its longest-lived isotope has a half-life of 3.8 days that can form helium and polonium.

The atomic radius of the noble gases increases from one period to another due to the increase in the number of electrons. The size of the atom is related to several properties. For example, the ionization potential decreases as the radius increases because the valence electrons in larger atoms are farther from the nucleus and are therefore not as tightly bound by the atom. The noble gases have the highest ionization potentials of each period, reflecting their stable electronic configuration and their lack of chemical reactivity. However, some of the heavier noble gases have ionization potentials low enough to be comparable to those of other elements and molecules. Chemist Neil Bartlett, attempting to create the noble gas compound, noted that the ionization potential of xenon was similar to that of the oxygen molecule, so he attempted to oxidize xenon using platinum hexafluoride, an oxidizing agent so strong that it is capable of to react with oxygen. The noble gases cannot accept an electron to form stable anions. This means that they have a negative electron affinity.

Figure of ionization potential compared to the atomic number. Noble gases have the greatest ionization potential for each period.

The macroscopic physical properties of noble gases are determined by the weak Van der Waals forces that exist between atoms. The attractive forces increase with the size of the atom as a result of the increase in polarizability and the decrease in ionization potential. This leads to systematic group trends. For example, as you go down the groups of the periodic table, the atomic radius and interatomic forces increase. In the same way, higher melting and boiling points, enthalpy of vaporization and solubility are acquired. The increase in density is due to the increase in atomic mass.

The noble gases behave like ideal gases under normal conditions of pressure and temperature, but their abnormal tendencies to the ideal gas law provide important clues to the study of molecular forces and interactions. The Lennard-Jones potential, often used to model intermolecular forces, was deduced in 1924 by John Lennard-Jones from experimental data for argon before the development of quantum mechanics provided the necessary tools to understand intermolecular forces from of first principles. Theoretical analysis of these forces became feasible because the noble gases are monatomic, and therefore isotropic (independent of direction).

Chemical properties

In the first six periods of the periodic table, the noble gases are exactly the members of group 18 (8A) of the table (formerly known as group 0). However, this is no longer true in the seventh period (due to relativistic effects): the next member of group 18, oganesson, is probably not as noble a gas. In contrast, group 14 member Flerovio exhibits similar properties to those of the noble gases.

Noble gases are colorless, odorless, tasteless and non-flammable under normal conditions. In the past, they were assigned group 0 of the periodic table because they were believed to have zero valence, that is, their atoms cannot combine with other elements to form compounds. However, it was later discovered that some do form compounds, causing this designation to be abandoned. Very little is known about the properties of the newest member of group 18, oganesson (oganesson). The noble gases have shells filled with valence electrons. Valence electrons are the outermost electrons in atoms and are usually the only ones involved in chemical bonding. Atoms with valence shells filled with electrons are extremely stable and therefore do not tend to form chemical bonds and have little tendency to gain or lose electrons. However, the heavier noble gases, such as radon, are less firmly bound by electromagnetic force than lighter ones, such as helium, making it easier to remove outer electrons from heavy noble gases. Because such a shell is complete, the noble gases can be used according to electron configuration notation to give a 'noble gas notation'. To do this, first write the nearest noble gas that precedes the element in question, and continue the electronic configuration from that point. For example, the electronic notation for carbon is 1s² 2s² 2p², and its noble gas notation is [He] 2s² 2p². This notation makes it easier to identify elements, and is shorter than writing the full atomic orbital notation.

Compounds

Three-dimensional model of xenon tetrafluoride (XeF₄)

Noble gases have extremely low reactivity; despite this, a large number of noble gas compounds have been formed. Neutral compounds in which helium and neon are present in chemical bonds have not formed (although there is theoretical evidence for some helium compounds), while xenon, krypton, and argon have only low reactivity. reactivity follows the order Ne < I have < Ar < Kr < Xe < rn.

In 1933, Linus Pauling argued that the heavier noble gases could form compounds with fluorine and oxygen. Similarly, he argued for the existence of krypton hexafluoride (KrF₆) and xenon hexafluoride (XeF₆), and speculated that XeF₈ could exist as an unstable compound, also suggesting that xenic acid (H₂XeO₄) could form perxenate salts. These predictions have been shown to be generally true. accurate, except that XeF₈ is currently believed to be thermodynamically and kinetically unstable. Xenon compounds are the most numerous of the noble gas compounds ever formed. Most of them have the xenon atom in the +2, +4, +6 or +8 oxidation state attached to highly electronegative atoms such as fluorine or oxygen, as in xenon fluoride (XeF₂), xenon tetrafluoride (XeF_4nmnvb), xenon hexafluoride (XeF₆), xenon tetroxide (XeO₄) and perxenate sodium (Na₄XeO₆). Some of these compounds have been used in chemical synthesis as oxidizing agents; XeF₂, in particular, is commercially available and can be used as a fluorinating agent. As of 2007, some five hundred xenon compounds bound to other elements had been identified, including organoxenone (carbon-bonded) compounds., as well as xenon bound 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 temperatures low in noble gas matrices, or in jet streams of noble gases.

In theory, radon is more reactive than xenon, and therefore should form chemical bonds more easily than xenon. However, due to the high radioactivity and short half-life of the radon isotopes, only a few fluorides and oxides of radon have been formed in practice. Krypton is less reactive than xenon, but various compounds have been observed with krypton in the +2 oxidation state. Krypton difluoride is the most notable and easy to characterize. Compounds in which krypton forms a single bond with nitrogen and oxygen have also been characterized, but are only stable below −60 °C and −90 °C, respectively. Krypton atoms have been observed chemically bonding to other nonmetals (hydrogen, chlorine, carbon), as well as some late transition metals (copper, silver, gold), but only or at low temperatures. Similar conditions were used to obtain the first few compounds of argon in 2000, such as argon fluorohydride (HArF), and some attached to late transition metals. As of 2007, no stable neutral molecules with covalently bonded neon or helium atoms were known.

Noble gases, including helium, can form stable molecular ions in the gas phase. The simplest is hydrohelium, HeH⁺, discovered in 1925. Composed of the two most abundant elements in the universe, hydrogen and helium, it is believed to occur naturally in the interstellar medium., although it has not yet been detected. In addition to these ions, there are many known neutral excimers of these gases. There are compounds like ArF and KrF that are only stable when in an excited electronic state, and some of them are used in excimer lasers.

In addition to compounds in which a noble gas atom is involved in a covalent bond, noble gases also form non-covalent compounds. Clathrates, first described in 1949, consist of a noble gas atom trapped within cavities in the crystal structure of certain organic and inorganic substances. The essential condition for them to form is that the guest atoms (those of the noble gas) must be of the right size to fit into the cavities of the host's crystalline structure. For example, argon, krypton, and xenon form clathrates with hydroquinone, but helium and neon do not, as they are too small or have insufficient polarizability to be retained. Neon, argon, krypton, and xenon they also form clathrate hydrates; this means that the noble gases are trapped within the helium shell of these compounds.

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 C₆₀, a spherical molecule composed of 60 carbon atoms, noble gases, is exposed to elevated pressure, complexes such as He@C₆₀ (@ indicates that He is contained within C₆₀, but not covalently bound). Endohedral complexes with helium, neon, argon, krypton, and xenon were obtained in 2008. These compounds are used in the study of the structure and reactivity of fullerenes using nuclear magnetic resonance of the noble gas atom.

Noble gas compounds, such as xenon difluoride (XeF₂), are considered to be hypervalent, as they violate the octet rule. Bonding in these compounds can be explained with a three-center, four-electron model. This model, first proposed in 1951, considers the bonding of three collinear atoms. For example, the bonds of XeF₂ are described by a set of three molecular orbitals derived from the p orbitals of each atom. Bonds result from the combination of a p orbital of Xe with a half-filled p orbital of each F atom, resulting in a filled bonding orbital, an unfilled bonding orbital, and an antibonding orbital. The highest occupied molecular orbital is found on the two terminal atoms. This represents a localization of charge facilitated by the high electronegativity of fluorine. The chemistry of the heavier noble gases, krypton and xenon, is well determined. That of the lightest, helium and argon, is still in an early state, while no neon compound has yet been identified.

Abundance and production

The abundance of noble gases in the universe decreases as their atomic number increases. Helium is the most common element in the universe after hydrogen, with a mass ratio of about 24%. Most of the helium in the universe was formed during primordial nucleosynthesis, but the amount of helium is constantly increasing due to hydrogen fusion in stellar nucleosynthesis (a process carried out by nuclear reactions that originated in stars during their evolutionary process, and that precedes a supernova by gravitational collapse). The abundance on Earth shows 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, since due to the small mass of this atom, helium cannot be retained by the Earth's gravitational field. Earth's helium derives from the alpha decay of heavy elements such as uranium or thorium in the earth's crust, and tends to accumulate in natural gas deposits. Argon, on the other hand, increases in abundance as a result of the alpha decay of potassium-40, which is also found in the earth's crust, to form argon-40, which is the most abundant argon isotope on Earth despite being relatively rare in the solar system. This process is the basis of the potassium-argon dating method. Xenon has a relatively low abundance in the atmosphere, which has become known as the "vanishing xenon problem"; one theory is that the missing xenon could be trapped in minerals within the Earth's crust. Radon is formed in the lithosphere by the alpha decay of radium. It can seep into buildings through foundations and accumulate in poorly ventilated areas. Due to its high radioactivity, radon poses a significant health risk; In the United States alone, it is associated with an estimated 21,000 lung cancer deaths each year.

Neon, argon, krypton, and xenon are obtained from air using the methods of gas liquefaction, to convert the elements to a liquid state, and fractional distillation, to separate the mixtures into their components. Helium is generally produced by separating it from natural gas, and radon is isolated from the radioactive decay of radium compounds. The price of the noble gases is influenced by their natural abundance, with argon being the cheapest and xenon being the most expensive. expensive. This is illustrated in the table on the right, with prices in 2004 USD for laboratory amounts of each gas.

Uses

Today liquid helium is used to cool superconductor magnets in magnetic resonance imaging scanners.

The noble gases have very low boiling and melting points, making them useful as cryogenic refrigerants. In particular, liquid helium, which boils at 4.2 K, is used for superconducting magnets, such as which are used for magnetic resonance imaging and nuclear magnetic resonance. Liquid neon, although it does not reach temperatures as low as liquid helium, also has applications in cryogenics, since it has a refrigeration capacity more than 40 times higher that of liquid helium and more than three times that of liquid hydrogen.

Helium is used as a component of breathable gases to replace nitrogen, thanks to its low solubility in fluids, especially lipids. Gases are absorbed into the blood and body tissues when there is pressure, such as in scuba diving, causing an anesthetic effect known as "depth sickness." Due to its low solubility, little helium enters cell membranes, and when helium is used to replace part of the breathing gases, such as trimix or heliox, a reduction in the narcotic effect of the gas at depth is achieved. The low solubility of helium offers further advantages for the condition known as decompression sickness. Less dissolved gas in the body means fewer gas bubbles are formed during the pressure drop during ascent. Another noble gas, argon, is considered the best choice as a dry suit inflation gas in scuba diving.

Since the Hindenburg disaster of 1937, helium has replaced hydrogen as the lift gas in airships and balloons, thanks to its lightness and non-combustibility, despite a reduction in buoyancy of 8.6%. In many applications, noble gases are used to form an inert atmosphere. Argon is used in the synthesis of air-sensitive compounds that are, at the same time, sensitive to nitrogen. Solid argon is also used to study highly unstable compounds, as reactive intermediates, by trapping them in an inert matrix at very low temperatures. Helium is used as a carrier medium in gas chromatography, as a filler gas in thermometers, and in devices to measure radiation, such as the Geiger counter and the bubble chamber. Both helium and argon are commonly used to shield welding arcs and the surrounding base metal from the atmosphere during welding and gouging, as well as in other metallurgical processes and the production of silicon for the semiconductor industry.

Noble gases are commonly used for lighting due to their lack of chemical reactivity. Argon, mixed with nitrogen, is used as a filler gas for incandescent light bulbs. Krypton is used in high-output light bulbs, which have a higher color temperature and higher efficacy by slowing the filament's evaporation rate. rather than argon, halogen lamps, in particular, use krypton mixed with small amounts of iodine or bromine compounds. The noble gases have characteristic colors when used in discharge lamps, such as neon headlamps, which They produce an orange-red color. Xenon is commonly used in xenon headlamps which, due to their nearly continuous spectrum resembling daylight, are used in movie projectors and as automobile headlights.

Noble gases are used in excimer lasers, which are based on short-lived, electronically excited molecules known as excimers. The excimers used in lasers can be dimers of noble gases such as Ar ₂, Kr ₂ or Xe ₂, or more commonly, the noble gas it is combined with a halogen in excimers such as ArF, KrF, XeF or XeCl. These lasers produce ultraviolet light which, due to its short wavelength (193 nm for ArF and 248 nm for KrF), allows for highly accurate imaging. Excimer lasers have many industrial, medical, and scientific uses. They are used in microlithography and microfabrication, essential for integrated circuit manufacturing, and for laser surgery, including laser angioplasty and eye surgery. Some noble gases have direct use in medicine. Helium is sometimes used to improve ease of breathing in asthma patients. Xenon is used as an anesthetic due to its high lipid solubility, which makes it more potent than the usual nitrous oxide, and because it is easily removed by the body, allows for faster recovery. Nuclear magnetic resonance imaging uses xenon in combination with other gases. Radon, which is highly radioactive and only available in trace amounts, is useful in radiotherapy treatment.

Dump Color

• Colours produced by the different noble gases in neon tubes
• 

  Helio

• 

  Neon

• 

  Argon (with a mercury translucent)

• 

  Krypton

• 

  Xenón

Noble gas fluorescence with a power supply of 5 kV, 20 mA and 25 kHz

The color of the gas discharge emission depends on several factors, including the following:

• discharge parameters (local value of current density and electric field, temperature, etc;
• purity of gas (even a small fraction of certain gases can affect color);
• material of the discharge tube wrapping.

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Notes and references