Why are noble gases mentioned there?
As Noble gases the elements of the 8th main group of the periodic table are called (formerly also: zero group; according to the newer numbering of the IUPAC: group 18). There are the elements: helium, neon, argon, krypton, xenon and radon and ununoctium
All noble gases are colorless, odorless, non-flammable and hardly water-soluble gases. They occur atomically instead of molecularly, since they are chemically almost impossible to form compounds. The reason for this is that the energy levels (outdated: “shells”) of the atom are closed (i.e. completely filled with electrons). For a more precise understanding, one needs quantum mechanics; a few noble gas compounds (such as XePtF6) and inclusion compounds (clathrates) could be generated in the meantime.
After the discovery of the previously unknown helium line in the spectrum of sunlight, the noble gases were discovered by Sir William Ramsay between 1894 and 1905 - helium together with John William Strutt, 3rd Baron Rayleigh, krypton, neon and xenon together with Morris William Travers - and classified in the periodic table. In 1904 he received the Nobel Prize in Chemistry for this. We owe this discovery not only to numerous possible applications of the noble gases in lighting, welding and space technology, but also to the discovery of the periodically repeating properties of the elements. This led to the development of the "Periodic Table of the Elements" and an understanding of the relationships between the structure of atoms and the chemical properties of the elements that make up the matter of our universe. Chemically almost non-reactive substances thus became the basis of chemical knowledge and research results in modern chemistry.
Occurrence and representation
After hydrogen, helium is the second most abundant element in the universe. See the Main ProductsCosmochemistry
Noble gases occur naturally in the earth's atmosphere, i.e. our air. But only 0.00016% of the 5.24 ml of helium per m³ of air consists of the nuclide 3Helium. The content in the air is:
|argon||0,934||% By volume||=||9,34||l / m3|
|neon||18,18||ppm||=||18,18||ml / m3|
The radioactive decay product radon occurs only in traces in the ppt range (parts per trillion) of around 1000 atoms per liter of soil air - especially in some underground tunnels, caves and, among other things, in cellars.
The noble gases are represented in "air separation plants (LZA)" and low-pressure oxygen plants through fractional distillation of the air or the raw argon fraction of the LZA (air liquefaction). Because of their rarity, krypton, xenon and neon are much more expensive than argon and helium.
Most helium is obtained from natural gas, in which it is present up to approx. 8%, especially American sources have a high proportion of helium. When the natural gas is cooled to –205 ° C, only helium remains in gaseous form. Argon is also produced as a by-product in the synthesis of ammonia (see Haber-Bosch process), since it is enriched with approx. 10% in the gas mixture.
The name "noble gas" comes from the fact that these elements of main group VIII are, like the noble metals, very inert.
Its atoms have eight electrons in the outer shell. An exception to this is helium, which has only two electrons in the outer shell; since the first shell can no longer bind electrons, the atom also has the highest possible number of outer electrons.
This so-called Noble gas configuration From an energetic point of view, it is in many cases to be regarded as the ideal state of an atom. The endeavor to achieve an energetically more favorable state can be cited as the main cause of chemical reactions. This creates molecules or ions. Put simply, atoms often (but not always) strive for a noble gas configuration. However, the truth is that it is a lot more complicated. There are countless chemical compounds in which not all atoms have the noble gas configuration, but which are nonetheless stable, e.g. E.g .: PF5, SF6 or the xenon fluorides mentioned below. In addition, many ions or molecules in which the noble gas configuration is achieved are thermodynamically unstable, such as CO (carbon monoxide), N2O5 (Nitrous oxide) or even the well-known ions O2- (Oxide) and N3- (Nitride). The latter can only be isolated as salts in combination with cations. The lattice energy of these salts compensates for the energy that has to be applied to generate these anions.
In contrast to all other gaseous elements or all non-metals, the noble gases do not occur in the air in molecular form, but only in monatomic form. There are, however, some artificially created noble gas compounds. Isolable binary (two-element) connections are XeF6, XeF4, XeF2, KrF2, XeO3, XeO4. The first representatives were synthesized in the mid-1960s. Many are less stable than the elementary noble gases (e.g. KrF2 and the xenon oxides). The fluorides of xenon are thermodynamically stable, which means that they are formed voluntarily and with the release of energy from the elements xenon and fluorine.
In accordance with its low atomic mass, helium is the noble gas with the lowest boiling point (4.22 K) and the lowest density (0.1785 kg / m)3) and the lowest water solubility. It is so light that it constantly escapes from the earth's atmosphere into space - but helium is also constantly being replenished by the solar wind.
As the only substance at all, helium does not solidify under normal pressure, even at very low temperatures, and a pressure of at least 24.5 bar must be applied even close to absolute zero in order to solidify liquid helium.
Liquid helium becomes “superfluid” near absolute zero. In this state, which can only be explained by quantum theory, it flows absolutely frictionless (Onnes effect), in Rollin films about 100 atomic layers thick, even over the walls of the vessel.
Helium was named after the sun (Greek: "helios"), as it was discovered in the spectrum of solar prominences during the total solar eclipse of 1868 by P. Janssen (spectral analysis). P. Janssen considered the newly discovered element to be a metal (until then spectra had only been found in metals), which is why the name was given the usual ending for metals -ium appended instead of the usual ending for noble gases -on.
It was not until 1895 that helium could be isolated on earth (William Ramsay) by heating minerals containing uranium and thorium in a vacuum (porcelain tube, 1200 ° C). In 1917 it could then also be obtained by liquefying natural gas and air.
Melting temperature: -272.2 ° C or 0.96 K (at a pressure of 24.5 bar)
Boiling temperature: -269.0 ° C or 4.2 K
Neon (content in the air approx. 16 ml / m3) is the third lightest of all gases after hydrogen and helium (density 0.8999 kg / m3 at 273 K). Although it has a higher atomic mass than nitrogen and oxygen, they only occur as molecules.
It is known in particular from lighting technology because it emits scarlet light in fluorescent tubes. However, it is also increasingly being used as a coolant in refrigeration technology
Neon means the "new" (from the Greek "neos", new); it was discovered relatively late for such a simple chemical element, not until 1898 by William Ramsay and Morris William Travers.
Melting temperature: -248.6 ° C or 24.6 K
Boiling temperature: -246.1 ° C or 27.1 K
Argon, the most common (air contains about 0.9%) and therefore the cheapest noble gas, has a weight of 1.784 kg / m3 at 273 K a density that is only slightly higher than that of nitrogen and oxygen, which at 78% and 21%, respectively, make up practically the remainder of the air.
In the laboratory, argon can therefore - if one neglects the traces of residual noble gases - also be obtained if air dried with soda lime and purified of carbon dioxide is passed over glowing copper and then heated with magnesium so that the oxygen is converted into copper (II) oxide and the nitrogen as magnesium nitride Mg3N2 be bound. The residual gas (raw argon) consists of 99.8% argon.
Argon comes from the ancient Greek word "argos", which means "sluggish" and refers to the reaction behavior of this noble gas. It was discovered by John William Strutt and William Ramsay in 1894.
Melting temperature: -189.4 ° C or 83.8 K
Boiling temperature: -185.9 ° C or 87.3 K
Krypton, the "hidden" (Greek: "kryptos"), is already heavier and therefore easier to liquefy than air (boiling point around 120 K, density 3.749 kg / m)3 at 273 K). Only 1.14 ml of krypton are contained in 1 cubic meter of air, but it slowly accumulates as trace gases in the atmosphere (by-product from the plutonium and uranium fission), from which it is recovered in oxygen systems as a Kr-Xe concentrate (lamp filling gas). With the help of fluorine gas, the strongest of all oxidizing agents, it has now been possible to force the chemical compound krypton difluoride (redox reaction) - a corrosive, poisonous gas that reacts immediately back to krypton and metal fluorides in contact with metals.
Krypton comes from the ancient Greek word "kryptos", which means "hidden". Like Neon, it was discovered in 1898 by William Ramsay and Morris William Travers.
Melting temperature: -157.4 ° C or 115.8 K
Boiling temperature: -153.4 ° C or 119.8 K
Xenon is one of the rarest elements on earth, over 3 times heavier than air (density 5.9 kg / m3 (273 K)) and has such a low speed of sound that the human voice can be heard in very deep bass after inhaling.
Xenon also has an anesthetic effect in certain concentration ranges and is currently being tested as a better alternative to previous anesthetics, which are still associated with risks.
Along with radioactive radon, xenon is the only noble gas that forms thermodynamically stable compounds (see below). One of them, xenon difluoride, is even used as an (expensive) strong oxidizing agent in organic synthesis.
Xenon comes from the ancient Greek word "xenos", which means foreign or unknown. It was also discovered in 1898 by William Ramsay and Morris William Travers.
Melting temperature: -111.8 ° C or 161.4 K
Boiling temperature: -108.1 ° C or 165.0 K
Radon is the heaviest elementary gas in the earth's atmosphere, density 9.73 kg / m3 (273 K) and also the rarest gas ever. In solid and liquid form, it luminesces due to its radioactivity. The decay products of radon are also radioactive (health hazard in uranium mining!). B. used in Bad Gastein, Karlsbad and Ischia. The longest-lived isotope, radon-222, has a half-life of 3.824 days.
The name radon was derived from radium from which it gassed, the name of which is derived from the Latin word "radius" - ray; both elements are highly radioactive. It was discovered in 1900 by Friedrich Ernst Dorn, but it was not until 1908 that William Ramsay and Robert Whytlaw-Gray isolated it and determined it more precisely. They first named the element niton from the Latin word "nitens" - luminous. The name radon has only been in use since 1923.
Melting temperature: -71.2 ° C or 202 K
Boiling temperature: -62.2 ° C or 211 K
Other noble gases
Since the relativistic effect already plays an important role in the 7th period, calculations have shown that the next homologue Ununoctium is probably not a noble gas. On the other hand, Ununquadium could be a noble gas, which is also indicated by tracer experiments.
In May 2006, however, according to a press release from the Paul Scherrer Institute, experiments showed that Ununbium behaves chemically similar to mercury (Hg) and can therefore be assigned to the same group of the periodic table. This statement is based on the observation of only 2 atoms.
Noble gases are used for neon advertising because they emit characteristic colors in gas discharge tubes:
Fluorescent tubes or Cold cathode lamps are thin glass tubes filled with gas under low pressure, the ends of which are sealed with electrodes. When a high voltage is applied, the gas is excited to glow. The first practically usable fluorescent tubes were developed by the Serb Nikola Tesla (see also: gas discharge lamp, incandescent lamp). The positive column of an electrical discharge is used in a fluorescent tube. Since the fluorescent tube, unlike a fluorescent tube, works with unheated electrodes, it requires high operating voltages of 400 V (at approx. 30 mm diameter) to 1000 volts (at approx. 80 mm diameter) per meter. VDE regulations limit the permissible voltage to 7.5 kV. A stray field transformer is chosen as the ballast
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