Space in the popular consciousness is represented as a kingdom of cold and emptiness (remember the song: “It’s cosmic cold here, the color of the sky is different”?). However, from about the middle of the 19th century, researchers began to realize that the space between the stars was at least not empty. A clear sign of the existence of interstellar matter is the so-called dark clouds, shapeless black spots, especially clearly visible in the light strip of the Milky Way. In the 18th–19th centuries, it was believed that these were real “holes” in the distribution of stars, but by the 1920s, an opinion had developed: the spots indicate the presence of colossal clouds of interstellar dust that prevent us from seeing the light of the stars located behind them (photo 1).

In the middle of the 19th century, a new era in astronomy began: thanks to the work of Gustav Kirchhoff and Robert Bunsen, spectral analysis appeared, which made it possible to determine the chemical composition and physical parameters of gas in astronomical objects. Astronomers quickly appreciated the new opportunity, and the 1860s saw a boom in stellar spectroscopy. At the same time, largely thanks to the efforts of the remarkable observer William Heggins, evidence of the presence of gas not only in stars, but also in the space between them, was accumulating.

Heggins was a pioneer in the scientific study of nonstellar matter. From 1863 he published the results of spectroscopic studies of several nebulae, including the Great Nebula of Orion, and demonstrated that the spectra of nebulae in the visible range were very different from the spectra of stars. The radiation of a typical star is a continuous spectrum, overlain by absorption lines produced in the stellar atmosphere. And the spectra of nebulae obtained by Heggins consisted of several emission lines, with virtually no continuous spectrum. It was a spectrum of hot rarefied gas, the parameters of which are completely different from the parameters of gas in stars. Heggins' main conclusion: observational confirmation was obtained of Herschel's assumption that in space, in addition to stars, there is diffuse matter distributed over significant volumes of space.

In order for the intrinsic glow of interstellar gas to be observed in the optical range, it must not only be hot, but also quite dense, and not all interstellar matter meets these conditions. In 1904, Johannes Hartmann noticed that cooler and/or thinner interstellar gas reveals its presence by leaving its own absorption lines in stellar spectra, which are born not in the stellar atmosphere, but outside it, on the way from the star to the observer.

By the 1930s, the study of the emission and absorption lines of interstellar gas had made it possible to study its chemical composition quite well and establish that it consists of the same elements that are found on Earth. Several lines in the spectra could not be identified for a long time, and Heggins suggested that this was a new chemical element - nebulium (from lat. nebula- cloud), but it turned out to be only doubly ionized by oxygen.

By the early 1930s, it was believed that all lines in the spectrum of interstellar gas had been identified and assigned to specific atoms and ions. However, in 1934, Paul Merrill reported four unidentified lines in the yellow and red regions of the spectrum. Previously observed interstellar lines had a very small width, as befits atomic lines formed in low-density gas, but these were wider and more diffuse. Almost immediately it was suggested that these were absorption lines not of atoms or ions, but of molecules. But which ones? Exotic molecules, such as sodium (Na2), and familiar diatomic compounds, discovered in comet tails by the same Heggins back in the 19th century, such as the CN molecule, were proposed. The existence of interstellar molecules was finally established in the late 1930s, when several unidentified lines in the blue region of the spectrum were unambiguously associated with compounds CH, CH + and CN.

A feature of chemical reactions in the interstellar medium is the dominance of two-particle processes: stoichiometric coefficients are always equal to unity. At first, the only way to form molecules seemed to be “radiative association” reactions: in order for two atoms to collide and combine into a molecule, it is necessary to remove excess energy. If a molecule, having formed in an excited state, manages to emit a photon before decay and go into an unexcited state, it remains stable. Calculations carried out before the 1950s showed that the observed content of these three simple molecules can be explained on the assumption that they are formed in radiative association reactions and destroyed by the interstellar radiation field - the total radiation field of the stars of the Galaxy.

The range of concerns of astrochemistry at that time was not particularly wide, at least in the interstellar medium: three molecules, a dozen reactions between them and their constituent elements. The situation ceased to be calm in 1951, when David Bates and Lyman Spitzer recalculated the equilibrium abundances of molecules, taking into account new data on the rates of radiative association reactions. It turned out that atoms bond into molecules much more slowly than previously thought, and therefore the simple model misses the prediction of CH and CH+ content by orders of magnitude. Then they suggested that two of these molecules appear not as a result of synthesis from atoms, but as a result of the destruction of more complex molecules, specifically methane. Where did methane come from? Well, it could have formed in stellar atmospheres and then entered the interstellar medium as part of dust grains.

Later, cosmic dust began to be attributed to a more active chemical role than the role of a simple carrier of molecules. For example, if for the efficient occurrence of chemical reactions in the interstellar medium there is not enough a third body that would remove excess energy, why not assume that it is a speck of dust? Atoms and molecules could react with each other on its surface and then evaporate, replenishing the interstellar gas.

Properties of the interstellar medium

When the first molecules were discovered in the interstellar medium, neither its physical properties nor even its chemical composition were well known. The very discovery of CH and CH+ molecules was considered in the late 1930s to be important evidence of the presence of carbon and hydrogen there. Everything changed in 1951, when the radiation of interstellar atomic hydrogen was discovered, the famous radiation at a wavelength of about 21 cm. It became clear that hydrogen is the most abundant in the interstellar medium. According to modern concepts, interstellar matter is hydrogen, helium and only 2% by mass of heavier elements. A significant portion of these heavy elements, especially metals, is found in dust particles. The total mass of interstellar matter in the disk of our Galaxy is several billion solar masses, or 1–2% of the total mass of the disk. And the mass of dust is about a hundred times less than the mass of gas.

Matter is distributed heterogeneously throughout interstellar space. It can be divided into three phases: hot, warm and cold. The hot phase is a very rarefied coronal gas, ionized hydrogen with a temperature of millions of kelvins and a density of the order of 0.001 cm–3, occupying approximately half the volume of the galactic disk. The warm phase, which accounts for another half of the volume of the disk, has a density of about 0.1 cm–3 and a temperature of 8000–10,000 K. The hydrogen in it can be either ionized or neutral. The cold phase is really cold, its temperature is no more than 100 K, and in the densest areas the frost is down to several kelvins. The cool neutral gas occupies only about a percent of the disk's volume, but its mass accounts for about half the total mass of interstellar matter. This implies significant density, hundreds of particles per cubic centimeter or higher. Significant in interstellar terms, of course - for electronic devices this is a wonderful vacuum, 10–14 torr!

The dense, cold neutral gas has a ragged cloud structure, the same one that can be seen in clouds of interstellar dust. It is logical to assume that dust clouds and gas clouds are the same clouds in which dust and gas are mixed with each other. However, observations have shown that the regions of space in which the absorption effect of dust is maximum do not coincide with the regions of maximum intensity of atomic hydrogen radiation. In 1955, Bart Bock and his co-authors suggested that in the densest regions of interstellar clouds, the same ones that become opaque in the optical range due to a high concentration of dust, hydrogen is not in an atomic, but in a molecular state.

Since hydrogen is the main component of the interstellar medium, the names of the various phases reflect the state of hydrogen. An ionized environment is an environment in which hydrogen is ionized; other atoms can remain neutral. A neutral environment is an environment in which hydrogen is neutral, although other atoms may be ionized. Dense compact clouds believed to be composed primarily of molecular hydrogen are called molecular clouds. It is here that the true history of interstellar astrochemistry begins.

Invisible and visible molecules

The first interstellar molecules were discovered due to their absorption lines in the optical range. At first, their set was not very large, and simple models based on radiative association reactions and/or reactions on the surfaces of dust grains were enough to describe them. However, back in 1949 I.S. Shklovsky predicted that the radio range is more convenient for observing interstellar molecules; in it, not only absorption, but also emission of molecules can be observed. To see the absorption lines, you need a background star, whose radiation will be absorbed by interstellar molecules. But if you look at a molecular cloud, you will not see the background stars, because their radiation will be completely absorbed by the dust that is part of the same cloud! If the molecules emit themselves, you will see them wherever they are, and not just where they are carefully illuminated from behind.

The emission of molecules is associated with the presence of additional degrees of freedom. A molecule can rotate, vibrate, and perform more complex movements, each of which is associated with a set of energy levels. Moving from one level to another, a molecule, just like an atom, absorbs and emits photons. The energy of these movements is low, so they are easily excited even at low temperatures in molecular clouds. Photons corresponding to transitions between molecular energy levels do not fall in the visible range, but in the infrared, submillimeter, millimeter, centimeter... Therefore, studies of molecular radiation began when astronomers had instruments for observations in long-wavelength ranges.

True, the first interstellar molecule discovered by observations in the radio range was still observed in absorption: in 1963, in the radio emission of the supernova remnant Cassiopeia A. This was the absorption line of hydroxyl (OH) - wavelength 18 cm, and soon hydroxyl was discovered in radiation. In 1968, an ammonia emission line of 1.25 cm was observed, a few months later they found water - a line of 1.35 cm. A very important discovery in the study of the molecular interstellar medium was the discovery in 1970 of the emission of a carbon monoxide (CO) molecule at a wavelength of 2.6 mm.

Until this time, molecular clouds were to a certain extent hypothetical objects. The most common chemical compound in the Universe - the hydrogen molecule (H 2) - has no transitions in the long-wave region of the spectrum. At low temperatures in a molecular environment, it simply does not glow, that is, it remains invisible, despite all its high content. The H2 molecule does, however, have absorption lines, but they fall in the ultraviolet range, in which they cannot be observed from the surface of the Earth; we need telescopes installed either on high-altitude rockets or on spacecraft, which significantly complicates observations and makes them even more expensive. But even with an extra-atmospheric instrument, molecular hydrogen absorption lines can only be observed in the presence of background stars. If we take into account that, in principle, there are not so many stars or other astronomical objects emitting in the ultraviolet range and, in addition, dust absorption reaches a maximum in this range, it becomes clear that the possibilities of studying molecular hydrogen using absorption lines are very limited.

The CO molecule has become a salvation - unlike, for example, ammonia, it begins to glow at low densities. Its two lines, corresponding to transitions from the ground rotational state to the first excited state and from the first to the second excited state, fall into the millimeter range (2.6 mm and 1.3 mm), still accessible to observation from the Earth's surface. Shorter wavelength radiation is absorbed by the earth's atmosphere, longer wavelength radiation produces images of less clarity (for a given lens diameter, the longer the observed wavelength, the worse the angular resolution of the telescope). And there are a lot of CO molecules, so many that apparently most of the carbon in molecular clouds is in this form. This means that the CO content is determined not so much by the characteristics of the chemical evolution of the medium (unlike CH and CH + molecules), but simply by the number of available C atoms. And therefore, the CO content in a molecular gas can be considered, at least to a first approximation, constant.

Therefore, it is the CO molecule that is used as an indicator of the presence of molecular gas. And if you come across, for example, a map of the distribution of molecular gas in the Galaxy, it will be a map of the distribution of carbon monoxide, not molecular hydrogen. The admissibility of such a widespread use of CO has recently been increasingly questioned, but there is nothing special to replace it. So we have to compensate for possible uncertainty in the interpretation of CO observations with caution in its implementation.

New approaches to astrochemistry

In the early 1970s, the number of known interstellar molecules began to be measured in the dozens. And the more they were discovered, the clearer it became that the previous chemical models, which did not explain the content of the first trio CH, CH + and CN very confidently, do not work at all with an increased number of molecules. A new view (it is still accepted) on the chemical evolution of molecular clouds was proposed in 1973 by William Watson and independently by Eric Herbst and William Klemperer.

So, we are dealing with a very cold environment and a very rich molecular composition: about one and a half hundred molecules are known today. Radiative association reactions are too slow to produce observable abundances of even diatomic molecules, let alone more complex compounds. Reactions on the surfaces of dust grains are more efficient, but at 10 K, a molecule synthesized on the surface of a dust grain will in most cases remain frozen to it.

Watson, Herbst and Klemperer suggested that in the formation of the molecular composition of cold interstellar clouds, the decisive role is played not by radiative association reactions, but by ion-molecular reactions, that is, reactions between neutral and ionized components. Their speeds do not depend on temperature, and in some cases even increase at low temperatures.

There's just one little thing to do: the cloud matter needs to be ionized a little. Radiation (the light of stars close to the cloud or the combined radiation of all the stars in the Galaxy) does not so much ionize as dissociate. In addition, due to dust, radiation does not penetrate into molecular clouds, illuminating only their periphery.

But in the Galaxy there is another ionizing factor - cosmic rays: atomic nuclei accelerated by some process to a very high speed. The nature of this process has not yet been fully revealed, although the acceleration of cosmic rays (those that are interesting from the point of view of astrochemistry) most likely occurs in shock waves accompanying supernova explosions. Cosmic rays (like all matter in the Galaxy) consist mainly of fully ionized hydrogen and helium, that is, protons and alpha particles.

When the particle encounters the most common molecule, H2, it ionizes it, turning it into an H2+ ion. It, in turn, enters into an ion-molecular reaction with another H2 molecule, forming an H3+ ion. And it is this ion that becomes the main engine of all subsequent chemistry, entering into ion-molecular reactions with oxygen, carbon and nitrogen. Then everything goes according to the general scheme, which for oxygen looks like this:

O + H 3 + → OH + + H 2
OH + + H 2 → H 2 O + + H
H 2 O + + H 2 → H 3 O + + H
H 3 O + + e → H 2 O + H or H 3 O + + e → OH + H 2

The last reaction in this chain - the reaction of dissociative recombination of a hydronium ion with a free electron - leads to the formation of a molecule saturated with hydrogen, in this case a water molecule, or to the formation of hydroxyl. Naturally, dissociative recombination can also occur with intermediate ions. The end result of this sequence for the major heavy elements is the formation of water, methane and ammonia. Another option is possible: the particle ionizes an atom of an impurity element (O, C, N), and this ion reacts with an H2 molecule, again with the formation of OH +, CH +, NH + ions (further with the same stops). Chains of different elements, naturally, do not develop in isolation: their intermediate components react with each other, and as a result of this “cross-pollination”, most of the carbon passes into CO molecules, the oxygen remaining unbound in CO molecules into water and O molecules 2, and the main reservoir of nitrogen becomes the N2 molecule. The same atoms that are not included in these basic components become components of more complex molecules, the largest of which, known today, consists of 13 atoms.

Several molecules do not fit into this scheme, the formation of which in the gas phase turned out to be extremely ineffective. For example, in the same 1970, in addition to CO, a significantly more complex molecule, methanol, was discovered in significant quantities. For a long time, the synthesis of methanol was considered to be the result of a short chain: a CH 3 + ion reacted with water to form protonated methanol CH 3 OH 2 +, and then this ion recombined with an electron, splitting into methanol and a hydrogen atom. However, experiments have shown that it is easier for the CH 3 OH 2 + molecule to fall apart in the middle during recombination, so the gas-phase mechanism for methanol formation does not work.

However, there is a more important example: molecular hydrogen is not formed in the gas phase! The scheme with ion-molecular reactions only works if there are already H 2 molecules in the medium. But where do they come from? There are three ways to form molecular hydrogen in the gas phase, but all of them are extremely slow and cannot work in galactic molecular clouds. The solution to the problem was found in returning to one of the previous mechanisms, namely reactions on the surfaces of cosmic dust particles.

As before, the dust particle in this mechanism plays the role of a third body, providing conditions on its surface for the union of atoms that cannot combine in the gas phase. In a cold environment, free hydrogen atoms freeze to dust particles, but due to thermal vibrations, they do not sit in one place, but diffuse over their surface. Two hydrogen atoms, meeting during these wanderings, can combine to form an H 2 molecule, and the energy released during the reaction tears the molecule away from the dust grain and transfers it into the gas.

Naturally, if a hydrogen atom encounters on the surface not its fellow atom, but some other atom or molecule, the result of the reaction will also be different. But are there other components in the dust? There is, and this is indicated by modern observations of the densest parts of molecular clouds, the so-called nuclei, which (it is possible) in the future will turn into stars surrounded by planetary systems. Chemical differentiation occurs in the nuclei: from the densest part of the nucleus, mainly the emission of nitrogen compounds (ammonia, N 2 H + ion) emanates, and carbon compounds (CO, CS, C 2 S) glow in the shell surrounding the core, therefore, on the radio emission maps such the nuclei look like compact spots of nitrogen compound emission, surrounded by rings of carbon monoxide emission.

The modern explanation for differentiation is as follows: in the densest and coldest part of the molecular core, carbon compounds, primarily CO, freeze to dust grains, forming icy mantle shells on them. In the gas phase, they are preserved only at the periphery of the core, where, perhaps, radiation from the stars of the Galaxy penetrates, partially evaporating the icy mantles. With nitrogen compounds the situation is different: the main nitrogen-containing molecule N2 freezes into dust not as quickly as CO, and therefore in the gas phase even the coldest part of the core there remains enough nitrogen much longer to provide the observed amount of ammonia and N2H+ ion.

Chemical reactions also take place in the icy mantles of dust grains, mainly associated with the addition of hydrogen atoms to frozen molecules. For example, the sequential addition of H atoms to CO molecules in the ice shells of dust grains leads to the synthesis of methanol. Slightly more complex reactions, in which other components besides hydrogen are involved, lead to the appearance of other polyatomic molecules. When a young star lights up in the depths of the core, its radiation evaporates the mantle of dust particles, and the products of chemical synthesis appear in the gas phase, where they can also be observed.

Successes and problems

Of course, in addition to ion-molecular and surface reactions, other processes occur in the interstellar medium: neutral-neutral reactions (including radiative association reactions), photoreactions (ionization and dissociation), and processes of exchange of components between the gas phase and dust grains. Modern astrochemical models have to include hundreds of different components interconnected by thousands of reactions. What is important is this: the number of simulated components significantly exceeds the number that is actually observed, since it is not possible to create a working model from observed molecules alone! In fact, this has been the case since the very beginning of modern astrochemistry: the H 3 + ion, the existence of which was postulated in the models of Watson, Herbst and Klemperer, was discovered in observations only in the mid-1990s.

All modern data on chemical reactions in the interstellar and circumstellar medium are collected in specialized databases, of which the two most popular are: UDFA (UMIST Database for Astrochemistry) and KIDA ( Kinetic Database for Astrochemistry).

These databases are essentially lists of reactions with two reactants, several products, and numerical parameters (one to three) that allow the reaction rate to be calculated as a function of temperature, radiation field, and cosmic ray flux. The sets of reactions on the surfaces of dust grains are less standardized, however, there are two or three options that are used in most astrochemical studies. The reactions included in these sets make it possible to quantitatively explain the results of observations of the molecular composition of objects of different ages and under different physical conditions.

Today, astrochemistry is developing in four directions.

Firstly, the chemistry of isotopomers, primarily the chemistry of deuterium compounds, attracts much attention. In addition to H atoms, the interstellar medium also contains D atoms, in a proportion of approximately 1:100,000, which is comparable to the content of other impurity atoms. In addition to H2 molecules, HD molecules are also formed on dust particles. In a cold environment the reaction
H 3 + + HD → H 2 D + + H 2
is not balanced by the reverse process. The H 2 D + ion plays a role in chemistry similar to the role of the H 3 + ion, and through it deuterium atoms begin to spread through more complex compounds. The result turns out to be quite interesting: with a general D/H ratio of about 10 –5, the ratio of the content of some deuterated molecules to the content of non-deuterated analogues (for example, HDCO to H 2 CO, HDO to H 2 O) reaches percentages and even tens of percentages. A similar direction for improving models is taking into account differences in the chemistry of carbon and nitrogen isotopes.

Secondly, reactions on the surfaces of dust grains remain one of the main areas of astrochemistry. Here, a lot of work is being done, for example, to study the characteristics of reactions depending on the properties of the surface of the dust particle and its temperature. The details of the evaporation of organic molecules synthesized on it from a speck of dust are still unclear.

Thirdly, chemical models are gradually penetrating deeper into studies of the dynamics of the interstellar medium, including studies of the processes of birth of stars and planets. This insight is very important because it allows the numerical description of the motions of matter in the interstellar medium to be directly correlated with observations of molecular spectral lines. In addition, this problem also has an astrobiological application related to the possibility of interstellar organic matter reaching the forming planets.

Fourthly, there is more and more observational data on the content of various molecules in other galaxies, including galaxies at high redshifts. This means that we can no longer isolate ourselves within the framework of the Milky Way and must understand how chemical evolution occurs with a different elemental composition of the medium, with other characteristics of the radiation field, with other properties of dust grains, or what chemical reactions took place in the pre-galactic environment, when all the set of elements was limited to hydrogen, helium and lithium.

At the same time, many mysteries remain around us. For example, the lines found in 1934 by Merrill have still not been identified. And the origin of the first interstellar molecule found - CH + - remains unclear...

Cosmochemistry Cosmochemistry is the science of the chemical composition of cosmic bodies, the laws of abundance and distribution of chemical elements in the Universe, the processes of combination and migration of atoms during the formation of cosmic matter. Geochemistry is the most studied part of cosmochemistry. Cosmochemistry is the science of the chemical composition of cosmic bodies, the laws of abundance and distribution of chemical elements in the Universe, the processes of combination and migration of atoms during the formation of cosmic matter. Geochemistry is the most studied part of cosmochemistry.

Chemistry of the Earth The composition of the earth's crust includes: O – 46.6% Ca – 3.63% Al – 8.13% Na – 2.83% Si – % K – 2.59% Fe – 5.0% Mg – 2.0% Total - 98.59%

Chemical composition of a meteorite Chemical analyzes of meteorites that fell on our planet have yielded remarkable results. If we calculate the average content of the most common elements on Earth in all meteorites: iron, oxygen, silicon, magnesium, aluminum, calcium, then they account for exactly 94%, i.e. there are equal amounts of them in meteorites as in the composition globe.

Chemistry of interstellar space Not so long ago, science assumed that interstellar space is empty. All the matter in the Universe is concentrated in the stars, and there is nothing between them. Only within the solar system, somewhere along unknown paths, do meteorites and their mysterious cousins, comets, wander. Not so long ago, science accepted that interstellar space was empty. All the matter in the Universe is concentrated in the stars, and there is nothing between them. Only within the solar system, somewhere along unknown paths, do meteorites and their mysterious cousins, comets, wander. The chemistry of interstellar space is surprisingly complex. The simplest radicals were discovered in space: for example, methine (CH), hydroxyl (OH). Where there is hydroxyl, there must be water, and it has actually been found in interstellar space. In space there is water, organic molecules (formaldehyde), ammonia. These compounds, reacting with each other, can lead to the formation of amino acids.

Lunar chemistry Moonstones are special - their composition is affected by a lack of oxygen. There was no free water or atmosphere on the Moon. All volatile compounds that arose during magmatic processes flew into space. Stone meteorites are composed of simple silicates, the number of minerals in them barely reaches a hundred. There are slightly more minerals in lunar rocks than in meteorites—probably several hundred. And more than 3 thousand minerals have been discovered on the surface of the Earth. This indicates the complexity of terrestrial chemical processes compared to lunar ones.

Chemical composition of the planets Mercury - the planet closest to the Sun, Mercury, is covered with silicate rocks similar to those on Earth. The composition of the atmosphere of Venus is carbon dioxide (CO2) about 97%, nitrogen (N2) no more than 2%, water vapor (H2O) about 1%, oxygen (O2) no more than 0.1%.

Chemical composition of planets The atmosphere of this planet consists of carbon dioxide, there is some nitrogen, oxygen and water vapor. Soviet and American scientists sent automatic research stations to Mars. Mars is a cold, lifeless, dusty desert. The most interesting, amazing and mysterious planet from a chemical point of view is Jupiter. Jupiter is 98% hydrogen and helium. Water, hydrogen sulfide, methane and ammonia were also detected.

Chemical composition of the planets The atmosphere of Uranus consists of approximately 83% hydrogen, 15% helium and 2% methane. Like other gas planets, Uranus has bands of clouds that move very quickly. The structure and set of elements that make up Neptune are probably similar to Uranus: various “ices” or solidified gases containing about 15% hydrogen and a small amount of helium. The atmosphere of Saturn is mainly hydrogen and helium.

METALS IN SPACE Titanium today is the most important structural material. This is due to the rare combination of lightness, strength and refractoriness of this metal. Based on titanium, many high-strength alloys have been created for aviation, shipbuilding and rocketry. Titanium is the most important structural material today. This is due to the rare combination of lightness, strength and refractoriness of this metal. Based on titanium, many high-strength alloys have been created for aviation, shipbuilding and rocketry.

Fullerenes in space fullerenes branched chains of hydrocarbons fullerenes branched chains of hydrocarbons Fullerenes were found for the first time outside the Milky Way Fullerenes were found for the first time outside the Milky Way fullerenes were found in meteorites fullerenes were found in meteorites

“The prevalence of elements in space is studied by cosmochemistry, and their distribution on Earth is studied by geochemistry. Studying the abundance of elements in space is a rather difficult task, since..."

Element prevalence

in nature

Studying the prevalence of elements in space

cosmochemistry, and their prevalence on Earth is geochemistry.

Study of the abundance of elements in space –

quite a difficult task, since matter in space

space is in a different state (stars,

planets, dust clouds, interstellar space, etc.).

Sometimes the state of a substance is difficult to imagine. For example,

It is difficult to talk about the state of matter and elements in neutron stars, white dwarfs, black holes at colossal temperatures and pressures. Nevertheless, science knows quite a lot about what elements and in what quantities are present in space.

In interstellar space there are ions and atoms of various elements, as well as groups of atoms, radicals and even molecules, for example molecules of formaldehyde, water, HCN, CH3CN, CO, SiO2, CoS, etc.

There are especially many calcium ions in interstellar space.

In addition to it, atoms of hydrogen, potassium, carbon, sodium ions, oxygen, titanium and other particles are scattered in space.

The first place in abundance in the Universe belongs to hydrogen.

Chemical composition of stars The chemical composition of stars depends on many factors, including temperature. As the temperature increases, the composition of particles existing in the star's atmosphere becomes simpler. Thus, spectral analysis of stars with temperatures of 10,000-50,000°C shows lines of ionized hydrogen and helium and metal ions in their atmospheres. Radicals are already found in the atmospheres of stars with a temperature of 5000°C, and even oxide molecules are found in the atmospheres of stars with a temperature of 3800°C. The chemical composition of some stars with temperatures of 20,000-30,000 ° C is given in Table. 6.1.

It can be seen that, for example, in the Pegasus star, for 8700 hydrogen atoms there are 1290 helium atoms, 0.9 nitrogen atoms, etc.

The spectra of stars of the first 4 classes (the hottest) are dominated by lines of hydrogen and helium, but as the temperature decreases, lines of other elements and even lines of compounds appear. These are also simple compounds: oxides of zirconium, titanium, as well as radicals CH, OH, NH, CH2, C2, C3, CaH, etc. The outer layers of stars consist mainly of hydrogen. On average, for every 10,000 hydrogen atoms there are about 1000 helium atoms, 5 oxygen atoms and less than 1 atom of other elements.

There are stars with a high content of one or another element:

silicon, iron, manganese, carbon, etc. Stars with an anomalous composition are quite diverse. Young red giant stars contain increased amounts of heavy elements. Thus, one of these stars contains 26 times more molybdenum than the Sun.

Table 6.1 Chemical composition of some class B stars Element Relative number of atoms in the star Scorpius Perseus Pegasus Hydrogen 8350 8300

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reactions that develop in a star during its life.

The initial composition of the star is close to the composition of the interstellar matter (cloud of gas and dust) from which the star arose.

And the composition of the gas and dust clouds is not the same, which could lead to differences in the composition of the elements contained in the star.

Spectral analysis shows that the presence of many elements in the composition of stars can only be caused by nuclear reactions occurring in them (barium, zirconium, technetium). There are stars in which hydrogen has turned into helium. Their atmosphere consists of helium. Carbon, neon, titanium, nitrogen, oxygen, silicon, and magnesium are found in such helium stars. Helium stars are known that contain practically no hydrogen, which burned up as a result of nuclear reactions.

Carbon stars are very interesting. These are relatively cool stars (giants and supergiants), their surface temperatures range from 2500-6000 ° C.

At temperatures below 3500° C, with equal amounts of oxygen and carbon in the atmosphere, most of these elements are bound into carbon monoxide CO. Among other carbon compounds, CN and CH radicals are present in the atmospheres of such stars.

A study of the abundance of elements in space showed that as the atomic mass of an element increases, its abundance decreases. In addition, elements with even ordinal numbers are more common than those with odd numbers.

The abundance of elements in space is shown in Fig. 6.1.

Element abundance in the solar system

The chemical composition of the Sun is studied using spectral analysis methods. This is a very difficult job, since under the conditions existing on the Sun, the atoms of elements are highly ionized (for example, an iron atom loses up to 9 electrons).

The atmosphere of the Sun is in constant motion.

The temperatures of the photosphere, chromosphere, and solar corona vary sharply. Nevertheless, the chemical composition of the Sun has been established quite completely. 72 elements have been discovered on the Sun. The contents of 60 elements are determined quite reliably, but for elements with atomic masses above 57 the data are less accurate.

The Sun contains the most hydrogen – almost 75% of its mass.

Helium contains about 24%, only 1-2% accounts for all other elements. Although 1% of the solar mass is not so little. The mass of the Sun is 1.99.1033 g. The hundredth part of this mass is 1.99.1031 g, or 1.99.1025 t, which is 3350 times the mass of the Earth.

There is quite a lot of oxygen, carbon, nitrogen, sodium, iron, nickel in the Sun, and little lithium. Boron and fluorine are found in combination with hydrogen. Radium, uranium, bismuth, rhenium are negligibly small, and radioactive elements obtained artificially under Earth conditions (promethium, astatine), as well as halogens, except fluorine, have not been detected.

In the solar atmosphere, for every oxygen atom there is:

hydrogen 560 atoms;

aluminum 0.0040 atom;

carbon 0.37 atoms;

silicon 0.037 atoms;

nitrogen 0.76 atoms;

sulfur 0.016 atoms;

magnesium 0.062 atoms;

potassium 0.00029 atom;

sodium 0.0035 atom;

calcium 0.0031 atoms.

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At first, opinions were expressed that all the planets of the Solar system have the same composition, but a comparison of densities showed that the composition differs (see.

Mercury, Venus, Earth, Mars, Moon are solid bodies.

They are formed by silicate, aluminosilicate, carbonate and other minerals that make up their surface layers. Inside these planets is a core formed by heavier rocks containing elements with high atomic mass. Mercury contains a ferromagnetic core and has a strong magnetic field.

The total amount of metallic iron, according to some data, in Mercury is about 58%. Venus and Mars, like the Earth, have iron cores surrounded by a mineral, predominantly silicate, shell. Venus has a lot of carbonates, the thermal decomposition of which led to the accumulation of carbon dioxide in the atmosphere of this planet. According to the Soviet space stations "Venera-4" - "Venera-7", the atmosphere of Venus consists of 97% carbon dioxide, contains about 2% nitrogen, 1% water vapor and no more than 0.1% oxygen. The temperature on the surface of the planet is about 500 ° C, and the pressure is about 100 atm.

The planet Mars has an atmosphere much thinner than Earth's. The atmospheric pressure on Mars is only 0.08 of Earth's. The main components of its atmosphere are nitrogen and carbon dioxide.

Oxygen and water vapor are approximately 1000 times less than in the earth's atmosphere. It is possible that the chemical composition of the compounds that form the surface of Mars is similar to that on Earth. This is confirmed in numerous experiments to simulate Martian conditions. This is also confirmed by photographs taken from a fairly close distance from the Mars and Mariner space stations.

The giant planets Jupiter, Saturn, Uranus, and Neptune are formed by less dense substances. They are based on hydrogen, helium, methane, ammonia and other gases.

The existence of a solid core on these planets cannot be considered proven. Spectral studies of Jupiter, Saturn, Uranus and Neptune have shown the presence of methane in their atmospheres.

Ammonia was also found in the atmospheres of Jupiter and Saturn, which may be present on Uranus and Neptune, but in a solid state. The study also showed the presence of hydrogen (about 60%), helium (36%), neon (about 3%).

In addition, the atmosphere contains complex molecules:

hydrogen cyanide, nitrogen dioxide in the form of N2O4, water, hydrogen sulfide, high molecular weight molecules (pyrene, coronene, chrysene, etc.). However, despite many years of research, the chemical composition of the giant planets is not well understood.

Prevalence of chemical elements on Earth

Many scientists have studied the prevalence of chemical elements on Earth, starting with the alchemists (Theophrastus, Pliny, etc.). But only in the XVII-XIX centuries.

Experimental data on chemical processes in the earth's crust appeared and they began to be interpreted from the perspective that we now call geochemical. In the 17th century R. Boyle, studying the chemistry of the atmosphere and natural waters, and the Dutchman H. Huygens came to an understanding of life as a cosmic phenomenon. In the 17th century, M.V. Lomonosov substantiated the importance of chemistry for geology and explained the processes of formation of coal, oil, peat and other minerals in his famous books “On the Layers of the Earth” and “On the Birth of Metals.” A. Lavoisier laid the foundations of the geochemistry of the atmosphere and natural waters. The work of the Swedish chemist I. Berzelius in the field of chemical analysis of rocks, ores, minerals and waters was of great importance for the accumulation of factual material on geochemistry.

He discovered thorium, cerium, selenium, and was the first to obtain silicon, titanium, tantalum, zirconium, etc. in a free state.

Works published in the 19th century came close to geochemistry. German scientists K. Bischof and I. Breithaup on the chemistry of the earth's crust. They looked at the chemical composition of the earth's crust and the circulation of substances in it. In those same years, the term “geochemistry” began to be used. Science owes its appearance to the Swiss chemist H. Schönbein, who wrote in 1842 that it is necessary, before talking about real geological science, to have geochemistry, which must investigate the chemical nature and origin of the masses that form the globe. But the real birth of geochemistry as a science occurred in the first half of the 20th century.

(1908-1911). Place of birth: Department of Mineralogy, Moscow University. Made it a science V.I.

Vernadsky (1861-1945). Vernadsky interpreted mineralogy as the chemistry of compounds of the earth's crust. Using the results of spectral analysis, he came to the conclusion about the general dispersion of chemical elements.

Vernadsky said:

"In every drop and speck of matter on the earth's surface, as the subtlety of our research increases, we discover more and more new elements. We get the impression of the microcosmic nature of their dispersion. In a grain of sand or a drop, as in a microcosm, the general composition of the cosmos is reflected. It can be found all those elements that are observed on the globe, in the celestial spaces. The question is connected only with the improvement and refinement of research methods. With their improvement, we find sodium, lithium, strontium where they have not been seen before; with their refinement, we discover them in smaller samples than they did before."

The first course in geochemistry was taught in 1912 by Vernadsky’s student A.E. Fersman (1883-1945). In 1933-1939.

V. M. Goldschmidt (Norway) made a great contribution to geochemistry. He pointed out that the size of the atoms or ions is crucial for the inclusion of chemical elements in a crystal lattice. He explained the co-occurrence of magnesium and nickel, potassium and lead and thereby laid the foundations for the geochemistry of minerals. After his work, it became possible to predict the accumulation of elements in the earth’s crust and conduct a targeted search for minerals in nature.

Back in 1815, the English mineralogist W. Philipps tried to determine the average content of 10 chemical elements in the earth's crust. His work was continued by the Frenchmen Elie de Beaumont and A. Daubray. But their research did not attract attention.

In the 80s XIX century F.U. worked a lot on the problems of determining the average composition of the earth's crust. Clark is the head of the chemical laboratory of the American Geological Committee in Washington. Having selected 880 of the most accurate analyzes of rocks, in 1889 he

determined the average content of 10 chemical elements in the solid earth's crust. Clark obtained the following results:

Element Content, Element Content, % % Oxygen 46.28 Magnesium 2.77 Silicon 28.02 Potassium 2.47 Aluminum 8.14 Sodium 2.43 Iron 5.58 Titanium 0.33 Calcium 3.27 Phosphorus 0.10 =99 .39% Clark interpreted geochemistry as a set of information about the chemical composition of the earth's crust. Continuing his research, he increased the accuracy of definitions, the number of analyzes and the number of elements. A summary of the average content of elements in the earth's crust, published in 1924, provided data on 50 elements.

Taking into account Clark's merits in the development of geochemistry and studies of the abundance of elements, Fersman in 1923 proposed denoting the average content of a chemical element in the earth's crust, on the Earth as a whole, as well as on planets and in space, by the term "Clark". As suggested by Vernadsky, the clarke tables contain the values ​​of mass (weight) and atomic clarkes.

The meaning of introducing atomic clarks is as follows.

Let there be a geological system consisting of hydrogen and fluorine, and for every hydrogen atom there is one fluorine atom. If you define atomic clarks, they will be the same for both elements. But, if we determine the contribution of hydrogen and fluorine to the mass of the system, it turns out that, in accordance with the values ​​of the atomic masses of hydrogen and fluorine, of the total amount 1H + 19F = 20HF, hydrogen will be only 5%, and fluorine – 95%. Thus, mass and atomic clarkes can differ significantly. To convert mass clarkes to atomic ones, the mass clarke value of each element must be divided by the atomic mass and the sum of these values ​​​​considered as 100%. Then the share in this sum of the content of each element will correspond to its atomic clarke.

More than 100 years have passed since Clark's first table was published. During this time, a tremendous amount of work was done, and the general picture of the distribution of elements in the earth's crust emerged quite clearly. First of all, Vernadsky’s brilliant assumption about the dispersed state of all chemical elements was confirmed. For iodine, hafnium, scandium, rubidium, indium, cesium, radium and some other rare elements, the dispersed state is the main one, since they do not form or almost do not form their own minerals. Only for oxygen, silicon, aluminum, iron, sodium, potassium, magnesium, the main form of occurrence is its own minerals. The position on the general dispersion of chemical elements by the Soviet geochemist N.I. Safronov proposed to call it the Clark-Vernadsky law.

Modern methods of analysis and instruments have made it possible to clarify the content of elements in the earth’s crust (Table 1.3).

As can be seen from the table, half of the earth's crust consists of oxygen. Thus, the earth's crust is an “oxygen sphere”. In second place is silicon (clarke 29.5), in third place is aluminum (8.05). If you add iron (4.65), calcium (2.96), potassium (2.50), sodium (2.50), magnesium (1.87), titanium (0.45), you get 99, 48%, i.e. almost the entire earth's crust. The remaining 80 elements account for less than 1%. Elements whose content does not exceed 0.01-0.0001% are called rare. If rare elements do not form their own minerals, then they are called “rare trace elements” (Br, In, Ra, U, Re, Hf, Se, etc.).

Thus, uranium and bromine have almost the same clarke values ​​(2.5.10-4 and 2.1.10-4), but uranium is a rare element, since 104 uranium minerals and uranium deposits are known, and bromine is dispersed (has only one mineral ).

In geochemistry there is also the concept of “microelements”, which means elements contained in small quantities (0.01%) in a given system. Thus, aluminum is a microelement in a living organism and a macroelement in silicate rocks.

It has been established that clarkes are largely independent of the chemical properties of the elements. How does the element’s core affect its abundance? Back in 1923

V. M. Goldshmidt formulated the basic law of geochemistry: the overall abundance of an element depends on the properties of its atomic nucleus, and the nature of the distribution depends on the properties of the outer electron shell of its atom.

Fersman obtained a graph of the dependence of atomic clarkes on the nuclear charge for even and odd elements of D. I. Mendeleev’s periodic system (Fig. 6.2). He found out that with the complication of the atomic nucleus, its increase

Rice. 6.2. Logarithms of atomic clarkes (according to A.I. Fersman)

The Clarke masses of the elements decrease, but these curves turned out to be nonmonotonic. Light atoms (those at the beginning of the periodic table) are more common. Their nuclei contain a small number of nucleons (protons and neutrons). Indeed, after iron (Z = 26) there is not a single common element.

This was also pointed out by D.I. Mendeleev. In 1869

simultaneously with the periodic law, he formulated the rule: elements with low atomic weights are generally more abundant than heavy elements.

Another pattern was established in 1914 by G.

Oddo (Italy) and V. Garkinson (USA) in 1915-1928. They noticed that elements with even atomic numbers and even atomic masses predominate in the earth's crust. Among neighboring elements, even ones always have higher clarks than odd ones (Fig. 6.2). For the first 9 elements in terms of prevalence, even clarks are 86.43%, and odd clarks are 13.03%. The clarkes of elements whose atomic mass is divisible by 4 are especially large. Among atoms of the same element, isotopes with a mass number divisible by 4 predominate. Fersman designated this structure as 4q, where q

– an integer. Below is the abundance ratio of various isotopes of oxygen and sulfur:

O - 99.76 S – 55.01 O – 0.04 S – 0.75 O – 0.20 S – 4.22 S – 0.02.

According to Fersman, type 4q nuclei make up 83.39% of the earth's crust.

Less common are 4q+3 nuclei (12.7%). There are very few cores 4q+l and 4q+2 (1%). It was also noted that among the even elements, starting with helium, every sixth has the highest clarkes: oxygen (No. 8), silicon (No. 14), calcium (No. 20), iron (No. 26).

For odd-numbered elements, there is a similar rule (starting with hydrogen, No. 1):

nitrogen (No. 7); aluminum (No. 13); potassium (No. 19); manganese (No. 25). Nuclei containing 2, 8, 20, 28, 50, 82, 126 protons or neutrons are especially stable. These numbers are called magic numbers. The most stable are doubly magic nuclei containing magic numbers of protons and neutrons (208Pb).

Thus, the abundance of elements in the earth’s crust is associated primarily with the structure of the atomic nucleus.

The earth's crust is dominated by nuclei with a small and even number of protons and neutrons. The reason for this lies in the stellar stage of the existence of earthly matter. Over 4.5 billion years ago, the substance of our planet was heated to tens of millions of degrees. At such temperatures, neither atoms nor molecules can exist, and the substance was a hot plasma with free electrons and nuclei. Nuclear reactions took place in the plasma

– the nuclei of chemical elements were formed from protons and neutrons. The formation of the most stable nuclei is most likely, and these are nuclei containing a small and even number of protons and neutrons. Nuclei, overflowing with protons and neutrons, are unstable and decay. These are uranium, thorium, radium and other radioactive elements that decay to form lead and helium. But even among the light elements, not all have high clarke values. For example, beryllium has an atomic number of 4, and its clarke is 3.8.10-4%. The clarke of helium is even smaller, although in space it ranks second in abundance (after hydrogen). There is little lithium (3.2.10 boron (1.2.10-3%), carbon (2.3.10-2%). This is explained by the fact that these atoms in the central parts of stars are nuclear fuel and are destroyed during nuclear reactions.

Control questions

1. What elements are most common in outer space?

2. What reactions serve as the source of heavy elements in space?

3. What methods are used to study the chemical composition of stars?

4. How many chemical elements have been discovered in the Sun?

5. What is evidence of the different chemical composition of the planets of the solar system?

6. What elements are found in the atmospheres of the giant planets?

7. What defines the subject of geochemistry?

8. Which scientist made the greatest contribution to the development of geochemistry?

9. Define clarke.

10. What are the most common elements in the earth's crust?

11. Which elements are called rare and which are scattered?

12. What numbers are called magic numbers?

13. What determines the abundance of elements in the earth’s crust?

14. Formulate the basic laws of geochemistry?

15. Why are some elements with small atomic masses and even numbers widespread?

additional literature

1. Spitsyn V.I., Martynenko L.I. Inorganic chemistry.

Part 1. M.: Moscow State University Publishing House, 1991. P. 378-391.

2. Garusevich G. A. Fundamentals of general geochemistry. M.: Higher School, 1968. 363 p.

3. Perelman A.I. Geochemistry. M.: Higher school. 1979. 423 p.

4. Lutz B. G. Chemical composition of the continental crust and upper mantle of the Earth. M.: Nedra, 1976. 152 p.

5. Lavrukhina A.K. Nuclear reactions in cosmic bodies.

M.: Nauka, 1972.187 p.

6. Safronov V. S. Evolution of the preplanetary cloud and the formation of the Earth and planets. M.: Nedra, 1969. 264 p.

7. Aller L. Prevalence of chemical elements. M.:

Nedra, 1963. 254 p.

8. Nikolaev L. A. Chemistry of space. M.: Education, 1974.

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inorganic and physical chemistry, TSU named after. G.R. Derzhavina Represented by a member of the editorial board, Professor V.I. Konovalov Key words and phrases: corrosion inhibition; corrosion..."research that extends to the beginnings, causes and elements, through their understanding (after all, we are then sure that..."

On Earth - oxygen, in space - hydrogen

The Universe contains the most hydrogen (74% by mass). It has been preserved since the Big Bang. Only a small part of the hydrogen managed to turn into heavier elements in stars. On Earth, the most abundant element is oxygen (46–47%). Most of it is bound in the form of oxides, primarily silicon oxide (SiO 2). Earth's oxygen and silicon originated in massive stars that existed before the birth of the Sun. At the end of their lives, these stars exploded in supernovae and ejected the elements they formed into space. Of course, the explosion products contained a lot of hydrogen and helium, as well as carbon. However, these elements and their compounds are highly volatile. Near the young Sun, they evaporated and were blown out by radiation pressure to the outskirts of the Solar System.

Ten Most Common Elements in the Milky Way Galaxy*

* Mass fraction per million.

The universe hides many secrets in its depths. For a long time, people have sought to unravel as many of them as possible, and, despite the fact that this does not always work out, science is moving forward by leaps and bounds, allowing us to learn more and more about our origins. So, for example, many will be interested in what is the most common one in the Universe. Most people will immediately think of water, and they will be partly right, because the most common element is hydrogen.

The most abundant element in the Universe

It is extremely rare for people to encounter hydrogen in its pure form. However, in nature it is very often found in association with other elements. For example, when it reacts with oxygen, hydrogen turns into water. And this is far from the only compound that includes this element; it is found everywhere not only on our planet, but also in space.

How did the Earth appear?

Many millions of years ago, hydrogen, without exaggeration, became the building material for the entire Universe. After all, after the big bang, which became the first stage of the creation of the world, nothing existed except this element. elementary because it consists of only one atom. Over time, the most abundant element in the universe began to form clouds, which later became stars. And already inside them reactions took place, as a result of which new, more complex elements appeared, giving rise to planets.

Hydrogen

This element accounts for about 92% of the atoms in the Universe. But it is found not only in stars, interstellar gas, but also in common elements on our planet. Most often it exists in a bound form, and the most common compound is, of course, water.

In addition, hydrogen is part of a number of carbon compounds that form oil and natural gas.

Conclusion

Despite the fact that it is the most common element throughout the world, surprisingly, it can be dangerous for humans because it sometimes catches fire when it reacts with air. To understand how important a role hydrogen played in the creation of the Universe, it is enough to realize that without it nothing living would have appeared on Earth.