The fascinating carbon atom
Life on our planet is made possible by two phenomena that constitute a rarity in the world of chemistry: hydrogen bonding (erroneously called hydrogen bridging) and carbon catenation .
In the case of the water molecule, H2Or, hydrogen bonding is a force of attraction that occurs between the hydrogen of one molecule and the oxygen of another molecule. This attraction is due to the fact that hydrogen is electropositive and oxygen is electronegative. Without hydrogen bonding, water could not exist in a liquid or solid state at Earth's ambient temperature. It would be a gas, as would all compounds whose molecules have a molar mass as low as that of water. On the other hand, catenation is the ability of an element to form chains; that is, to chemically bond with itself. Carbon is not the only element that has this capacity, but it is the one that tends to do so the most, and in the most varied forms.
The carbon atom
The symbol for the carbon atom is "C". In the English language, "carbon" is not the same as "carbon". "Carbon" is the name of the element, and "carbon" is a solid formed mainly by chains of carbon atoms. The carbon found on Earth was created about 5000 million years ago, during the period of formation of the solar system, when nuclear fusion chemistry prevailed, and proved to be relatively stable. This allowed it to contribute an amount representing 0.02% by weight of all the elements. Although this percentage seems low, carbon is the twelfth most abundant element on our planet. We will continue to discuss the carbon cycle below, but first it is important to point out What is carbon?
Carbon element
Carbon belongs to group 14 of the periodic table, whose elements are: carbon (C), silicon (Si), germanium (Ge), tin (Sb) and lead (Pb). The first three are nonmetals, and the last two are metals. All of these elements share the ability to catenate, but none of them do so as easily as carbon. In addition to concatenating, carbon can do so by multiple bonding, which means bonding to each other by double and triple bonds. The latter property is common to nitrogen and oxygen, but in such cases, catenation is relatively rare. Carbon atoms can bond to each other in a variety of ways, and in a number of atoms, impossible for any other element. They can form chains of thousands of atoms or rings of all sizes; these chains and rings can have branches. Other atoms are attached to the carbons of these chains and rings; mainly hydrogen, oxygen, fluorine, chlorine, bromine, iodine, nitrogen, sulfur, phosphorus.... This particular characteristic is what allows so many carbon compounds to exist. The number of compounds containing carbon is several times greater than the number of substances that do not contain it. Carbon is very important for the preservation of life on the planet, one of the most important contributions is photosynthesis, which is explained below:
The emergence of life and the process of conversion ofCO2 into organic molecules through photosynthesis thanks to the carbon cycle.
During the formation of the Earth, its atmosphere was composed mainly of water vapor, carbon dioxide and nitrogen, along with other gases emitted by volcanic action. Life began with plants about 3 billion years ago, in the warm waters of the oceans and seas, and originally in primitive plant forms. This form of life evolved due to its ability to photosynthesize, taking carbon dioxide from the atmosphere as a raw material and replacing it with oxygen. In the process of photosynthesis, the plant convertsCO2 into the cellulose chains and other molecules that make up the plant, which, as we will see below, chemists have called organic molecules. The earliest forms of plants and algae grew in massive abundance over millions of years. Animal life forms evolved much later, probably around 2000 million years ago, and were totally dependent on the oxygen generated by the flora of that time. Herbivorous animals feed on plants, and carnivorous animals feed on other animals. Therefore, all living things, plants and animals, start from CO2 as raw material to form our tissues. We can be aware, then, that all our tissues wereCO2. The main compound present in the human body is water, but in second place are organic molecules based on carbon chains. Therefore, oxygen represents the major part of the mass of the human body (65%), but in second place is carbon (18%). Six elements make up 99% of the mass of the human body: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. The content of the elements aluminum and silicon, although very abundant on Earth, is very low in the human body.
Organic chemistry or carbon chain chemistry
Originally, chemical compounds were divided into two groups: inorganic and organic, depending on where they came from. Inorganic compounds were those that came from minerals, and organic compounds were those obtained from plant and animal sources, i.e., from materials produced by living organisms. Until about 1850, many chemists believed that organic compounds must have their origin in living organisms and, consequently, could never be synthesized from inorganic substances. All compounds from organic sources contained the element carbon. Even after it was established that these compounds did not necessarily have to come from living sources, since they could be synthesized in the laboratory, it was convenient to keep the name organic to describe them, and so to date, compounds are classified as inorganic and organic. Organic compounds have been grouped into families that, in general, have no equivalents among the inorganics. Organic chemistry has developed methods to decompose complicated molecules, to rearrange atoms to generate new molecules, to add atoms to existing molecules or to substitute new atoms for old ones. Their goal is to synthesize new molecules that provide solutions or improvements to human activities. Currently, about 16 million organic compounds are known, and every year another 500,000 are discovered.
The formation of oil fields, and coal mines
Oil and natural gas are organic compounds that formed from organic matter accumulated in sediments of the geological past, and in association with inorganic matter from the seas over millions of years. On the other hand, around 500 million years ago, the flora had evolved considerably, and moved from the warm waters of the seas, to the land. As we entered the Carboniferous era, massive growth occurred in the form of tropical rainforests. At this time, the continents were also slowly moving northward through the warmer climates of the equatorial regions, with their torrential storms. Continental drift with the corresponding depressions and uplifts of the earth's crust caused growing areas of these rainforests to slowly, over millions of years, become submerged in river estuaries and the sea. Not all the trees in the rainforests that grew over a period of about 300 million years formed coal mines. Probably only one in a thousand billion (1 x1015) trees ended up in a coal mine. The rest simply decomposed into gaseous compounds and minerals.
Coal formation
An important stage in the formation of coal from the material in these rainforests was the swamp, with its botanical matter decomposed by aerobic and anaerobic bacteria to create the residual material that became coals under subsequent time, temperature and pressure influences associated with burial of material, usually at deep distances. This carbonization process continued beneath the earth as the continents moved northward to where they are today. The properties of coals from different coal-bearing areas of the world are not identical, including coals found at different levels within the same beta. Some coals were formed long after the end of the Carboniferous era, i.e., in the Cretaceous period, associated with the dinosaurs.
Anthracite, mineral and lignite coals.
Older or more mature coals are anthracites, which essentially do not melt when heated. The mineral coals, i.e. those of intermediate age, melt when heated. These coals are used to make metallurgical coke for the iron and steel production industries. The youngest coals are lignites and brown coals, which are relatively rich in oxygen and hydrogen. Therefore, the raw materials of the coal industry are clearly associated with the world's fossil ores. Due to the wide range of geological conditions that existed in the formation of both oil and coal, it is understood that these materials exhibit considerable variation in their physical and chemical properties. Such differences lead to different uses of these materials, particularly in the iron and aluminum production industries.
Carbon isotopes and carbon-14 as a method for measuring the age of remains of organic origin
What defines each element is its atomic number, which corresponds to the number of protons contained in its nucleus. The atomic number of carbon is 6. But each element can have a different number of neutrons. Atoms of the same element whose nucleus has a different number of neutrons are called isotopes. This causes the isotopes to differ in atomic mass. Natural carbon has three isotopes. The most common is carbon-12 or 12C, which accounts for 98.89 % of all carbon on Earth. Its nucleus consists of 6 protons and 6 neutrons. Carbon-13 (13C) is also stable, representing 1.11% of the carbon present on Earth, and its nucleus contains 6 protons and 7 neutrons. And carbon-14 (14C) is a radioactive isotope of carbon, present in a very small amount. Its nucleus contains 6 protons and 8 neutrons.
Carbon life time
The half-life of carbon-14 is 5730 years. Half-life is the time it takes for the concentration of an element or compound that decays - as in the case of radioactive elements - to decrease by half. Carbon-14 is constantly being formed as a result of reactions that occur between neutrons from cosmic rays and nitrogen atoms in the upper layers of the atmosphere. The neutron replaces one of the protons of a nitrogen atom and converts it into a carbon-14 atom. In this way, the production of carbon-14 is constant and is present in the atmosphere in very small quantities. The carbon-14 atoms react with gaseous oxygen to form radioactive carbon dioxide molecules, which are absorbed by plants in photosynthesis. Creatures that eat plants and creatures that feed on creatures that eat plants all contain the same proportion of radioactive carbon-14. When the organism dies, carbon ingestion ceases, and the carbon already present in the organism decays. Therefore, the age of an object can be determined by measuring the amount of carbon-14 present in a sample of the object. This method provides an absolute scale for dating objects between 1,000 and 20,000 years old. W. F. Libby was awarded the Nobel Prize in chemistry in 1960 for the development of the radiocarbon dating technique.
Coal allotropes
Allotropy is the property of some simple substances to have different molecular structures. Molecules consisting of a single element and having different molecular structures are called allotropes. Throughout much of history, two common allotropes of carbon have been known: graphite and diamond. Both are crystalline (i.e., they consist of an ordered molecular structure) and the atoms are bonded with strong covalent bonds. Recently, however, a whole new family of allotropes, such as fullerenes, has been identified.
Diamond
Diamond has a tetrahedral structure, in which each carbon atom is bonded to four other carbon atoms by covalent bonds. In other words, its crystals form a volume in three spatial dimensions: length, width and depth. The stability of the bond between its atoms gives it very particular characteristics: it is the hardest natural material on Earth; its stability prevents electrons from moving through it, making it an electrical insulator; however, the rigid bond between its atoms makes it an excellent thermal conductor: about five times better than copper (and this is because the vibration of an atom that receives heat is transmitted to the others with great efficiency, due to the rigidity of the structure). Its density is 3.5 g/cm3.
Graphite
The structure of graphite is very different from that of diamond. Graphite is composed of sheets of carbon atoms called "graphene", parallel to each other. The distance between the carbon sheets is relatively large, so the attraction between the layers is very weak. This explains its most interesting properties: ability to conduct electricity, because the electrons move along the plates; it is an excellent lubricant because the sheets of carbon atoms can slide over each other; it adsorbs (traps by intermolecular attractions) gas molecules between the layers. For this reason, many chemists argue that the graphite sheets actually slide on "ball bearings", which are the gas molecules. Graphite is used in lubricants, as an electrode and as mixtures of graphite and clay in lead pencils. The higher the proportion of clay, the "harder" the pencil. The ordinary mixture is designated "HB". Mixtures with more clay (harder) are designated by various "H" numbers, e.g., "2H", and mixtures with higher graphite content (softer) are assigned various "B" numbers.
Graphene plate
Graphite can be converted into diamond at high pressures (50,000 atmospheres) and temperatures (1600oC). In fact, it is a process that is applied industrially. The diamonds obtained do not have the aesthetic characteristics suitable for use as gems, but are applied in drills for drilling very hard materials.
The discovery of a new series of carbon allotropes should be regarded as an unexpected finding. Fullerenes constitute a family of structures in which the carbon atoms are arranged in a spherical or ellipsoidal structure. To build such structures, the carbon atoms form five- and six-membered rings in a pattern similar to the lines on a soccer ball (the first name given to C60 was futbolene). The 60-membered sphere, C60, buckminsterfullerene, is the easiest to prepare and, from an aesthetic point of view, the most beautiful, as it is a perfect sphere. The 70-membered sphere, C70, is the next common fullerene available. The ellipsoidal structure of this allotrope resembles a football or rugby ball.
This family of allotropes is named after R. Buckminster Fuller, a 20th century genius. Its name is particularly associated with the geodesic dome, an architectural design of enormous strength, which has the same structural arrangement as the C60 molecule. Fullerenes can also form tubes with the same type of structure ("buckitubes"). Now that we know about the existence of these molecules, they are popping up everywhere. Ordinary soot contains fullerenes, and they have been found in natural graphite deposits. Some astrochemists argue that these molecules exist in great abundance in interstellar space.
The chemistry of these novel molecules is now a field of intense research, and the molecules are already commercially available.
Amorphous or semigraphitic carbon forms
An amorphous carbon is one in which the carbon chains that make it up do not have a crystalline arrangement, as in the case of graphite or diamond. And a semi-graphitic carbon is a carbon in which a certain proportion of it is graphitic. The main uses of carbon are as an energy source and as a reducing agent. An impure form of coal is used for these purposes: coke. This material is produced by heating hard coal in the absence of air, a process in which the complex structure of the coal is destroyed, the hydrocarbons evaporate and the residue is a porous, low-density, silvery solid with an almost metallic appearance. The compounds that evaporate represent a huge problem as they are carcinogenic. Coke is used in iron production. Carbon black is a finely pulverized form of carbon. It is a micrographite produced by incomplete combustion of organic materials and is used in extraordinarily large quantities. Carbon black is mixed with rubber to give tires strength and reduce wear. About 3 kg are used per average tire, and it is the carbon content that gives it its black color. Another form of carbon that is known as activated carbon has a very large surface area, typically between 500 and 1500m2/g. Its large surface area makes it a great adsorbent of covalent compounds (a typical characteristic of organic molecules). Carbon blocks are of industrial importance as electrodes in electrochemical and thermodynamic processes. For example, around 7.5 million tons of carbon are used every year in aluminum processing alone. And, of course, in the summer there is always an increase in charcoal consumption in domestic meat roasters.
Carbonates and bicarbonates
The carbon atom also forms inorganic compounds that are very common in the earth's crust and in both fresh and salt water: carbonates, CO3-2. And bicarbonates, HCO3-1. The most common are sodium, calcium and magnesium. These compounds, together with the hydroxides, are what is known as "alkalinity" in water.
What is the carbon cycle called?
Carbon atoms can form organic molecules at a given time, and inorganic molecules at another time. The series of transformations that this element undergoes is called the "carbon cycle". This text does not pretend to mention exhaustively the main characteristics of the compounds in which the carbon atom participates. Some of these compounds are of enormous interest to human beings. There is, for example, the subject of the greenhouse effect of carbon dioxide,CO2, in the Earth's atmosphere; the subject of biochar, which is so beneficial for the cultivation of many plants; the subject of carbon monoxide, CO, with such high toxicity for aerobic animals; the subject of carbides...
Author:
Germán Groso Cruzado
Bibliography of carbon cycle contents
Choppin, G. R., B. Jaffe, L. Summerlin and L. Jackson, CHEMISTRY, Cultural Publications, Mexico, 1974. Carbon Cycle. Marsh, H., E. A. Heintz and F. Rodríguez-Reinoso (Eds.), INTRODUCTION TO CARBON TECHNOLOGIES, Publicaciones de la Universidad de Alicante, Alicante, 1997. Carbon cycle. Morrison, R. T. and R. N Boyd, QUÍMICA ORGÁNICA, 3rd Ed., Fondo de Cultura Interamericano, Mexico, 1976. Carbon cycle. Rayner-Canham, G., QUÍMICA INORGÁNICA DESCRIPTIVA, 2nd Ed., Pearson Educación, México, 2000. Carbon Cycle.
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