The fascinating carbon atom

Life on our planet is possible thanks to two phenomena that constitute a rarity in the world of chemistry: the hydrogen bridge and carbon catenation.

In the case of the water molecule, H2Or, the hydrogen bond is an attractive force that occurs between hydrogen in one molecule, and oxygen in another molecule. This attraction is because hydrogen is electropositive, and oxygen is electronegative. Without the hydrogen bridge, water could not exist in a liquid or solid state at Earth's ambient temperature. It would be a gas, like all compounds whose molecules have a molecular weight as low as that of water.

On the other hand, catenation is the ability of an element to form chains; that is to say, to chemically bond with itself. Carbon is not the only element that has this ability, but it is the one that most tends to do so, and in the most varied forms.

The carbon atom

 

The symbol for the carbon atom is "C". In the Spanish language, “carbon” is not the same as “carbon”. "Carbon" is the name of the element, and "carbon" is a solid made up mainly of 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, in which nuclear fusion chemistry prevailed, and it proved to be relatively stable. This allowed him to contribute an amount that represents 0.02% by weight of all the elements. Although this percentage seems low, carbon is the 12th most abundant element on our planet.

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 non-metals, and the last two are metals. All of these elements share the ability to catenate, but none of them does it as easily as carbon.

In addition to being concatenated, carbon can do so by multiple bonding, which means bonding with each other by double and triple bonds. This last property is common to nitrogen and oxygen, but in such cases, catenation is relatively rare.

Carbon atoms can bond together 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 ramifications. Other atoms are attached to the carbons in 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 that contain carbon is several times greater than the number of substances that do not.

The rise of life and the CO conversion process2 into organic molecules through photosynthesis

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 life form evolved due to its ability to photosynthesize, taking carbon dioxide from the atmosphere as raw material, and replacing it with oxygen. In the process of photosynthesis, the plant converts CO2 in the cellulose chains and other molecules that make it up, and which, as we will see later, 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 2 billion 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 beings, plants and animals, start from CO2 as raw material to form our fabrics. We can be aware, then, that all of our tissues were CO2.

The main compound present in the human body is water, but in second place are organic molecules based on carbon chains. Thus, oxygen accounts for most of the mass of the human body (65%), but second is carbon (18%). The 99 % of human body mass is made up of six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. The content of the elements aluminum and silicon, although they are very abundant on Earth, is very low in the human body.

The organic chemistry or chemistry of carbon chains

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 ones, those that were obtained from plant and animal sources, that is, 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 it is thus that to date, the compounds are classified as inorganic and organic .

Organic compounds have been grouped into families that generally have no equivalents among the inorganic ones.

Organic chemistry has developed methods to break down complicated molecules, to rearrange atoms and generate new molecules, to add atoms to existing molecules, or to replace new atoms with old ones. Its objective is to synthesize new molecules that provide solutions or improvements to human activities. Currently about 16 million organic compounds are known, and each year another 500,000 are known.

The formation of oil fields, and of coal mines

Oil and natural gas are organic compounds that were formed from organic matter accumulated in sediments from 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 went from the warm waters of the seas, to the land. As we entered the Carboniferous era, massive growth occurred in the form of rainforests. At this time, also the continents were slowly heading north, through the warmer climates of the equatorial regions, with their torrential storms. The continental displacement with the corresponding depressions and uplifts of the earth's crust, caused that growing areas of these tropical forests, slowly and through millions of years, were submerged in estuaries of rivers and in the sea. Not all forest trees that grew over a period of about 300 million years formed coal mines. Probably only one in a billion (1 x 1015) of trees ended up in a coal mine. The rest simply decomposed into gaseous and mineral compounds.

An important stage in the formation of carbon, from the material of these tropical forests, was the swamp, with its botanical matter decomposed by aerobic and anaerobic bacteria, to create the residual material that became coals, under subsequent influences of time, temperature and pressure, associated with burial of material, usually at deep distances.

This carbonization process continued underground, as the continents moved north to the position they are in today. The properties of the coals of the different coal areas of the world are not identical, including the coals that are found at different levels within the same beta. Some coals were formed long after the Coal Age had ended; that is, in the Cretaceous period, associated with dinosaurs.

The oldest or most mature coals are anthracites, and they essentially do not melt on heating. Mineral coals, that is, 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 minerals. Due to the wide range of geological conditions that existed in the formation, both oil and coal, it is understood that these materials exhibit considerable variation in their physical and chemical properties. These differences lead to different uses of these materials, particularly in the iron and aluminum production industries.

The isotopes of carbon, and carbon-14 as a method to measure the age of remains of organic origin

What defines each element is the atomic number, which corresponds to the number of protons contained in its nucleus. Carbon's atomic number is 6. But each element can have a different number of neutrons. Isotopes are the atoms of the same element, whose nucleus has a different amount of neutrons. This causes the isotopes to differ in their atomic mass.

Natural carbon has three isotopes. The most common is carbon-12 or 12C, which represents 98.89 % of all the carbon that exists on Earth. Its nucleus is made up of 6 protons and 6 neutrons. Carbon-13 (13C) is also stable, represents 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 tiny amount. Its nucleus contains 6 protons and 8 neutrons.

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 decomposes - as is the case of radioactive substances - to decrease by half. Carbon-14 is constantly being formed as a result of reactions that occur between neutrons in cosmic rays and nitrogen atoms in the upper layers of the atmosphere. The neutron replaces one of the protons on a nitrogen atom, and turns 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. Carbon-14 atoms react with oxygen gas 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 body dies, the ingestion of carbon ceases, and what is already present in the body disintegrates. 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 offers an absolute scale for dating objects between 1,000 and 20,000 years old. WF Libby was awarded the Nobel Prize in Chemistry in 1960 for the development of the radiocarbon dating technique.

Allotropes of carbon

Allotropy is the property of some simple substances to have different molecular structures. Molecules made up of a single element and possessing a different molecular structure are called allotropes.

Throughout much of history, two common allotropes of carbon have been known: graphite and diamond.

Both are crystalline, (that is, they are made up of an ordered molecular structure) and the atoms are bonded with strongly covalent bonds. However, a whole new family of allotropes, such as the fullerenes, has recently been identified.

Diamond

Diamond has a tetrahedral structure, in which each carbon atom is linked to four others by covalent bonds. That is, its crystals form a volume in the three spatial dimensions: long, wide and deep. 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 union between its atoms makes it an excellent thermal conductor: around five times better than copper (and this is so because the vibration of an atom that receives heat is transmitted to the others with great efficiency, due to 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 made up 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, due to electrons moving along the plates; it is an excellent lubricant because the sheets of carbon atoms can slide over each other; adsorbs (traps by intermolecular attractions) gas molecules between the layers. For this reason, many chemists argue that graphite sheets actually slide on "ball bearings," which are gas molecules.

Graphite is used in lubricants, as an electrode, and as graphite-clay mixtures in lead pencils. The higher the proportion of clay, the “harder” the pencil. The ordinary mixture is designated as "HB". The more clay (harder) mixtures are designated by various "H" numbers, eg, "2H", and the higher graphite (softer) mixtures are assigned various "B" numbers.

Graphene plate

Graphite can turn into diamond at high pressures (50,000 atmospheres) and temperatures (1600orC). In fact, it is a process that is applied industrially. The diamonds obtained do not have the appropriate aesthetic characteristics to be used as gemstones, but are applied in bits to drill very hard materials.

The discovery of a new series of carbon allotropes should be considered an unexpected finding. Fullerenes are a family of structures in which the carbon atoms are organized in a spherical or ellipsoidal structure. To build these kinds of 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 football). The 60-membered sphere, C60, the buckminsterfulerene, 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 especially associated with the geodesic dome, an architectural design of enormous resistance, 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 of the existence of these molecules, they appear everywhere. Ordinary soot contains fullerenes, and they have been found in natural deposits of graphite. Some astrochemists argue that these molecules exist in great abundance in interstellar space.

The chemistry of these novel molecules is today a field of intense research, and the molecules are now commercially available.

Amorphous or semi-graphic 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-graphic carbon is a carbon in which a certain proportion of it is graphitic.

The main uses of coal 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 coal in the absence of air, a process in which the complex structure of the coal is destroyed, the hydrocarbons evaporate and a porous, low-density, silvery, almost metallic-looking solid remains as a residue. Compounds that evaporate pose a huge problem as they are carcinogenic. Coke is used in the production of iron.

Carbon black is a finely powdered form of carbon. It is a micrographite that is produced by the 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 for each average tire, and it is the carbon content that gives it its black color.

Another form of charcoal is known as activated carbon has a very large surface area, typically between 500 and 1500 m2/ g. Its large surface area makes it a great adsorbent for covalent compounds (typical characteristic of organic molecules).

Carbon blocks are of industrial importance as electrodes in electrochemical and thermodynamic processes. For example, about 7.5 million tons of coal are used each year in aluminum mills alone. And of course, in the summer the consumption of charcoal in domestic meat grills always increases.

Carbonates and bicarbonates

The carbon atom also forms very common inorganic compounds in the earth's crust and in both fresh and salty waters: carbonates, CO3-2. And bicarbonates, HCO3-1. The most common are sodium, calcium and magnesium. These compounds, along with the hydroxides, are what are known as "alkalinity" in water.

Carbon atoms can form organic molecules at one time, and inorganic molecules at another time. The series of transformations that this element undergoes is called the “carbon cycle”.

This text does not intend to exhaustively mention the main characteristics of the compounds in which the carbon atom participates. Some of these compounds are of enormous interest to humans. There is, for example, the issue of the greenhouse effect of carbon dioxide, CO2, in the Earth's atmosphere; the issue of biochar that is so beneficial for the cultivation of many plants; that of carbon monoxide, CO, with such high toxicity for aerobic animals; the issue of carbides ...

 

Bibliography

Choppin, GR, B. Jaffe, L. Summerlin and L. Jackson, CHEMISTRY, Cultural Publications, Mexico, 1974

Marsh, H., EA Heintz and F. Rodríguez-Reinoso (Eds.), INTRODUCTION TO CARBON TECHNOLOGIES, Publications of the University of Alicante, Alicante, 1997.

Morrison, RT and R. N Boyd, ORGANIC CHEMISTRY, 3rd Ed., Inter-American Culture Fund, Mexico, 1976

Rayner-Canham, G., DESCRIPTIVE INORGANIC CHEMISTRY, 2nd Ed., Pearson Education, Mexico, 2000

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