The carbon atom it is perhaps the most important and emblematic of all the elements, because thanks to it the existence of life is possible. It encloses in itself not only a few electrons, or a nucleus with protons and neutrons, but also star dust, which ends up incorporated and forms living beings.
Likewise, carbon atoms are found in the earth's crust, although not in an abundance comparable to metallic elements such as iron, carbonates, carbon dioxide, oil, diamonds, carbohydrates, etc., they are a part of its physical and chemical manifestations.
But what is the carbon atom like? A first inaccurate sketch is the one seen in the image above, whose characteristics are described in the following section.
Carbon atoms run through the atmosphere, the seas, the subsoil, plants and any animal species. Its great chemical diversity is due to the high stability of its bonds and the way in which they are arranged in space. Thus, you have on the one hand the smooth and lubricating graphite; and on the other, the diamond, whose hardness surpasses that of many materials.
If the carbon atom did not have the qualities that characterize it, organic chemistry would not exist completely. Some visionaries see in it the new materials of the future, through the design and functionalization of their allotropic structures (carbon nanotubes, graphene, fullerenes, etc.).
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The carbon atom is symbolized by the letter C. Its atomic number Z is 6, therefore it has six protons (red circles with the symbol "+" in the nucleus). In addition, it has six neutrons (yellow circles with the letter "N") and finally six electrons (blue stars).
The sum of the masses of its atomic particles gives an average value of 12.0107 u. However, the atom in the image corresponds to the carbon 12 isotope (12C), which consists of d. Other isotopes, such as 13C and 14C, less abundant, vary only in the number of neutrons.
Thus, if these isotopes were drawn the 13C would have an additional yellow circle, and the 14C, two more. This logically means that they are heavier carbon atoms..
In addition to this, what other characteristics can be mentioned in this regard? It is tetravalent, that is, it can form four covalent bonds. It is located in group 14 (IVA) of the periodic table, more specifically in block p.
It is also a very versatile atom, capable of bonding with almost all the elements of the periodic table; especially with itself, forming linear, branched and laminar macromolecules and polymers.
What is the structure of a carbon atom? To answer this question, first you have to go to its electronic configuration: 1stwo2stwo2 Ptwo or [He] 2stwo2 Ptwo.
Therefore, there are three orbitals: the 1stwo, the 2stwo and the 2ptwo, each with two electrons. This can also be seen in the image above: three rings with two electrons (blue stars) each (do not mistake the rings for orbits: they are orbitals).
Note, however, that two of the stars have a darker shade of blue than the remaining four. Why? Because the first two correspond to the inner layer 1stwo or [He], which does not participate directly in the formation of chemical bonds; while the outer shell electrons, 2s and 2p, do.
The s and p orbitals do not have the same shape, so the illustrated atom does not agree with reality; in addition to the great disproportion of the distance between the electrons and the nucleus, which should be hundreds of times greater.
Therefore, the structure of the carbon atom consists of three orbitals where electrons "melt" into blurred electronic clouds. And between the nucleus and these electrons there is a distance which reveals the immense "vacuum" inside the atom..
It was mentioned earlier that the carbon atom is tetravalent. According to its electronic configuration, its 2s electrons are paired and the 2p electrons are unpaired:
One p orbital is available, which is empty and filled with an additional electron on the nitrogen atom (2p3).
According to the definition of the covalent bond, it is necessary that each atom contributes an electron for its formation; however, it can be seen that in the baseline state of the carbon atom, it only has two unpaired electrons (one in each 2p orbital). This means that in this state it is a divalent atom, and therefore, it forms only two bonds (-C-).
So how is it possible for the carbon atom to form four bonds? To do this, you must promote an electron from the 2s orbital to the higher-energy 2p orbital. This done, the resulting four orbitals are degenerate; in other words, they have the same energy or stability (note that they are aligned).
This process is known as hybridization, and thanks to it, the carbon atom now has four sp orbitals3 with one electron each to form four bonds. This is due to its characteristic of being tetravalent.
When the carbon atom has an sp hybridization3, orients its four hybrid orbitals to the vertices of a tetrahedron, which is its electronic geometry.
Thus, one can identify a carbon sp3 because it only forms four simple bonds, as in the methane molecule (CH4). And around it a tetrahedral environment can be observed.
The overlap of sp orbitals3 it is so effective and stable that the single C-C bond has an enthalpy of 345.6 kJ / mol. This explains why there are endless carbonate structures and an immeasurable number of organic compounds. In addition to this, carbon atoms can form other types of bonds.
The carbon atom is also capable of adopting other hybridizations, which will allow it to form a double or even triple bond.
In sp hybridizationtwo, as seen in the image, there are three sp orbitalstwo degenerate and a 2p orbital remains unchanged or "pure". With the three sp orbitalstwo 120º apart, the carbon forms three covalent bonds drawing a trigonal plane electronic geometry; while with the 2p orbital, perpendicular to the other three, it forms a π bond: -C = C-.
In the case of sp hybridization, there are two sp orbitals 180º apart, in such a way that they draw a linear electronic geometry. This time, they have two pure 2p orbitals, perpendicular to each other, which allow the carbon to form triple bonds or two double bonds: -C≡C- or ·· C = C = C ·· (the central carbon has sp hybridization ).
Note that always (generally) if the bonds around the carbon are added it will be found that the number is equal to four. This information is essential when drawing Lewis structures or molecular structures. A carbon atom forming five bonds (= C≡C) is theoretically and experimentally inadmissible.
How are carbon atoms classified? More than a classification by internal characteristics, it actually depends on the molecular environment. That is, within a molecule its carbon atoms can be classified according to the following.
A primary carbon is one that is bound only to one other carbon. For example, the molecule of ethane, CH3-CH3 It consists of two bonded primary carbons. This signals the end or beginning of a carbon chain.
It is one that is linked to two carbons. Thus, for the propane molecule, CH3-CHtwo-CH3, the middle carbon atom is secondary (the methylene group, -CHtwo-).
The tertiary carbons differ from the rest because branches of the main chain emerge from them. For example, 2-methylbutane (also called isopentane), CH3-CH(CH3) -CHtwo-CH3 has a tertiary carbon highlighted in bold.
And finally, quaternary carbons, as their name implies, are linked to four other carbon atoms. The neopentane molecule, C(CH3)4 possesses a quaternary carbon atom.
The average atomic mass of the 12C is used as a standard measure for calculating the masses of the other elements. Thus, hydrogen weighs one twelfth of this isotope of carbon, which is used to define what is known as atomic mass unit u.
Thus, the other atomic masses can be compared with that of the 12C and the 1H. For example, magnesium (24Mg) weighs approximately twice the weight of a carbon atom, and 24 times more than a hydrogen atom.
Plants absorb COtwo in the photosynthesis process to release oxygen into the atmosphere and act as plant lungs. When they die, they turn into charcoal, which after burning, releases CO againtwo. One part returns to the plants, but another ends up in the sea beds, nourishing many microorganisms.
When the microorganisms die, the solid remaining after its biological decomposition sediments, and after millions of years, it is transformed into what is known as oil..
When humanity uses this oil as an alternative energy source to burning coal, it contributes to the release of more COtwo (and other undesirable gases).
On the other hand, life uses carbon atoms from deep within its foundations. This is due to the stability of its bonds, which allows it to form chains and molecular structures that make up macromolecules as important as DNA..
The 13C, although it is in a much smaller proportion than that of the 12C, its abundance is sufficient to elucidate molecular structures by carbon 13 nuclear magnetic resonance spectroscopy.
Thanks to this analysis technique, it is possible to determine which atoms surround the 13C and which functional groups they belong to. Thus, the carbon skeleton of any organic compound can be determined..
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