por What is carbon?
He carbon It is a non-metallic chemical element whose chemical symbol is C. It owes its name to carbon, vegetable or mineral, where its atoms define various structures. It forms a wide range of organic and inorganic compounds, and also occurs in a considerable number of allotropes.
Carbon is found in all living things; all its biomolecules owe their existence to the stability and strength of CC bonds and their high tendency to concatenate. It is the element of life, and with its atoms their bodies are built.
The organic compounds with which biomaterials are built consist practically of carbon skeletons and heteroatoms. These can be seen with the naked eye in the wood of the trees; and also, when lightning strikes them and roasts them. The remaining inert black solid also contains carbon; but it is charcoal.
carbon properties
The physical or chemical properties found in solids, minerals, or carbonaceous materials are subject to many variables. Among them are: the composition or degree of impurities, the hybridizations of the carbon atoms, the diversity of the structures, and the morphology or size of the pores.
When describing the properties of carbon, most of the texts or bibliographical sources are based on graphite and diamond.
Because? Because they are the best known allotropes for this element and represent solids or materials of high purity; that is, they are practically made of nothing more than carbon atoms (although with different structures, as will be explained in the next section).
The properties of charcoal and mineral carbon differ in their origins or compositions, respectively. For example, brown coal (poor in carbon) as a fuel crawls in comparison to anthracite (rich in carbon). And what about the other allotropes: nanotubes, fullerenes, graphenes, graphins, etc.
However, chemically they have one point in common: they oxidize with an excess of oxygen in CO2:
C + O2 => CO2
However, the speed or temperature required to oxidize are specific to each of these allotropes.
graphite vs diamond
A brief comment will also be made here regarding the very different properties for these two allotropes:
Electronic structure and configuration
Hybridizations
The electronic configuration for the carbon atom is 1s22s22p2, also written as [He]2s22p2 (top image). This representation corresponds to its ground state: the carbon atom isolated and suspended in a vacuum such that it cannot interact with others.
It can be seen that one of its 2p orbitals lacks electrons, which accepts an electron from the 2s orbital of lower energy through electronic promotion; and thus, the atom acquires the capacity to form up to four covalent bonds through its four sp3 hybrid orbitals.
Note that all four sp3 orbitals are degenerate in energy (aligned on the same level). orbitals p Pure orbitals are more energetic, which is why they rank higher than the other hybrid orbitals (on the right of the image).
If there are three hybrid orbitals, it is because one orbital remains. p not hybridizing; therefore, there are three sp2 orbitals. And when there are two of these hybrid orbitals, two orbitals p are available to form double or triple bonds, being sp carbon hybridization.
Such electronic aspects are essential to understand why carbon can be found in infinities of allotropes.
oxidation numbers
Before continuing with the structures, it is worth mentioning that, given the valence electronic configuration 2s22p2, carbon can have the following oxidation numbers: +4, +2, 0, -2 and -4.
Because? These numbers correspond to the assumption that there is an ionic bond such that you form the ions with the respective charges; that is, C4+, C2+, C0 (neutral), C2- and C4-.
For carbon to have a positive oxidation number it must lose electrons; and to do so, it necessarily has to be bonded to highly electronegative atoms (such as oxygen).
Meanwhile, for carbon to have a negative oxidation number, it must gain electrons by bonding to metal atoms or less electronegative than it (such as hydrogen).
The first oxidation number, +4, means that carbon has lost all of its valence electrons; The 2s and 2p orbitals remain empty. If the 2p orbital loses its two electrons, carbon will have an oxidation number of +2; if it gains two electrons, it will have -2; and if it gains two more electrons completing its valence octet, -4.
examples
For example, for CO2 the oxidation number of carbon is +4 (because oxygen is more electronegative); while for CH4, it is -4 (because hydrogen is less electronegative).
For CH3OH, the oxidation number of carbon is -2 (+1 for H and -2 for O); while for HCOOH, it is +2 (check that the sum equals 0).
Other oxidation states, such as -3 and +3, are also likely, especially for organic molecules; for example, in methyl groups, -CH3.
Molecular geometries
In the image above, not only the hybridization of the orbitals for the carbon atom was shown, but also the resulting molecular geometries when several atoms (black spheres) are linked to a central one. This central atom, in order to arrange a certain geometric environment in space, must have the respective chemical hybridization that allows it.
For example, for the tetrahedron the central carbon is sp3 hybridized; because such is the most stable arrangement for the four sp3 hybrid orbitals. In the case of sp2 carbons, they can form double bonds and have a trigonal planar environment; and thus, these triangles define a perfect hexagon. And for an sp hybridization, the carbons adopt a linear geometry.
Thus, the geometries observed in the structures of all allotropes are governed simply by tetrahedrons (sp3), hexagons or pentagons (sp2), and lines (sp).
The tetrahedrons define a 3D structure, while the hexagons, pentagons and lines, 3D or 2D structures; the latter come to be the planes or sheets similar to the walls of the honeycombs:
And if we fold said hexagonal wall (pentagonal or mixed), we will obtain a tube (nanotubes) or a ball (fullerenes), or another figure. The interactions between these figures give rise to different morphologies.
Amorphous or crystalline solids
Leaving aside the geometries, hybridizations, or morphologies of the possible structures of carbon, its solids can be classified globally into two types: amorphous or crystalline. And between these two classifications their allotropes are distributed.
Amorphous carbon is simply that which presents an arbitrary mixture of tetrahedrons, hexagons or lines, unable to establish a structural pattern; such is the case of coal, vegetable or activated carbon, coke, soot, etc.
While crystalline carbon consists of structural patterns made up of any of the proposed geometries; for example, diamond (three-dimensional network of tetrahedrons) and graphite (stacked hexagonal sheets).
Where is the carbon found?
In addition to being the common component chemical element in all forms of life, carbon occurs in nature in three crystalline forms: diamond, graphite, and fullerene.
There are also various amorphous mineral forms of coal (anthracite, lignite, coal, peat), liquid (varieties of oil) and gaseous (natural gas) forms.
Applications
Again, like the properties and structure, the uses or applications are consistent with the allotropes or mineralogical forms of carbon. However, there are certain generalities that can be mentioned, in addition to some well-known points. Such are:
-Carbon has been used for a long time as a mineral reducing agent in obtaining pure metals; for example, iron, silicon and phosphorus, among others.
-It is the cornerstone of life, and organic chemistry and biochemistry are the studies of this reflex.
-It has also been a fossil fuel that allowed the first machines to turn their gears. In the same way, the carbon gas for the old lighting systems was obtained from it. Coal was synonymous with light, heat and power.
-Mixed as an additive with iron in different proportions allowed the invention and improvement of steel.
-Its black color took place in art, especially graphite and all the writings made with its lines.
Risks and Precautions
Carbon and its solids do not represent any health risk. Who has ever worried about a bag of coal? They sell them in droves inside the aisles of some markets, and as long as there’s no fire nearby, their black blocks won’t burn.
Coke, on the other hand, can pose a risk if its sulfur content is high. When combusted, it will release sulfur gases that, in addition to being toxic, contribute to acid rain. And while CO2 in small amounts can’t suffocate us, it does have a huge impact on the environment as a greenhouse gas.
From this perspective, carbon is a “long-term” danger, since its combustion alters the climate of our planet.
And in a more physical sense, solids or carbonaceous materials, if they are pulverized, are easily transported by air currents; and consequently, they are introduced directly to the lungs, which can irreparably damage them.
For the rest, it is very common to consume «charcoal» when a food is overcooked.
References
Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. amines. (10th ed.). WileyPlus.