CARBON ATOM: CHARACTERISTICS, STRUCTURE, HYBRIDIZATION - CHEMISTRY - 2025

The carbon atom 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.

Also, 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.

Source: Gabriel Bolívar

But what is the carbon atom like? An inaccurate first sketch is the one seen in the image above, whose characteristics are described in the next 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, 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.).

Characteristics of the carbon atom

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 (¹² C) isotope, which consists of d. Other isotopes, such as ¹³ C and ¹⁴ C, less abundant, vary only in the number of neutrons.

Thus, if these isotopes were drawn, ¹³ C would have an additional yellow circle, and ¹⁴ C would have 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.

Structure

What is the structure of a carbon atom? To answer this question, first you have to go to its electronic configuration: 1s ² 2s ² 2p ² or 2s ² 2p ².

Therefore, there are three orbitals: 1s ², 2s ^2, and 2p ², 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 1s ² o, which does not participate directly in the formation of chemical bonds; while the electrons in the outer shell, 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 "void" inside the atom.

Hybridization

It was mentioned earlier that the carbon atom is tetravalent. According to its electronic configuration, its 2s electrons are paired and the 2p unpaired:

Source: Gabriel Bolívar

There is one available p orbital, which is empty and filled with an additional electron at the nitrogen atom (2p ³).

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 ground 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 ³ orbitals with one electron each to form four bonds. This is due to its characteristic of being tetravalent.

sp

When the carbon atom has an sp ³ hybridization, it orients its four hybrid orbitals to the vertices of a tetrahedron, which is its electronic geometry.

Thus, an sp ³ carbon can be identified because it only forms four simple bonds, as in the methane molecule (CH ₄). And around this a tetrahedral environment can be observed.

The overlap of the sp ³ orbitals is so effective and stable that the single bond CC has an enthalpy of 345.6 kJ / mol. This explains why there are endless carbonate structures and an immeasurable number of organic compounds. Besides this, carbon atoms can form other types of bonds.

sp

Source: Gabriel Bolívar

The carbon atom is also capable of adopting other hybridizations, which will allow it to form a double or even triple bond.

In sp ² hybridization, as seen in the image, there are three degenerate sp ² orbitals and one 2p orbital remains unchanged or "pure". With the three sp ² orbitals 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.

Classification

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.

Primary

A primary carbon is one that is bound only to one other carbon. For example, the ethane molecule, CH ₃ –CH ₃ consists of two bonded primary carbons. This signals the end or beginning of a carbon chain.

Secondary

It is one that is linked to two carbons. Thus, for the propane molecule, CH ₃ - CH ₂ –CH ₃, the middle carbon atom is secondary (the methylene group, –CH ₂ -).

Tertiary

The tertiary carbons differ from the rest because branches of the main chain emerge from them. For example, 2-methylbutane (also called isopentane), CH ₃ - CH (CH ₃) –CH ₂ –CH ₃ has a tertiary carbon highlighted in bold.

Quaternary

And finally, quaternary carbons, as their name implies, are linked to four other carbon atoms. The neopentane molecule, C (CH ₃) _4, has a quaternary carbon atom.

Applications

Atomic mass unit

The average atomic mass of ¹² C 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 the atomic mass unit u.

Thus, the other atomic masses can be compared with that of ¹² C and ¹ H. For example, magnesium (²⁴ Mg) weighs approximately twice that of a carbon atom, and 24 times more than a hydrogen atom.

Carbon cycle and life

Plants absorb CO ₂ in the photosynthesis process to release oxygen into the atmosphere and act as plant lungs. When they die, they become charcoal, which after burning, releases CO _{2 again}. One part returns to the plants, but another ends up in the sea beds, nourishing many microorganisms.

When the microorganisms die, the solid remaining in its biological decomposition sediments, and after millions of years, it is transformed into what is known as oil.

When mankind uses this oil as an alternative energy source to burning coal, it contributes to the release of more CO ₂ (and other undesirable gases).

On the other hand, life uses carbon atoms from the very bottom. 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.

NMR spectroscopy

The ¹³ C, although it is at a much lower proportion of the ¹² C, their abundance is sufficient to elucidate molecular structures by nuclear magnetic resonance spectroscopy carbon 13.

Thanks to this analysis technique it is possible to determine which atoms surround the ¹³ C and which functional groups they belong to. Thus, the carbon skeleton of any organic compound can be determined.

References

1. Graham Solomons TW, Craig B. Fryhle. Organic Chemistry. Amines. (10th edition.) Wiley Plus.
2. Blake D. (May 4, 2018). Four Characteristics of Carbon. Recovered from: sciencing.com
3. Royal Society of Chemistry. (2018). Coal. Taken from: rsc.org
4. Understanding Evolution. (sf). Journey of a carbon atom. Recovered from: evolution.berkeley.edu
5. Encyclopædia Britannica. (March 14, 2018). Coal. Recovered from: britannica.com
6. Pappas S. (September 29, 2017). Facts About Carbon. Recovered from: livescience.com