Carbon is the central element of organic chemistry and is considered the origin of life. It is the second most common element of the body and is essential for the human organism. Its electron configuration allows the carbon atom to form single, double and triple bonds. This makes carbon a very diverse element so that it pays off to take a closer look as a medical practitioner. In this article, you will get a brief overview of the properties of carbon
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Elements of the Human Body

Picture: “Elements of the Human Body” by philschatz. License: CC BY 4.0

Chemical Properties of Carbon

Carbon has the symbol C (for carbon) and is a tetragen. This means that it is in the fourth group of the periodic system of elements. Within this group, it is the first element and, thus, is in the second period. It has the atomic number six and is a non-metal. Moreover, it has a melting point of 3550 °C and its boiling temperature is 4827 °C.  A chemically pure carbon may be prepared in the laboratory through the termic decomposition of sucrose in absence of oxygen. The physical and chemical properties of carbon are also dependent on the crystalline structure of the element.

Carbon has a relative atomic mass of 12 grams per mole. However, there are two additional isotopes of carbon: C13 and C14. Hereby, C13 is the more frequent isotope and is considered stable. C14, on the other side, is very rare and unstable. Its half-life time is 5730 years. It is used for radiometric dating of organic substances. Additionally, the density for graphite is estimated to be 2.25 g/cm³ and that for diamond is approximated to be 3.51 g/cm³.

Pure carbon mostly organizes in crystalline form and, thus, forms either graphite or diamonds as a modification. The diamond is the hardest natural substance and is also used as a cutting material beside its use as gem. Under normal pressure and temperature conditions, graphite is the thermodynamically stable form of pure carbon. Due to its electronic conductivity, it is used as an electrode in the industry. Equally important is the fact that carbon as a free element has numerous uses such as its application a black fume pigment in inks for printers and rims for motor vehicles. Graphite, an allotrope of carbon, is used as a lubricant, for pencil tips, as an electrode in dry cells and as a component for high temperature crucibles. An amorphous form of carbon referred to as vegetal carbon is utilized predominantly for industrial purposes for the manufacture of bleaching agents and as a gas absorbent.

In its less crystalline form, carbon is also known as soot. Mostly, soot forms as a byproduct of combustions processes. The regular inhalation of soot can lead to pneumoconiosis and cardiac damages. Besides, evidence-based studies on workers who come into direct physical contact with carbon particles may experience inflammation on the hair follicles as well as occurrences of oral mucosal lesions.

Often, the medical practitioner encounters carbon as a compound with oxygen. Under high temperatures, these two elements form carbon monoxide CO or carbon dioxide CO2. The latter is primarily important in the context of respiration and blood circulation.

Elements of the Human Body

Picture: “Elements of the Human Body” by philschatz. License: CC BY 4.0

In nature, carbon can only scarcely be found as an element in its pure form, but rather as a compound with other elements. In this sense, carbon can form a larger number of different compounds than all other elements combined together. Most frequently, carbon binds with hydrogen. Together with oxygen, nitrogen and sulfur, these two elements form the basis of organic chemistry.

Electron Configuration and Hybridization of Carbon

Atom Model of Carbon

Carbon is in the second period of the fourth main group of the periodic system of elements. This means that carbon has two electron shells around its core of six protons and neutrons each. Also, it means that it has four valence electrons, which can form bonds.

Overall, carbon thus consists of a core with six protons, six neutrons and a two-layered electron shell with six electrons in total.

Electron Configuration of Carbon

With the atom orbital model, the distribution of the electrons around the core can be described in more detail. Therefore, the 1s level is completely filled with two electrons. The same situation is present in the 2s level. The two remaining electrons of carbon are distributed on two of the three p-orbitals.

This way, the electron configuration for carbon is 1s2 2s2 2p2. The superscript number always states the amount of electrons in the orbital. The s-orbitals are spherical, while the p-orbitals have a double droplet form with point symmetric arrangement.

Carbon atom

Image: “Elektronenkonfiguration des Kohlenstoffatoms im Grundzustand” by Ichwarsnur. License: CC-BY-SA 4.0

Hybridization of Carbon

Four sp3 orbitals are directed to the same tetrahedral angle to each other from

Picture: “Four sp3 orbitals are directed to the same tetrahedral angle to each other from.” by Sven. License: CC BY-SA 3.0

Hybridization means the fusion of two electron energy levels to a single joint level. In case of carbon, the two energy levels of the second shell can hybridize. This way, the 2s- and 2px-, y-, z-orbitals become energetically equal sp3-orbitals.

These four orbitals have an asymmetric dumbbell form and seem to be enlarged in a club-like shape to one side. The greater main orbitals arrange to have the greatest possible distance between each other due to electric repulsion. This results in a tetrahedral form with the typical distance angle of 109.5°.

These sp3-orbitals can overlap with the sp3-orbitals of another carbon atom and, thus, form a covalent bond. This bond is also referred to as sigma-bond. Analogous to this, a tetrahedral form results from the arrangement of the atoms. This is referred to as the so-called diamond grid of carbon.

It is not necessary that all p-orbitals participate in hybridization. Also, only two of them can hybridize with the 2s-orbitals. They then form the three sp2-orbitals. They arrange in one plane and are thus two-dimensional. The p-orbital, which does not participate in hybridization, is oriented rectangular to this plane and forms a droplet form both over and under the triangle formed by the sp2-orbitals.

Three sp2 orbitals are directed in a plane symmetry ( trigonal ) from each other

Picture: “Three sp2 orbitals are directed in a plane symmetry ( trigonal ) from each other.” by Sven. License: CC BY-SA 3.0

The sp2-orbitals of carbon can form covalent bonds with other sp2-hybridized carbon atoms. They then lie in one plane and have a triangular structure. One speaks of the so-called plane of graphite.

The non-hybridized p-orbitals can interact with each other and form pi-bonds, which are less energetic compared to the sigma-bonds. Above and below the sigma bond, they form a kind of electron cloud. Due to this cloud, the rotatability of the molecule is lost.

However, the interactions of the pi-bonds are so weak that the graphite planes can be shifted against each other. This also is the reason why a pencil, which contains graphite, can be used for writing.

Note: Sigma- and pi-bonds together form the double bond C = C.

Thus, it is not surprising that there also is the possibility that only one of the three 2p-orbitals participates in hybridization. Together with the 2s-orbital, it then forms the so-called sp1-orbitals. They are arranged linearly, that is in series. The two non-hybridized p-orbitals are located both orthogonally to each other and orthogonally to the sp1-orbitals.

The sp1-orbitals of carbon can form covalent bonds with other sp1-hybridized carbon atoms. They are then one-dimensionally arranged in a line. Together with the non-hybridized p-orbitals, a triple bond C C forms.

Note: Non-hybridized p-orbitals form the requirement for multiple bonds.

Reactive Particles of Carbons

Reactive particles are the special form of an atom, which mostly occur as instable intermediate products with high reactivity. Reactive particles are very short-lived and mostly react immediately.

Carbenium Ion

The carbenium ion is the positively charged carbon ion C+. It is an electrophile particle, since it has an electron gap due to its positive charge. As an electrophile particle, the carbenium ion can attack molecules with great electron density. This process is called electrophile attack. Hereby, mostly anions, double bonds, or lone electron pairs of electrons are attacked.

Carbenium ion initiation

Picture: “Carbenium ion initiation” by MatChem121. License: CC BY 3.0

The stability of the carbenium ion depends on its substituents. Thus, the methyl-cation is the most instable form of the carbenium ion. Primary carbenium ions are characterized by greater stability. Even more stable, however, are secondary carbenium ions. The tertiary carbenium ion is the most stable form of the carbenium ion.

Note: The more substituents the carbenium ion has, the more stable it is. This rule does not apply to molecules stabilized by mesomerism (e.g.: benzyl).


The carbanion is the negatively charged carbon ion C. Due to its lone electron pair, it is a nucleophile particle. Carbanions mostly form via splitting off of an alpha-hydrogen proton, that is a proton, which is bound to the C atom next to the functional group.

Carbanion structure

Picture: “Carbanion structure” by Jü. Lizenz: Public Domain

As a nucleophile particle, the carbanion preferably attacks molecules with low electron density. This is called nucleophile attack.


Radicals are particles with an unpaired electron. They are very instable and can form further radicals out of other components. This mostly leads to a chain reaction.

For example, radicals form under ultraviolet radiation. A homolytic split occurs, at which the two binding electrons divide equally onto the products so that each of them has one unpaired electron.

Primary carbon radicals are the most instable carbon radicals. Secondary carbon show higher stability. This stability increases towards the tertiary carbon radicals.

Electronic Effects of Carbon

Electronic effects influence the distribution of charge of a molecule and, thus, have an effect on its reactivity.

Inductive Effects

Inductive effects or I-effects base on the sigma-bonds and only have an effect over short distances. To understand the inductive effects, it is important to know the electronegativity of the affected atoms.

Note: The further an element is in the upper-right corner of the periodic system of elements, the greater is its electronegativity.

If a carbon atom has a bond to a substituent with greater electronegativity, it pulls the electrons of the sigma-bond closer to it. Such a substituent is called I-substituent. Due to the attraction of the two binding electrons, it becomes slightly more negative, while the carbon becomes slightly more positive. Carbon now has a positive partial charge δ+.

Mesomeric Effects

Mesomeric effects or M-effects base on the pi-bonds. Their influences affect the whole molecule. A substituent with a lone pair can act as a +M-substituent if the lone pair participates in mesomerism of the molecule. This way, the electron density of the molecule’s core is increased. Thus, the pi-electrons are not firmly bound to a location, but they are de-locatable.

Vice-versa, -M-substituents can decrease electron density of a molecule if they pull the pi-electrons away from the molecules core.

Electrometric Effects

The electromeric effect refers to the displacement of electrons from one atom to another following the advent of reagent attacks within the pi-bond. In the event that the attacking species is an electrophile, the pi-electrons are drawn towards a positively charged atom. This occurrence is scientifically referred to as the +E effect. Contrariwise, the attacking component is a nucleophile, the electrons are delocalized away from the attacking reagent and into the pi-system. This phenomenon is referred to as the –E Effect. Nonetheless, the term “electrometric effect” has gradually become obsolete.

Review Questions

The correct answers can be found below the references.

1. Which of the following statements concerning the properties of carbon is correct?

  1. Carbon has numerous stable isomers.
  2. Carbon is in the fourth period of the second main group of the periodic system of elements.
  3. Carbon only appears in inorganic compounds.
  4. Carbon is a non-metal.
  5. Carbon has a molar mass of 14 grams per mole.

2. Which of the following statements concerning electron configuration of carbon is correct?

  1. Carbon consists of six protons, six neutrons and six electrons.
  2. Carbon has the electron configuration 1s2 2s2 2p4.
  3. Hybridization of all 2p-orbitals with the 2s-orbital can lead to double bonds.
  4. Hybridization of all 2p-orbitals with the 2s-orbitals lead to sp1-orbitals.
  5. The diamond grids resulting from hybridization have a triangular structure.

3. Which of the following statements concerning the reaction behavior of carbon is correct?

  1. The carbanion mostly performs electrophile attacks.
  2. The carbenium ion mostly performs nucleophile attacks.
  3. The carbanion is a nucleophile particle.
  4. The –I-effect pulls pi-binding electrons out of the molecule’s core.
  5. The mesomeric effect bases on the attraction of sigma-binding electrons due to the difference in electronegativity of the binding partners.



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