Carbon

Biological role and chemical stability

The term "carbon" refers exclusively to the chemical element, whereas its pure substances are represented by charcoal, diamond, graphite, and other allotropic modifications.

The most important feature of the element is the relatively low reactivity of its simple substances and compounds. This very factor makes the construction of living matter possible. Complex macromolecules (proteins, polysaccharides, polynucleotides) maintain their structural stability over time. If carbon compounds had high reactivity, these vital substances would rapidly degrade via oxygen oxidation or hydrolysis, which would completely preclude the existence of life.

Structural features and catenation

The tetravalent state is most characteristic of carbon. It is divalent extremely rarely: under normal conditions, such a state is found only in the carbon monoxide ($CO$) molecule.

A unique feature of carbon atoms is their ability to undergo catenation — the formation of stable macromolecules with an unlimited number of bonds between C atoms. These carbon chains can be:

• Linear;
• Branched;
• Ring-shaped (cyclic);
• Volumetric.

Bonds between carbon atoms can be single, double, and triple. Structural diversity is achieved through the widespread occurrence of three types of hybridization: $sp$, $sp^2$, and $sp^3$. Depending on the type of hybridization, molecules adopt a tetrahedral (methane $CH_4$), planar (ethylene $C_2H_4$), or linear (acetylene $C_2H_2$) shape. Molecules can also combine fragments with different geometries.

Electronegativity and the nature of chemical bonds

In the $sp^3$-hybridization state, the relative electronegativity of carbon ($\chi$) is 2.5. Upon transitioning to $sp^2$- and $sp$-hybridization states, this indicator increases slightly.

On the electronegativity scale, the element is located precisely in the middle between active metals (e.g., lithium with $\chi = 1$) and the most electronegative halogen — fluorine ($\chi = 4$). Due to this intermediate position, carbon does not form ionic bonds with any chemical element. Its compounds contain exclusively covalent and polar covalent bonds.

Isotopes and occurrence in nature

In the biosphere, the carbon content by mass is only 0.023%, but in the dry mass of living matter, its share reaches 18%. The maximum concentration of the element (over 80%) is observed in coal and oil deposits. In the Earth's crust, huge reserves of carbon are bound in the form of carbonates: calcium carbonate ($CaCO_3$) forms deposits of limestone, chalk, and marble. In the atmosphere, carbon is present as carbon dioxide ($CO_2$) at a concentration of 0.036%. Despite its small fraction, atmospheric $CO_2$ is critically important for the biosphere: during photosynthesis, it is converted into glucose.

In nature, the element is represented by three isotopes:

1. $^{12}C$ (98.93%) — stable and principal isotope. It is 1/12 of the mass of this atom that forms the basis of the atomic mass unit (a.m.u.).
2. $^{13}C$ (1.07%) — minor stable isotope.
3. $^{14}C$ ($10^{-10}$%) — radioactive isotope with a half-life of 5715 years. It is continuously regenerated in the atmosphere due to nuclear reactions of cosmic ray particles with nitrogen atoms.

Classification of carbon compounds

A vast number of compounds containing C–C bonds are studied by organic chemistry. The purview of inorganic chemistry includes oxides ($CO, CO_2$), unstable carbonic acid ($H_2CO_3$) and its salts (carbonates), carbonyls, as well as compounds of carbon with metals (carbides) and certain non-metals.

There is a classification challenge regarding borderline compounds. Formally, the presence of a C–C bond makes a substance organic. Nevertheless, some of them are traditionally considered at the intersection of disciplines or within the inorganic chemistry course:

• Calcium carbide ($CaC_2$): contains a triple bond between carbon atoms ($Ca^{2+} [C \equiv C]^{2-}$).
• Hexachloroethane ($C_2Cl_6$): contains a single bond ($Cl_3C — CCl_3$).

Thus, groups such as carbides and halides can belong to either organic or inorganic compounds depending on the presence of C–C bonds and the context of study.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Chemical properties of carbon using charcoal as an example
• Structure and practical application of allotropic modifications
• The mechanism of photosynthesis: from carbon dioxide to glucose
• Carbonic acid, carbonates, and their role in the Earth's crust
• Properties and physiological effects of carbon oxides (CO and CO₂)

Other topics in "Chapter 12. Group IVA Elements"

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