Why carbon can only form a covalent bond

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The special element carbon

Organic chemistry is the chemistry of carbon and there are more compounds of carbon than all the other elements combined. The outstanding position of carbon in comparison to all other elements calls for an explanation. Why are there so many carbon compounds that an entire branch of chemistry deals only with them?

The element carbon itself occurs in three modifications, some of which differ greatly from one another:

  • the diamond, which is perhaps the most popular modification;
  • the graphite, which we at least know from drawing pens (pencil) and
  • the fullerenes, which are a family of soccer ball-like molecules that can be produced in the laboratory using an electric arc.

In order to be able to recognize which properties this special feature of carbon is based on, it is necessary to take a look at the periodic table.

Periodic table

  • As an element of the second period, carbon has a small atomic diameter and tends to form multiple bonds, since in the case of single bonds the spatial requirement of four bonding partners must be satisfied.
  • As an element with medium electronegativity, it forms stable covalent bonds.
  • In addition, the electronegativity difference between carbon and hydrogen is very small, which means that the hydrocarbon bond is also stable and only slightly polarized.

This combination of moderate electronegativity and atomic size allows carbon to form strong, stable bonds with itself. Long, possibly branched chains or rings are formed, which in turn can contain multiple bonds. This property leads to a multitude of possible combinations and thus enables the almost infinite abundance of organic compounds.

The proportions between carbon and hydrogen mean that the carbon in their compounds is almost completely shielded by the hydrogen atoms. This gives the hydrocarbons (alkanes) additional stability compared to the element-homologous silicon-hydrogen compounds (silanes). Alkanes are stable to water, while silanes decompose through humidity.

This possibility for carbon to form chains and rings through bonds with itself leads to a vast number of connections. No other element has this special property. In combination with the elements hydrogen, oxygen and nitrogen alone, over 12 million compounds are known and all life on our earth is based on such "organic compounds".

Exercise 1

exercise 2

Exercise 3