What is the significance of the fractional oxidation number?

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Group overview

The Elements

The elements of group 16 or VI belong to the chalcogens. Main group:

  • Oxygen,
  • Sulfur,
  • Selenium,
  • Tellurium,
  • Polonium,
  • as well as the artificial Ununhexium.

Oxygen occupies a special position among the chalcogens.

The elements of the chalcogens are non-metals, semi-metals and metals. The non-metallic character decreases from oxygen to polonium. Oxygen and sulfur are typical non-metals. There are various modifications of selenium and tellurium, some of which have non-metallic and partly semi-metallic properties. Polonium is a typical metal, it is radioactive.

The group name

The group name is derived from the behavior of an important compound group of these metals, the ores. Chalcogens Greek: Ore, hence the name means ore builder.


In their compounds with electropositive elements, the chalcogens have the oxidation state -2, whereby the electronegativity decreases from oxygen to polonium. Compared to electronegative elements, the chalcogens have oxidation numbers of +2, +4 and +6 with the exception of the only divalent oxygen. By using the d orbitals or by intramolecular charge shifting, sulfur, selenium and tellurium can also form further covalent bonds instead of two and thus achieve oxidation states of +4 and +6 in compounds. The stability of the oxidation number +6 decreases from sulfur to tellurium due to the increasing atomic mass.

In the case of connections with electropositive elements, the resistance of the connection between polonium and oxygen increases. In the case of compounds with electronegative reactants such as halogen, for example, the resistance increases from oxygen to polonium.

Physical Properties

As in many other main groups, the first element, oxygen, differs significantly from the other elements in the group. With the second highest electronegativity of all elements, oxygen is significantly more electronegative than the rest of the elements in the group.

Occurrence and extraction

Oxygen occurs in the air, in water and in the earth's crust. In the air it is contained with 21% by volume, in the earth's crust it is the most common element with 46.6%.

Oxygen is mainly represented by fractional distillation of liquefied air (Linde process).

Elemental sulfur is widely found in deposits from which it is extracted using the Frasch process. Sulfur occurs in bound form in many minerals.

Selenium and tellurium also occur in the earth's strata. They are obtained as a by-product in copper extraction. Both elements are contained in the anode sludge as water-insoluble or analogous tellurides in electrolytic copper refining.

The isolation of polonium is very complex. That is why bismuth is extracted today by neutron bombardment in the nuclear reactor.