Physical properties

There is a transition of typical nonmetals (oxygen and sulfur) in the six group to metalloids (selenium and tellurium) and metallic behaviour of polonium.

Significant similarity of S, Se and Te is illustrated by their ability to form variety of allotropes.

In the series O—S—Se—Te—Po ionization energy and electronegativity diminish. Appropriately, oxidising ability of simple substances diminishes too and the reducing ability increases (it can be confirmed by standard electrodes potentials of the elements).

In the series O—S—Se—Te—Po metallic properties of elements increase, respectively. There is a change of aggregate states of simple substances: polymeric allotropes become more stable, insulators are transformed into semiconductors, the forbidden band width of semiconductors diminishes, and, finally, crystal lattice with metallic properties is formed in the case of elemental polonium. For instance, tellurium is a p-type semiconductor that shows a greater conductivity in certain directions which depends on atomic alignment. Selenium and tellurium (slightly) increase conductivity when exposed to light. These materials are called photoconductors.

The increasing metallic character in the series O—S—Se—Te—Po is also reflected in the less acidic character of oxides, and in lower stability of the oxidation state +6 for selenium compared to sulfur.

Significant similarity of Se, Te and S is illustrated by their ability to form variety of allotropes.

Sulfur. It is characteristic of sulfur, unlike oxygen, to form chains of the following type:

.

Two -bonds, which are formed by every sulfur atom in the chain S S S are stronger and energetically more advantageous (total energy of two -bonds is equal 2×260 kJ/mol), than double bonds in S₂ molecule (420 kJ/mol). For oxygen, on the contrary, double bonds in О₂ molecule are stronger (bond dissociation energy is 494 kJ/mol), than two separate -bonds in the chain О О О (2×210 kJ/mol). The possible explanation: oxygen atom radius is smaller than that of sulfur atom and oxygen has no vacant sublevel to place unpaired excitated electrons, therefore the electrostatic mutual repulsion of unshared electron pairs exhibited here is considerably stronger.

Determination of molecular mass of sulfur by the depression of freezing point of its solutions in benzene led to the conclusion that sulfur molecules consist of eight atoms (S₈). The molecule S₈, which has the form of a crown, is the most stable and configuration of sulfur as a simple substance:

here the valence angle is 108⁰. It is the evidence of sp³-hybridization of valence orbitals of sulfur.

A general scheme of sulfur molecular transformations looks like:

S₈ S₈ S₈ S_

-sulfur, rhombic, -sulfur, monoclinic, -sulfur, mobile liquid -sulfur, viscous liquid

yellow primrose straw-colour umber

Sulfur begins to boil at 444.6^oC and at 900^oC its vapours consist practically only of S₂ molecules. Dissociation of S₂ molecules on atoms begins at t>1500^oC. The number of atoms in a molecule of sulfur vapours gradually diminishes with elevation of the temperature:

S₈→S₆→S₄→S₂→S.

From 800 to 1400 °C sulfur vapours consist mainly of diatomic molecules, at 1700 °C of atoms.

Sulfur exists as a mixture of four isotopes in natural compounds:

₁₆³²S (95.084%), ₁₆³³S (0.74%), ₁₆³⁴S (4.16%), ₁₆³⁶S (0.016%).