12.7: Group VIA- Chalcogens

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As we approach the right-hand side of the periodic table, similarities among the elements within a group become greater again. This is true of group VIA. Except polonium, which is radioactive and usually omitted from discussion, all members of the group form X2– ions when combined with highly electropositive metals. The tendency to be reduced to the –2 oxidation state decreases significantly from top to bottom of the group, however, and tellurium shows some metallic properties. The group VIA elements are called chalcogens because most ores of copper (Greek chalkos) are oxides or sulfides, and such ores contain traces of selenium and tellurium. Atomic properties of the chalcogens are summarized in the table.
Table $$\PageIndex{1}$$: ​Properties of the Group VIA Elements
Element Symbol Electron Configuration Usual Oxidation State Radius/pm
Covalent Ionic (X2-)
Oxygen O [He]2s22p4 -2 66 140
Sulfur S [Ne]3s23p4 +6, +4, -2 104 184
Selenium Se [Ar]4s23d104p4 +6, +4, -2 117 198
Tellurium Te [Kr]5s24d105p4 +6, +4, -2 135 221
Symbol Ionization Energy/MJ mol–1 Density/ g cm–3 Electro- negativity Melting Point (in °C)
First Second Third
O 1.320 3.395 5.307 1.43×10-3 3.5 -218
S 1.006 2.257 3.367 2.06 2.5 119
Se 0.947 2.051 2.980 4.82 2.4 217
Te 0.876 1.800 2.704 6.25 2.1 450

At ordinary temperatures and pressures, oxygen is a gas. It exists in either of two allotropic forms: O2, which makes up 21 percent of the earth’s atmosphere, or O3 (ozone), which slowly decomposes to O2. O3 can be prepared by passing an electrical discharge through O2 or air:

$\text{3O}_2(g) \xrightarrow{\text{electrical discharge}} \text{2O}_3(g)$

This reaction occurs naturally as a result of lightning bolts. O3 is also produced by any device which produces electrical sparks. You may have noticed its distinctive odor in the vicinity of an electric motor, for example.

Ozone is formed in the earth’s stratosphere (between altitudes of 10 and. 50 km) by ultraviolet rays whose wavelengths are shorter than 250 nm:

$\text{O}_2 \xrightarrow{\text{ultraviolet light}} \text{2O}$

$\text{O} + \text{O}_2 \rightarrow \text{O}_3\label{3}$

The ozone itself absorbs longer-wavelength ultraviolet radiation (up to 340 nm), preventing these harmful rays fom reaching the earth’s surface. Otherwise these rays would increase the incidence of human skin cancer and cause other environmental problems. In recent years convincing evidence has been obtained to show that nitrogen oxide emissions from supersonic transport (SST) airplanes (which fly in the stratosphere) can reduce the concentration of ozone. Similar conclusions have been drawn regarding chlorofluorocarbons(sometimes referred to as CFCs) used as propellants in aerosol hair sprays and deodorants. Once in the atmosphere, a photochemical reaction causes atomic chlorine to be broken off from CFCs. This atomic chlorine can then participate in a catalytic ozone depleting reaction:

$\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2$

$\text{ClO} + \text{O}_3 \rightarrow \text{Cl} + \text{2O}_2$

Atomic chlorine is regenerated, meaning that each CFC molecule has the potential to deplete large amounts of ozone. In the 1980s, it was determined that use of chemicals such as CFCs were thinning stratospheric ozone. This is also when the "ozone hole" over Antarctica was discovered. In response to the depletion of ozone, and the danger presented by it, the Montreal Protocol on Substances that Deplete the Ozone Layer was signed by leaders of multiple countries, with the goal to phase out production and use of CFCs and other chemicals harmful to the ozone layer. Today, 191 countries have signed the protocol, and while it is projected to take until 2075 for ozone levels to return to normal, the Montreal Protocol has so far proven a success[1].

O3 is also an important component of photochemical smog. It is produced when O atoms (formed by breaking N—O bonds in NO2) react with molecules according to Eq. $$\ref{3}$$. O3 is a stronger oxidizing agent than O2. It reacts with unsaturated hydrocarbons (alkenes) in evaporated gasoline to produce aldehydes and ketones which are eye irritants. Rubber is a polymeric material which contains bonds, and so it too reacts with O3. Further, ground level ozone and the accompanying smog has proven a significant health concern, irritating and damaging the respiratory system and also having links to asthma[2]. So ozone is beneficial when in the upper atmosphere, but has adverse effects when at ground level.

Sulfur occurs in a variety of allotropic forms. At room temperature the most stable form is rhombic sulfur. This yellow solid consists of S8 molecules (seen in the Jmol below) packed in a crystal lattice which belongs to the orthorhombic system (listed on the page discussing crystal systems).

Figure $$\PageIndex{1}$$ S8 molecule: The initial ball and stick model can be manipulated in three dimensional space. Click on VdW radii to see a space filling model of the same molecule.

When heated to 96°C, solid rhombic sulfur changes very slowly into monoclinic sulfur, in which one-third of the S8 molecules are randomly oriented in the crystal lattice. When either form of sulfur melts, the liquid is at first pale yellow and flows readily, but above 160°C it becomes increasingly viscous. Only near the boiling point of 444.6°C does it thin out again. This unusual change in viscosity with temperature is attributed to opening of the eight-membered ring of S8 and formation of long chains of sulfur atoms. These intertwine and prevent the liquid from flowing. This explanation is supported by the fact that if the viscous liquid is cooled rapidly by pouring it into water, the amorphous sulfur produced can be shown experimentally to consist of long chains of sulfur atoms.

Both selenium and tellurium have solid structures in which the atoms are bonded in long spiral chains. Both are semiconductors, and the electrical conductivity of selenium depends on the intensity of light falling on the element. This property is utilized in selenium photocells, which are often used in photographic exposure meters.

Selenium is also used in rectifiers to convert alternating electrical current to direct current. Compounds of selenium and tellurium are of little commercial importance, and they often are toxic. Moreover, many of them have foul odors, are taken up by the body, and are given off in perspiration and on the breath. These properties have inhibited study of tellurium and selenium compounds.