Group VIA: Chalcogens

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 14:35


As we approach the right-hand side of the periodic tableA chart showing the symbols of the elements arranged in order by atomic number and having chemically related elements appearing in columns., similarities among the elements within a groupThose elements that comprise a single column of the periodic table. Also called family. become greater again. This is true of group VIA. Except polonium, which is radioactiveDescribes a substance that gives off radiation‐alpha particles, beta particles, or gamma rays‐by the disintegration of its nucleus. 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 stateA formally defined charge that an atom in a compound or ion would have if the compound or ion consisted entirely of monatomic ions. Based on a Lewis diagram, the charge that an atom would have if all electrons in bonds were assigned to the more electronegative atom or divided equally between atoms of the same electronegativity. 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.


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 gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container.. It exists in either of two allotropic forms: O2, which makes up 21 percent of the earth’s atmosphereA unit of pressure equal to 101.325 kPa or 760 mmHg; abbreviated atm. Also, the mixture of gases surrounding the earth., or O3 (ozone), which slowly decomposes to O2. O3 can be prepared by passing an electrical discharge through O2 or air:


3O2(g) \xrightarrow{\text{electrical discharge}} 2O3(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:


O2 \xrightarrow{\text{ultraviolet light}} 2O


O + O2 → O3      (1)


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 concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. 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:

Cl + O3 → ClO + O2

ClO + O3 → Cl + 2 O2

Atomic chlorine is regenerated, meaning that each CFC moleculeA set of atoms joined by covalent bonds and having no net charge. 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 atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. (formed by breaking N—O bonds in NO2) react with molecules according to Eq. (1). O3 is a stronger oxidizing agentA chemical species that accepts electrons in order to oxidize another species. In the process the oxidizing agent is itself reduced. than O2. It reacts with unsaturatedDescribes 1) a solution that contains less than the equilibrium concentration of a solute, or 2) an organic compound containing double or triple bonds. hydrocarbons (alkenes) in evaporated gasoline to produce aldehydes and ketones which are eye irritants. RubberA tough, elastic polymer obtained from the juices of certain tropical plants; a synthetic material having similar properties. is a polymeric material which contains Image:C-Cdouble bond.jpg 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 solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. consists of S8 molecules (seen in the Jmol below) packed in a crystal latticeAn orderly, repeating arrangement of points in 3-D space in which each p;oint has surroundings identical to every other point. A crystal's constituent atoms, molecules, and ions are arranged about each lattice point. which belongs to the orthorhombic system (listed on the page discussing crystal systems).


Figure 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 liquidA state of matter in which the atomic-scale particles remain close together but are able to change their positions so that the matter takes the shape of its container is at first pale yellow and flows readily, but above 160°C it becomes increasingly viscous. Only near the boiling pointThe temperature at which the vapor pressure of a liquid equals the pressure of the gas in contact with the liquid; usually this is atmospheric pressure. of 444.6°C does it thin out again. This unusual change in viscosityThe resistance of a liquid to flow when subjected to shear stress. 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 amorphousA solid in which the constituent atoms, molecules, or ions have no long-range repeating structure. 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.


Chemical Reactions and Compounds


Oxygen      Since oxygen has the second largest electronegativityThe tendency of an atom (nucleus and core electrons) within a molecule to attract electrons in bonds. among all the elements, it is found in the –2 oxidation state in most compounds. Important oxides have already been discussed in sections dealing with the elements from which they form, and so we will deal only with unusual oxidation states of oxygen here. One of these is the +2 state found in OF2, the most common compound in which oxygen is combined with the more electronegative fluorine. We have already mentioned the –½ and –1 states observed in alkali-metal superoxides and peroxides, but one important peroxide, hydrogen peroxide (H2O2), has not yet been discussed.

H2O2 can be prepared by electrolysisA process in which a reaction that would otherwise be non-spontaneous (reactant-favored) is forced to occur by passage of electric current. of solutions containing sulfate ions. H2O2 is a weak acidAn acid that ionizes only partially in a given solvent., and it can serve as an oxidizing agent (oxygen being reduced to the –2 state) or as a reducing agentA chemical species that donates electrons in order to reduce another species. In the process the reducing agent is itself oxidized. (oxygen being oxidized to the 0 state). Like the peroxide ion, the H2O2 molecule contains an O—O single bondAttraction between two atoms (nuclei and core electrons) that results from sharing a single pair of electrons between the atoms; a bond with bond order = 1.. This bond is rather weak compared with many other single bonds, and this contributes to the reactivity of H2O2. The compound decomposes easily, especially if exposed to light or contaminated with traces of transition metals. The decomposition


2H2O2(l) → 2H2O(l) + O2(g)


can occur explosively in the case of the pure liquid.


Sulfur      Although this element is only sixteenth in abundance at the surface of the earth, it is one of the few that has been known and used throughout history. Deposits of elemental sulfur are not uncommon, and, because they were stones that would burn, were originally called brimstone. Burning sulfur produces sulfur dioxide,


S8(s) + 8O2(g) → 8SO2(g)


This colorless gas has a choking odor and is more poisonous than carbon monoxide. It is the anhydrideA substance that reacts with water to form an acid or a base. A substance from which water has been removed. of sulfurous acid, a weak diproticDescribes an acid that can donate two hydrogen ions (protons) to a base. acid:


SO2(g) + H2O(l) → H2SO3(aq)


SO2 is also produced when almost any sulfur-containing substance is burned in air. Coal, for example, usually contains from 1 to 4% sulfur, and so burning coal releases SO2 to the atmosphere. Many metal ores are sulfides, and when they are heated in air, SO2 is produced. Copper, for example, may be obtained as the element by heating copper(I) sulfide:


Cu2S(s) + O2(g) \xrightarrow{\Delta } 2Cu(s) + SO2(g)


Since SO2 is so poisonous, its release to the atmosphere is a major pollutionThe contamination of the air, water, and earth by personal, industrial, and farm waste. problem. Once in the air, SO2 is slowly oxidized to sulfur trioxide, SO3:


2SO2(g) + O2(g) → 2SO3(g)


This compound is the anhydride of sulfuric acid, H2SO4:


SO3(g) + H2O(l) → H2SO4(aq)


Thus if air is polluted with SO2 and SO3, a fine mist of dilute droplets of can form. All three substances are very irritating to the throat and lungs and are responsible for considerable damage to human health.

The natural mechanism for removal of sulfur oxides from the air is solution in raindrops, followed by precipitationThe formation of a solid within a solution, often by the combination of cations and anions to form an insoluble ionic compound.. This makes the rainwater more acidic than it would otherwise be, and acid rainRain with pH less than 5.6 (the pH of unpolluted rainwater). Acid rain is caused by sulfur and nitrogen oxides formed by combustion of fossil fuels. is now common in industrialized areas of the United States and Europe. Acid rain can slowly dissolve limestone and marble, both of which consist of CaCO3:


CaCO3(s) + H3O+(aq) → Ca2+(aq) + HCO3(aq) + H2O(l)


Thus statues and buildings made of these materials may be damaged.

Despite the fact that a tremendous amount of sulfur is released to the environment by coal combustionVigorous combination of a material with oxygen gas, usually resulting in a flame. and ore smelting, this element is not usually recovered from such processes. Instead it is obtained commercially from large deposits along the U.S. Gulf Coast and from refining of sour petroleum. Sour petroleum contains numerous sulfur compounds, including H2S, which smells like rotten eggs. The deposits of elemental sulfur in Texas and Louisiana are mined by the Frasch process. Water at 170°C is pumped down a pipe to melt the sulfur, and the latter is forced to the surface by compressed air. Most of the H2S or S8 obtained from these sources is oxidized to SO2, passed over a vanadium catalystA substance that increases the rate of a chemical reaction but that undergoes no net change during the reaction. to make SO3, and dissolved in water to make H2SO4. In 2005 an estimated 190 billion kg of H2SO4 was produced in the world, making H2SO4 one of the most important industrial chemicals. About half of it is used in phosphate fertilizer production[3].

Pure H2SO4 is a liquid at room temperature and has a great affinity for H2O. This is apparently due to the reaction


H2SO4 + H2O → H3O+ + HSO4


Formation of H3O+ releases energy, and the reaction is exothermicDescribes a process in which energy is transferred to the surroundings as a result of a temperature difference.. ConcentratedIncreased the concentration of a mixture or solution (verb). Having a large concentration (adjective). H2SO4 is 93% H2SO4 and 7% H2O by massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object., corresponding to more than twice as many H2SO4 as H2O molecules. Since many H2SO4 molecules still have protons to donate, concentrated H2SO4 also has a great affinity for H2O. It is often used as a drying agent and can be employed in condensation reactions which give off H2O.

  1. "Montreal Protocol-One Page Factsheet." Environmental Protection Agency. 6 September 2007. http://www.epa.gov/ozone/downloads/MP20_FactSheet.pdf
  2. Kenneth Olden. "Statement on Health Effects of Air Pollution before the Senate Committee on Health, Education, Labor and Pensions, Subcommittee on Health. Department of Health and Human Services. 3 September 2003.http://www.hhs.gov/asl/testify/t020903.html
  3. Bala Suresh and Yoshio Inoguchi. "Sulfuric Acid". SRI Consulting. July 2006. http://www.sriconsulting.com/CEH/Public/Reports/781.5000/