R28

Elements Handbook

The Frasch Process

Elements HandbookHandbook

Group 6A

Oxygen

| discovered in 1772 by Carl Scheele and in 1774 by Joseph Priestly

Sulfur

| known since ancient times

Selenium

| discovered in 1817 by Jöns Jacob Berzelius

Tellurium

| discovered in 1782 by Franz Joseph Müller von Reichenstein

Polonium

| discovered in 1898 by Marie Curie

The unit cell in crystalline sulfur is an S

8

molecule.

Sources

• Large-scale production of oxygen is by fractional distillation of liquid air. Liquid oxygen is stored and shipped at its boiling point of

�

183ºC in vacuum-walled bottles.

• The Frasch process is used to mine sulfur from underground deposits. A well is drilled into a sulfur bed and a set of concentric tubes installed. Superheated water melts the sulfur. Compressed air forces it to the surface.

• Sulfur is also produced from hydrogen sulfide, H

2

S, and sulfur dioxide, SO

2

.

2H

2

S(

g

)

�

SO

2

(

g

) 2H

2

O(

l

)

�

3S(

s

)

• Selenium and tellurium are by-products of the processing of sulfide ores for other metals.

• Polonium is formed by the radioactive decay of radium in minerals such as pitchblende.

84

PoPolonium

(209)

28

1832186

52

TeTellurium

127.60

28

18186

34

SeSelenium

78.96

28

186

16

SSulfur

32.06

286

8

OOxygen

15.999

26

monoclinic rhombic

DensityMelting and Boiling Points

990 962

445

685

250

�250

500

0

750

1000

Te

mp

era

ture

(�C

)

452

254

113217

mp

bp

O Te PoS Se

�219

�183

O Te

6.25

Po

9.14

S

2.07

Se

4.284

2

0

6

8

10

De

nsit

y (

g/c

m3)

1.43 � 10�3

O Te

6.25

Po

9.14

S

2.07

Se

4.284

2

0

6

8

10

De

nsit

y (

g/c

m3)

1.43 � 10�3

Liquid Sulfur (119°C)Solid SulfurAir

(180°C)Hot Water

Hot sulfur frothCompressed air

Physical Properties

• Except for oxygen gas, O

2

, Group 6A elements are solid at room temperature.

• The metallic properties of Group 6A elements increase from top to bottom within the group.

• Polonium is a radioactive metal (half-life

�

140 days).

Appendix_A.fm Page 28 Wednesday, January 4, 2006 3:21 PM

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Group 6A R29

Atomic Properties

• Group 6A elements have an electron configuration that ends in ns2np4.

• For Group 6A elements, the most common oxidation numbers are �4, �6, and �2.

• Oxygen is paramagnetic because there are unpaired electrons in O2 molecules.

Liquid oxygen is held between the poles of a magnet because of its attraction to the magnet.

Important Compounds and Reactions

• Oxygen reacts with almost all other elements to form oxides. Example:

4K(s) � O2(g) 2K2O(s)�H � �363.2 kJ/mol

• Ozone, O3, is produced directly from oxygen, O2, during lightening strikes.

3O2(g) 2O3(g) �H � �285 kJ/mol

• Oxygen is necessary for releasing energy from fuels, such as glucose, in organisms.

C6H12O6(s) � 6O2(g) 6CO2(g) � 6H2O(g)�H � �2808 kJ/mol

• Oxygen is used to produce steel and to oxidize hydrogen in fuel cells.

• Sulfur compounds often have unpleasant odors. Hydrogen sulfide, H2S, smells like a rotten egg. It forms when metallic sulfides and hydrochloric acid react. Example:

FeS(s) � 2HCl(aq) H2S(g) � FeCl2(aq)

• Concentrated sulfuric acid, H2SO4, is a strong dehydrating agent. Example:

C12H22O11(s) 12C(s) � 11H2O(g)

• So people will know when there is a natural gas leak, ethyl mercaptan, CH3CH2SH, is added to supplies of odorless natural gas.

• Sodium thiosulfate, Na2S2O3, also known as hypo, is used in the development of film.

• The addition of cadmium selenide, CdSe, gives glass a beautiful ruby color.

When concentrated sulfuric acid is added to sucrose, water vapor (steam) and carbon are produced. The release of the vapor causes the carbon to expand.

First Ionization Energy Electronegativity

1314

869 813

999941

500

0

1000

1500

En

erg

y (

kJ/m

ol)

O Te PoS Se0

1.0

3.0

2.0

4.0

Ele

ctr

on

eg

ati

vit

y 3.5

2.1 2.0

2.5 2.4

O Te PoS Se

Atomic

radius (pm)

Ionic

radius (pm)

O S Se Te Po

O2– S2– Se2– Te2– Po4+

66 105 120 139 168

140 184 198 221 94

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R30 Elements Handbook

Soils in the Great Plains and Rocky Mountain regions often contain high levels of selenium. If cattle graze on plants that grow in those soils, they may develop chronic selenium poisoning. The symptoms include loss of hair, sore hoofs, lameness, and a lack of energy.

Acute selenium poisoning causes cattle to lose their vision and stumble aimlessly before dying from respiratory failure. Cowboys called this condition the “blind staggers.” ■

Elements HandbookHandbook

Group 6A

Se Selenium In Food

Selenium is an antioxidant that protects cell membranes from damage. In the United States, grain is grown on selenium-rich soils. Livestock and people who eat those grains are unlikely to be deficient in selenium.

An essential nutrient may be harmful in large doses. Too much selenium can damage the nervous system. It may also cause anxiety and fatigue.

This milkvetch (Astragalus bisulcatus) accumulates high levels of selenium.

S Sulfuric Acid

Pure sulfuric acid is a dense, colorless, oily liquid. Concentrated sulfuric acid is 98% H2SO4 and 2% H2O. Dilute sulfuric acid reacts with metals, oxides, hydroxides, or carbonates to form sulfates. The reaction with metals also releases hydrogen gas. Sulfuric acid can be used to produce other acids from their salts. For example, hydrogen chloride can be produced from sulfuric acid and sodium chloride.

H2SO4(l) � 2NaCl(s) Na2SO4(s) � 2HCl(g)

Sulfuric acid is produced mainly from sulfur dioxide. The process is called the contact process because the key reaction takes place when the reactants are in con-tact with the surface of the solid catalyst.

(1) Melted sulfur is burned in air.

S(l) � O2(g) SO2(g)

(2) Sulfur dioxide is oxidized in the presence of a vanadium oxide catalyst, V2O5.

2SO2(g) � O2(g) 2SO3(g)

(3) Sulfur trioxide dissolves in water and forms sulfuric acid.

Much of the sulfuric acid produced in North America is used to make fertilizers. Sulfuric acid is also used in petroleum refining, the production of other chemicals, and for pickling iron and steel. During pickling, oxides are removed from the surface of a metal. ■

¡V2O5

When sulfur burns in air, the product is the irritating gas sulfur dioxide, SO2.

This sidegill slug produces sulfuric acid, which discourages predators.

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R31

?? Did You Know...Did You Know...

O Ozone

Near Earth’s surface, ozone (O3) is a pollutant. In the stratosphere, ozone is literally a lifesaver. The ozone layer in the stratosphere absorbs 99% of the sun’s harmful ultraviolet (UV) radiation. In the 1970s, scientists began to suspect that the ozone layer might be threatened. They based their concerns on laboratory models. In 1985, their suspicions were confirmed when British researchers discovered a “hole” in the ozone layer over Antarctica. In the winter, the amount of ozone was reduced by almost one half.

The scientists traced the thinning of the ozone layer to chlorofluorocarbons (CFCs). These chemicals were used mainly as propellants in aerosol spray cans and as coolants in refrigerators and air conditioners. CFCs are highly stable and inert in the lower atmosphere. Over time, they drift up into the stratosphere, where they are broken down by solar radiation. Through a repeated cycle of reactions (2) and (3), a single chlorine atom can destroy as many as 100,000 molecules of ozone.

(1) CCl3F Cl• � •CCl2F

(2) Cl• � O3 ClO• � O2

(3) ClO• � O Cl• � O2

In 1978, the United States banned the use of CFCs in aerosols. By 1990, 40 nations had agreed to eliminate production of CFCs. The ban has had an effect. Concentrations of CFCs have begun to level off in the stratosphere and even decline in the lower atmosphere. ■

Scientists estimate that it may take decades for the ozone layer to

reach pre-1980 levels. On September 9, 2000, the “hole”

over Antarctica was record size. The amount of ozone is low in the blue areas.

S O Sulfites

Ancient Egyptians and Romans used sulfur dioxide (SO2) to preserve food. When SO2 reacts with water, hydrogen sulfite ions (HSO3

�) and sulfite ions (SO3

2�) form. The food industry uses the term sulfite to describe sulfur-based preservatives. Sulfites are antioxidants that slow down the discoloration or deterioration of food.

Restaurants used to spray fruit and lettuce in salad bars with sulfites to maintain the color of the fruit and keep the lettuce from wilting. Sulfites can cause asthma attacks and headaches. After some people died from severe reactions, the Food and Drug Administration banned the use of sulfites on fresh produce. Sulfites must be listed on the food label if a processed food contains at least 10 parts per million. ■

Organic compounds containing sulfur give onions their taste and smell. When an onion is cut, reactions occur that produce propanethial-S-oxide. When receptors in your eyes are exposed to this irritating gas, they trigger the production of tears.

Group 6A R31

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