R32 Elements Handbook

Elements HandbookHandbook

Group 7A: Halogens

Fluorine | discovered in 1886 by Henri Moissan

Chlorine | discovered in 1774 by Carl Wilhelm Scheele

Bromine | discovered in 1826 by Antoine-Jérôme Balard

Iodine | discovered in 1811 by Bernard Courtois

Astatine | discovered in 1940 by Dale R. Corson, K. R. Mackenzie, & Emilio Segrè

Physical Properties

• Halogens are nonmetals. At room temperature, fluorine and chlorine are gases and bromine is a liquid. Iodine and astatine are solids.

• Halogens are very reactive. The reactivity decreases from fluorine to astatine. Halogens do not exist in the elemental form in nature.

• Astatine isotopes are radio-active with short half-lives.

There are vapors in the bromine and iodine tubes because bromine is volatile and iodine sublimes easily at room temperature.

Sources

• Chlorine gas is made commercially by the electrolysis of brine.

2NaCl(aq) � 2H2O(l) Cl2(g) � H2(g) � 2NaOH(aq)

• Bromine is obtained from seawater by a displacement reaction with chlorine.

2NaBr(aq) � Cl2(g) 2NaCl(aq) � Br2(l)

• Iodine is found in brine and in sodium iodate, NaIO3, in deposits of sodium nitrate. Iodine is produced from NaIO3 by this redox reaction.

2NaIO3(aq) � 5NaHSO3(aq) I2(g) � 2Na2SO4(aq) � 3NaHSO4(aq) � H2O(l)

• Fluorine is made by the electrolysis of potassium fluoride, KF, dissolved in liquid hydrogen fluoride, HF.

Fluorite, CaF2, is the principal mineral of fluorine. The term fluorescent comes from this mineral, which glows in the presence of UV radiation.

85

AtAstatine

(210)

28

183218 7

53

Iodine

126.90

28

1818 7

35

BrBromine

79.904

28

18 7

17

ClChlorine

35.453

28 7

9

FFluorine

18.998

2 7

Br2

Cl2

2

Density

4.93

3.12

2

1

0

3

4

5

De

nsit

y (

g/c

m3)

1.81 � 10�3

3.21 � 10�3

F Cl Br

Melting and Boiling Points

0

�300

�200

100

�100

200

Te

mp

era

ture

(�C

)

114

184

59

�7�35

�101

mp

bp�188

F Cl Br

�220

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Group 7A: Halogens

R33

Atomic Properties

• Group 7A elements have an electron configuration that ends in

ns

2

np

5

.

• Halogens exist as diatomic molecules.

• Each halogen has the highest electronegativity in its period.

• The most common ionic charge for halogens is 1

�

. Except for fluorine, halogens also have positive oxidation numbers of

�

1,

�

3,

�

5, and

�

7.

Chlorine forms four anions with oxygen. The oxidation number of chlorine is different in each anion.

Important Compounds and Reactions

• Halogens form metal halides. Example:

2Na(

s

)

�

Cl

2

(

g

) 2NaCl(

s

)

�

H

�

�

411 kJ/mol

• Halogens form hydrogen halides. Example:

H

2

(

g

)

�

Cl

2

(

g

) 2HCl(

g

)

�

H

�

�

92.3 kJ/mol

• Dry bleach is a mixture of compounds represented by the formula CaCl(ClO). Dry bleach is used to bleach paper and textiles. It also removes stains and disinfects laundry.

Ca(OH)

2

(

aq

)

�

Cl

2

(

g

) CaCl(ClO)(

aq

)

�

H

2

O(

l

)

• Small amounts of oxygen are produced in the laboratory by heating potassium chlorate, KClO

3

. Potassium chlorate is an oxidizing agent in fireworks, matches, and explosives.

2KClO

3

(

s

) 2KCl(

s

)

�

3O

2

(

g

)

• Canisters of sodium chlorate are used on submarines to produce oxygen. They are carried on airplanes in case of an emergency.

• Chlorine is used to make the monomer vinyl chloride, CH

2

“

CHCl, which reacts to form the polymer polyvinyl chloride (PVC).

• Hydrofluoric acid (HF) is made from the mineral fluorite and sulfuric acid. Although HF is extremely dangerous, it has many uses, including cleaning metals and frosting glass.

CaF

2

(

s

)

�

H

2

SO

4

(

aq

) 2HF(

g

)

�

CaSO

4

(

s

)

• Nonstick pans are coated with a polymer of tetrafluoroethene, F

2

C

“

CF

2

.

• Tincture of iodine is a solution of iodine, I

2

, and potassium iodide, KI, in alcohol. It is an example of an iodine-based skin disinfectant.

Chlorine reacts vigorously with sodium to form solid sodium chloride.

Electronegativity

0

1.0

3.0

2.0

4.0

Ele

ctr

on

eg

ati

vit

y

4.0

2.52.2

3.02.8

F AtCl Br

First Ionization Energy

1681

1009 926

1256 1143

600

0

1200

1800

En

erg

y (

kJ/m

ol)

F AtCl Br

Atomic

radius (pm)

Ionic

radius (pm)

F Cl Br I At

F– Cl– Br– I–

62 102 120 140 140

133 181 196 220

Hypochlorite Chlorite

1�1�

Chlorate

1�

Perchlorate

1�

Appendix_A.fm Page 33 Friday, October 22, 2004 9:59 AM

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R34 Elements Handbook

Elements HandbookHandbook

Group 7A: Halogens

F Tooth Decay

Your teeth have a hard outer layer called enamel. Enamel is mainly calcium carbonate, CaCO3, and hydroxyapatite, [Ca3(PO4)2]3•Ca(OH)2. Lactic acid, C3H6O3, is the main cause of tooth decay. It forms when bacteria in saliva feed on sugars present in the sticky plaque on tooth surfaces. An increase in H� concentration causes the minerals in tooth enamel to decay faster.

Fluoride ions are added to the water supply in many cities. Most toothpastes contain fluoride ions. The ions replace hydroxide ions in hydroxyapatite to form fluoroapatite, [Ca3(PO4)2]3•CaF2. This replacement makes the enamel more resistant.

Fluoride ions alone won’t prevent tooth decay. You need to vigorously brush your teeth and floss to keep plaque from building up on tooth enamel. ■

The use of fluorides in drinking water and toothpastes has caused such a drastic reduction in tooth decay that many young adults have never had a cavity.

Cl Swimming Pool Chemistry

The person who maintains a swimming pool has two main goals. Prevent the growth of bacteria that cause diseases. Prevent the growth of algae that can foul the water and clog the filters.Chlorine compounds are used to dis-infect pool water. “Liquid chlorine” contains sodium hypochlorite, NaClO. “Dry chlorine” is calcium hypchlorite, Ca(ClO)2. When hypochlorite ions dissolve in water, hydrolysis occurs and weak hypochlorous acid, HClO, is produced.

ClO�(aq) � H2O(l) HOCl(aq) � OH�(aq)

The amount of undissociated hypochlorous acid in the pool water depends on the pH. If the pH is too high, the hydrolysis reaction will shift toward the reactants and reduce the concen-tration of HClO. If the pH is too low, too much acid will form. A high concentration of acid can cause eye irritation, damage plaster, and corrode the metal piping and filters in the pool.

If the pH of the pool water is too high, solid sodium hydrogen sulfate can be used to react with the OH� ions.

NaHSO4(s) � OH�(aq) Na�(aq) � SO42�(aq) � H2O(l)

If the pH is too low, sodium carbonate can be used to neutralize some of the acid.

Na2CO3(s) � 2H�(aq) 2Na�(aq) � H2O(l) � CO2(aq)

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Group 7A: Halogens R35

F Artificial Blood

The most important function of blood is its ability to deliver oxygen from the lungs to cells throughout the body, and carry carbon dioxide from the cells to the lungs. Blood transfusions have been the traditional response to blood loss due to severe injuries or surgery. Blood transfusions have saved millions of lives. But it takes time to determine the recipient’s blood type and less common blood types may not be available. Also, blood can carry disease-causing bacteria and viruses.

Some artificial blood contains perfluorocarbons (PFCs). PFCs are organic compounds in which all the hydrogen has been replaced by fluorine. PFCs can dissolve and transport large quantities of oxygen. PFCs are mixed with emulsifiers and salts that mimic the composition of blood, minus the cells.

Large amounts of PFCs can be manufactured and their purity controlled. PFCs can deliver oxygen to areas where blood flow is restricted by narrowed arteries or tumors. In current trials, artificial blood is used to delay the need for blood transfusions during surgery. Because PFCs carry less oxygen than hemo-globin does, the patient must breathe oxygen-rich air.

Because blood is so complex, scientists may never find a sub-stitute to perform all its functions. Therefore, there is still a great need for donations of human blood. ■

The liquid in the beaker is a perfluorocarbon that is saturated with oxygen. The mouse survived by absorbing oxygen from the liquid in its lungs.

I Iodized Salt

The thyroid gland produces hormones that help to control the body’s growth and the energy produced by cells. Trace amounts of iodine are needed to produce thyroid hormones. An adult needs about 150 mg of iodine daily.

Ocean fish are a good source of iodine. When people used to get most of their food from local sources, a person who lived far from the ocean often had an iodine deficiency.

To compensate for the lack of iodine, the thyroid gland might enlarge. A severe deficiency could cause mental retardation. Adding potassium iodide to table salt proved to be a simple solution to this public health problem.

In the United States, iodized salt was first sold in 1924. There is about 400 mg of iodine in a teaspoon of iodized salt.

The use of iodized salt has virtually eliminated the problem of iodine deficiency in the United States. ■

Did You Know...Did You Know...The purple dye prized by Roman emperors and other rulers contains bromine. The dye was extracted from the mollusk Murex brandaris, which lives in the Mediterranean Sea.

Group 7A: Halogens R35

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