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Elements Handbook

Elements HandbookHandbook

Group 1A: Alkali Metals

Lithium

| discovered in 1817 by Johan August Arfvedson

Sodium

| discovered in 1807 by Sir Humphry Davy

Potassium

| discovered in 1807 by Sir Humphry Davy

Rubidium

| discovered in 1861 by Robert Bunsen & Gustav Kirchhoff

Cesium

| discovered in 1860 by Robert Bunsen & Gustav Kirchhoff

Francium

| discovered in 1939 by Marguerite Perey

Physical Properties

• Alkali metals are silver-gray solids that are soft enough to cut with a knife. They are soft because they have only one valence electron.

• The presence of a single valence electron also explains the low melting points and boiling points of alkali metals.

Pure alkali metals are stored under oil because they will tarnish on exposure to oxygen or moisture in air.

Sources

• Alkali metals do not occur uncombined in nature because they are highly reactive.

• Sodium occurs widely as sodium chloride in underground salt and brine deposits. It is a major component of seawater.

• Sodium is the only alkali metal manufactured on a large scale. It is generally produced by the electrolysis of molten sodium chloride.

2NaCl(

l

) 2Na(

l

)

�

Cl

2

(

g

)

Lithium and potassium are produced by a similar process.

• The mineral sylvite, KCl, is a source of potassium.

3

LiLithium

6.941

21

11

NaSodium

22.990

2 8 1

19

KPotassium

39.098

2 8 8 1

37

RbRubidium

85.468

2 8

18 8 1

55

CsCesium

132.91

2 8

18 18 8 1

87

FrFrancium

(223)

2 8

18 32 18 8 1

DensityMelting and Boiling Points

Li

1342

Rb

700

Cs

670

Na

883

K

758

179300

0

600

900

1200

1500

Tem

per

atu

re (

�C)

39 2898 64

mpbp

Li

0.53

Rb

1.53

Cs

1.88

Na

0.97

K

0.86

0.5

0

1

1.5

2

Den

sity

(g

/cm

3 )

Steel cathode (�)

Molten NaCl

Molten Na

Iron screen

Na(l )

NaClCl2(g )

Graphite anode (�)

Manufacture of Sodium

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Group 1A: Alkali Metals

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Atomic Properties

• Alkali metals have an electron configuration that ends in

ns

1

.

• Alkali metals form ions with a 1

�

charge.

• The atoms of alkali metals are the largest in their periods.

• Cesium metal is used in photocells because its first ionization energy is very low.

Alkali metals can be identified by the colors produced when their compounds are heated in a flame.

Important Compounds and Reactions

• The alkali metals are the most reactive metals.

• All alkali metals react with water to form an alkaline solution. Example:

2K(

s

)

�

2H

2

O(

l

) 2KOH(

aq

)

�

H

2

(

g

)

• All alkali metals react with halogens to form an ionic halide. Example:

2Cs(

s

)

�

Cl

2

(

g

) 2CsCl(

s

)

�

H

�

�

442.8 kJ/mol

• Sodium hydrogen carbonate (baking soda) produces CO

2

(

g

) and steam, which causes baked goods to rise.

2NaHCO

3

(

s

) Na

2

CO

3

(

s

)

�

CO

2

(

g

)

�

H

2

O(

g

)

�

H

�

129 kJ/mol

• Lithium hydride is a convenient source for hydrogen.

LiH(

s

)

�

H

2

O(

l

) LiOH(

aq

)

�

H

2

(

g

)

• Potassium superoxide, KO

2

, is a source of oxygen in submarines. It removes CO

2

from the atmosphere as it produces oxygen.

4KO

2

(

s

)

�

2CO

2

(

g

) 2K

2

CO

3

(

s

)

�

3O

2

(

g

)

• Lithium carbonate, Li

2

CO

3

, is a prescription medicine for chronic depression.

• Sodium hypochlorite, NaClO, is used as a bleach and to disinfect swimming pools.

• Sodium hydrogen carbonate, NaHCO

3

, is used in fire extinguishers.

• Potassium nitrate, KNO

3

, is used in matches and chemical fertilizers.

• Sodium hydroxide, NaOH, is used as a drain cleaner. It is also used to produce other chemicals and in many industrial processes.

ElectronegativityFirst Ionization Energy

Lithium Sodium Potassium Rubidium Cesium

Atomic

radius (pm)

Ionic

radius (pm)

Li Na K Rb Cs

Li+ Na+ K+ Rb+ Cs+

156 191 238 255 273

60 95 133 148 169

Li

520

Rb

403

Cs

376

Na

496

K

419

150

0

300

450

600

En

erg

y (

kJ/m

ol)

Li

1.0

Rb

0.8

Cs Fr

0.7 0.7

Na

0.9

K

0.8

0.25

0

0.5

0.75

1.0

Ele

ctr

on

eg

ati

vit

y

Uses of NaOH

32%35%

14%13%

5%

4%

32%35%

14%13%

5%

4%

Pulp and Paper

Soaps and Detergents

Petroleum

Other Industries

Organic Chemicals

Inorganic Chemicals

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Elements Handbook

Elements HandbookHandbook

Group 1A: Alkali Metals

Na

Vapor Lamps

When streetlights have a golden glow, the source of the light is probably sodium vapor. Inside the lamp is a sealed tube containing a sodium-mercury alloy and a starter gas, such as xenon. Electrodes at each end of the tube are connected to an electric circuit. When the lamp is on, a continuous spark, or arc, forms between the electrodes. The arc produces enough heat to vaporize the sodium and mercury atoms. Within the arc, atoms ionize. Outside the arc, ions recombine with electrons and light is emitted—yellow for sodium and blue-green for mercury.

A sodium vapor lamp uses less energy than most other light sources and costs less to operate. But the lamp isn’t a perfect light source.The color of an object is visible only when light of that color is reflected off the object. So yellow lines on the pavement appear yellow under a sodium vapor lamp, but a red stop sign appears gray.

■

Na K

Restoring Electrolytes

“Don’t sweat it” may be good advice for handling stress, but not for maintaining a healthy body. The sweat you produce on a hot day or during exercise cools your body as it evaporates. Sweat consists mainly of water, sodium chloride, and small amounts of other inorganic salts. The salts are electrolytes that help keep the volume of body fluids constant. Electrolytes produce ions when they dissolve in water.

Body fluids contain sodium ions and potassium ions. Potassium ions are the principal cations inside cells. Sodium ions are the principal cations in the fluids outside of cells. The transmission of nerve impulses depends on the movement of sodium and potassium ions across the membranes of nerve cells. Potassium ions cause the heart muscle to relax between heartbeats.

Replacing the water lost during exercise is important, but not sufficient. Electrolytes must be replaced too. Some signs of electrolyte depletion are muscle cramps, nausea, and an inability to think clearly. Many athletes use sports drinks to replace electrolytes. Some experts recommend these drinks for people who lose more than 8 liters of sweat daily or exercise continuously for more than 60 minutes.

Table salt is the chief source of sodium in the diet. But large amounts are also found in unexpected places, such as eggs. For a healthy adult, the recommended daily intake of sodium chloride is about 5 grams per day—about half the amount many people consume. The daily recommended amount of potassium is about 1 gram. The word

daily

is important because your kidneys excrete potassium even when the supply is low.

Eating foods high in potassium and low in sodium is ideal. Such foods include bananas, chicken, and orange juice.

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Cs

Cesium Atomic Clock

Many watches contain a quartz crystal that vibrates at a constant rate. The vibration provides the “beat” that is trans-lated into the time you see displayed. A clock with a quartz crystal is more accurate than a mechanical clock, which has moving parts that can be worn down by friction. But a quartz crystal isn’t accurate enough for modern communication and navigation systems.

For greater accuracy, you need an atomic clock, which may gain or lose only one second in 20 million years! In most atomic clocks, cesium-133 atoms provide the “beat.” Unlike quartz crystals, all cesium-133 atoms are identical and they don’t wear out with use. The clock is designed so that the atoms repeatedly absorb and emit radiation. The emitted radiation has a frequency of exactly 9,192,631,770 cycles per second. The cycles are counted and translated into seconds, minutes, and hours. Since 1967, the second had been defined based on the cesium clock.

There is a standard cesium atomic clock at the National Institute of Standards and Technology in Fort Collins, Colorado. A short-wave radio station transmits accurate time signals from this clock to most of North America. Clocks and

watches advertised as “atomic” or “radio controlled” contain a tiny antenna and receiver that pick up

the signal and decode the information. The owner sets the time zone. The radio signal sets the time.

■

Global Positioning System (GPS) receivers can pinpoint any location on Earth to within a few meters. They depend on accurate time signals from atomic clocks in 24 Earth-orbiting satellites.

Na

Salt of the Earth

In the ancient world, table salt (NaCl) was an extremely valuable commodity. Before refrigeration, salt was used to preserve foods such as meats and fish. As salt draws the water out of bacteria, they shrivel up and die. Sailors (“salty dogs”) especially depended on salt pork and fish to survive long voyages.

Trade routes were established and roads were built to transport salt. In ancient China, coins were made of salt and taxes were levied on salt. Roman soldiers were paid an allotment called a

salarium argentum

, or “salt silver,” from which the word

salary

is derived.

■

Li

Bipolar Disorder

About two million people in the United States experience the extreme mood swings of bipolar disorder. During a manic phase, they think they can conquer the world. During a depression, they may feel hopeless. The ionic compound lithium carbonate often is used to control these symptoms. Lithium ions probably have an effect on the transmission of messages between brain cells.

■

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

Most alkali metals are stored under oil. But

lithium

is so light that it floats on oil. So a coat of petroleum jelly is applied to lithium before it is stored.

Group 1A: Alkali Metals

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