R18

Elements Handbook

*diamond *graphite

Elements HandbookHandbook

Carbon

| known since ancient times

Silicon

| discovered in 1824 by Jöns Jacob Berzelius

Germanium

| discovered in 1886 by Clemens Winkler

Tin

| known since ancient times

Lead

| known since ancient times

Physical Properties

• Group 4A elements are all solids at room temperature.

• The metallic properties of Group 4A elements increase from carbon to lead.

• Diamond, graphite, and buckminsterfullerene are three allotropes of carbon.

Graphite is more stable than diamond at STP, but the activation energy is too high for diamond to change to graphite at these conditions.

Sources

• Carbon is found in nature as an element, in Earth’s atmosphere as carbon dioxide, in Earth’s crust as carbonate minerals, and in organic compounds produced in cells.

• Silicon can be produced by the reduction of silicon dioxide (silica) with magnesium, carbon, or aluminum. Example:

SiO

2

(

s

)

�

2Mg(

s

) Si(

s

)

�

2MgO(

s

)

• Tin is prepared by reduction of the mineral cassiterite, SnO

2

.

SnO

2

(

s

)

�

2C(

s

) 2CO(

g

)

�

Sn(

s

)

• Lead is refined from the mineral galena, PbS. Galena is heated in air to form a mixture of PbO and PbSO

4

. Lead is produced through further reaction of these compounds with PbS.

About 90% of the minerals in Earth’s crust are silica and silicates. In silicates, each silicon atom is surrounded by three or four oxygen atoms. These units can be linked together in chains, sheets, rings, or crystals.

82

PbLead

207.2

28

183218 4

50

SnTin

118.69

28

1818 4

32

GeGermanium

72.59

28

18 4

14

SiSilicon

28.086

28 4

6

CCarbon

12.011

2 4

DensityMelting and Boiling Points

2623

1751

2850

2000

0

3000

1000

4000

5000

Tem

pera

ture

(�C

)

232 327

945

mp

bp

C Sn PbSi Ge

3280

1420

4827

>3562*

2.27*

7.26

11.34

2.34

5.32

3

0

6

9

12

Den

sit

y (

g/c

m3)

C Sn PbSi Ge

Phase Diagram of Carbon

0100

101

102

103

104

105

106

2000

Diamond(s)

Graphite(s)

Liquid

Vapor

4000 6000

Pre

ssu

re (

atm

)

Temperature (C�)

Fe2SiO4olivine

AlSi2O5OHpyrophyllite

Al2SiO5andalusite

Group 4A

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Group 4A R19

Atomic Properties

• Group 4A elements have an electron configuration that ends in ns2np2.

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

• Silicon and germanium are semiconductors.

When carbon and silicon form four covalent bonds, there is often sp3hybridization. The result is compounds and ions with tetrahedral structures.

Important Compounds and Reactions

• Group 4A elements are oxidized by halogens. Example:

Ge(s) � 2Cl2(g) GeCl4(l)

• Group 4A elements combine with oxygen to form oxides. Example:

Sn(s) � O2(g) SnO2(s)

• Complete combustion of hydrocarbons yields carbon dioxide and water. Example:

CH4(g) � 2O2(g) CO2(g) � 2H2O(l)�H � �890 kJ/mol

• Plants use carbon dioxide to produce carbohydrates and oxygen.

• Aqueous sodium silicate, Na2SiO3, is used as an adhesive for paper, as a binder in cement, and to stabilize shale during oil drilling.

SiO2(s) � 2NaOH(aq) Na2SiO3(aq) � H2O(l)

• Acetylene is a fuel used for welding. It forms when calcium carbide and water react.

CaC2(s) � 2H2O(l) C2H2(g) � Ca(OH)2(aq)

• Tungsten carbide, WC, is used on the cutting surfaces of drill bits and saw blades.

• Lead(IV) oxide, PbO2, is used as electrodes in lead acid car batteries.

• Tin(II) fluoride, SnF2, is used in toothpaste to prevent tooth decay.

Silicon dioxide, SiO2, is the sand on many beaches and is used to make glass.

First Ionization Energy Electronegativity

Atomic

radius (pm)

Ionic

radius (pm)

C Si Ge Sn Pb

C4+ Si4+ Ge4+ Sn4+ Pb4+

77 109 122 139 175

15 41 53 71 84

1086

708 715786 761

600

300

0

900

1200

En

erg

y (

kJ/m

ol)

C Sn PbSi Ge0

1.0

2.0

3.0

Ele

ctr

on

eg

ati

vit

y

C

2.5

Sn

1.8

Pb

1.9

Si

1.8

Ge

1.8

4�

methane molecule silicate ion

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R20 Elements Handbook

Elements HandbookHandbook

Group 4A

C Green Chemistry

The term green chemistry was coined in 1992. It describes the effort to design chemical processes that don’t use or produce hazardous substances. The goal is to protect the environment and conserve resources. For example, if a catalyst is used to reduce the temperature at which a reaction occurs, the process requires less energy.

Carbon dioxide is at the center of a green chemistry success story. Organic solvents are used to dissolve substances that are insoluble in water. Many of these solvents are toxic. It can be difficult to remove all traces of the toxic solvent from reaction products and safely recycle or dispose of the solvent. Supercritical carbon dioxide can replace some organic solvents.

A gas becomes a supercritical fluid at a temperature and pressure called its critical point. For carbon dioxide, this occurs at 31.1ºC and about 100 atmospheres. At its critical point, carbon dioxide is in a hybrid state. It has a high density (like a liquid) but it is easily compressed (like a gas). Many organic compounds dissolve in supercritical carbon dioxide. The solvent is easily separated from a reaction mixture because it evaporates at room temperature and pressure. It is also used to separate substances from mixtures. It can extract caffeine from coffee beans, dry-clean clothes, or clean circuit boards. ■

After the caffeine is extracted, the coffee beans are dried and roasted. The aqueous solution of caffeine is sold to soft drink manufacturers.

Si Optical Glass

Glass is a material with the structure of a liquid, but the hardness of a solid. In most solids, the particles are arranged in an orderly lattice. In solid glass, the molecules remain disordered, as in a liquid. The main ingredient in most glass is silica (SiO2), which is one of the few substances that can cool without crystallizing.

The glass used in eyeglasses, microscopes, and telescopes is called optical glass. It is purer than window glass and transmits more light. Optical glass can be drawn into long fibers that are used like tiny periscopes to view tissues deep within the human body.

In an optical fiber, light travels through a thin glass center called the core. A second glass layer reflects light back into the core. An outer plastic layer protects the fiber from damage.

When the fibers are bundled into cables, they often replace electrical cables in computer networks. They are also used to transmit television signals and phone calls over long distances. ■

Caffeine dissolves

inside coffee beans

soaked in water.

Caffeine diffuses into

supercritical CO2.

Water droplets leach

caffeine from CO2.

Extraction

Absorption

Decaffeinating Coffee

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R21

Si Semiconductors

With a cellular phone, you can call your friends from almost any location. You can play a video game, read e-mails, or get the latest news. You may even be able to take and send digital photos. How can such a complex device be small enough to fit in your pocket? Semiconductor technology is responsible.

Silicon is a semiconductor. In its pure form, it conducts an electric current better than most nonmetals but not as well as metals. But its ability to conduct can be changed dramatically by doping, or adding traces of other elements, to the silicon crystal.

Doping with arsenic produces a donor, or n-type, semiconductor. Each arsenic atom has five valence electrons, compared with four for silicon. So there are extra electrons in the crystal. Doping with boron produces an acceptor, or p-type, semiconductor. Because boron has only three valence electrons, there is a positive “hole” in the crystal for every boron atom. The extra electrons or holes are free to move and conduct an electric current.

Combinations of n-type and p-type semiconductors are used to build tiny electronic components. An integrated circuit containing millions of components can fit on a semiconductor wafer that is smaller than a fingernail! The resulting “chip” can control a computer, portable CD player, calculator, or cellular phone. ■

C Beyond Buckyballs

Buckminsterfullerene (C60) is one member of a family of fullerenes. These structures are closed-cage spherical or nearly spherical forms of elemental carbon. The cages are networks of 20 to 600 carbon atoms.

Scientists have verified the existence of nesting spheres of fullerenes. C60 can be nested inside C240, and this pair can be nested inside C540. These nesting structures are sometimes called bucky-onions because they resemble the layers of an onion. ■

Dr. Sumio Iijima discovered a tubular fullerene, or carbon nanotube, in Japan in 1991.

Did You Know...Did You Know...??One name for diamonds is “ice.” A diamond can quickly draw heat from your hand when you touch it. Such a high thermal conductivity is unusual for a substance containing covalent bonds.

Group 4A R21

A cell phone’s integrated circuit must process more and more data as features are added to the phone.

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Elements Handbook

Elements HandbookHandbook

Group 4A

C

Greenhouse Gases

There are gases in Earth’s atmosphere that are called greenhouse gases because they act like the glass in a greenhouse. Sunlight easily passes through these gases to Earth’s surface. Some of the solar energy is reflected off the surface as infrared radiation. This radiation is absorbed by greenhouse gases and radiated back to Earth. By trapping infrared radiation, the greenhouse gases keep Earth’s surface about 33

°

C warmer than it would be otherwise.

Carbon dioxide (CO

2

) is the most abundant greenhouse gas. It is released into the air as a product of cellular respiration and removed from the air during photosynthesis. Such interactions normally keep the amount of atmospheric CO

2

in check. But the burning of fossil fuels releases more than 20 billion metric tons of CO

2

every year. Also, as forests are cleared for agriculture, the ability of plants to remove CO

2

from the atmosphere is reduced.

With an increase in greenhouse gases, more infrared radiation is trapped, causing global warming.

Scientists agree that a rise of only a few degrees in Earth’s temperature could cause problems. They disagree on how severe the problems could be. Could climates change so that farm-lands become deserts? Could the melting of ice caps cause sea levels to rise until coastal cities are under water?

■

The glass in a greenhouse traps infrared radiation.

Polycrylonitrile (PAN) is used to make some carbon fibers. PAN fibers oxidize when they are heated in air. The chains of aromatic rings that result are then heated in the absence of air. The chains fuse into long thin ribbons of almost pure carbon, which pack together in stacked layers.

Composites reinforced with carbon fibers are stronger than steel, yet light in weight. These composites are used in sports equipment, such as hockey sticks and golf clubs. It is less tiring to swing a tennis racket made from a carbon-fiber composite than one made from wood or metal. Carbon-fiber baseball bats act more like wood bats than do aluminum bats.

■

Average Temperature at Earth’s Surface

19500

13.8

14.0

14.2

14.4

14.6

1975 2000Year

Tem

pera

ture

(�C

)

When a hockey stick has a composite shaft, there can be a quicker release of the shot.

Si

Composite Materials

Most composites contain two distinctly different materials. The materials can be arranged in layers as when a sheet of plastic is sealed between panes of glass. Or a composite may consist of a matrix in which fibers of a second material are embedded. Often, the matrix is plastic. The fibers can be carbon.

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C Carbon Monoxide

It is hard to detect colorless, odorless carbon monoxide gas. When it is inhaled, its molecules bind to the hemoglobin in red blood cells. They bind about 200 times more effectively than oxygen molecules do. So less oxygen reaches body tissues. Head-aches, dizziness, nausea, and drowsiness are symptoms of low-level carbon monoxide poisoning. Higher levels of carbon monoxide are fatal.

The incomplete combustion of fuel in gas furnaces and space heaters produces carbon monoxide. It also forms in internal combustion engines that are not well maintained. In the United States, cars have catalytic converters, which convert carbon monoxide to carbon dioxide.

Cigarette smoke contains carbon monoxide. The carbon monoxide from one cigarette can remain in a smoker’s blood for several hours. Smoking increases the risk of heart attacks because the heart must pump harder to deliver oxygen to cells when the level of oxygen is reduced. ■

C Recycling Plastics

At about 2 kg of waste per person per day, the United States leads the world in the production of solid waste. Luckily, the United States is also a leader in recycling. It is important to recycle plastics because they are made from crude oil, a non-renewable resource. Also, some plastics release toxic gases, such as hydrogen cyanide (HCN) and hydrogen chloride (HCl), when they burn in an incinerator. Finally, plastics are used for packaging material because they do not decay when exposed to sunlight, water, or microorganisms. The downside of this resistance to decay is that plastics can remain unchanged in dumps and landfills for decades.

Plastics are usually sorted by type before they are melted and reprocessed. The plastics industry has a code to identify common types of plastics. The numeral 1 is assigned to

polyethylene terephthalate (PET), which is used in soft-drink bottles. The numeral 2 refers to high-density polyethylene (HDPE), which is used in milk jugs and shampoo bottles. These are the two types in greatest demand. Carpets and clothing are made from recycled PET fibers. Recycled HDPE is used as a wood substitute for decks and benches. ■

Fleece clothing often contains post- consumer recycled (PCR) plastic.

Si Silicone Polymers

If you have worn hard contact lenses or used shaving cream, you have used a silicone. Silicone polymers have chains in which silicon and oxygen alternate. The properties of silicones depend on the groups bonded to the silicon atoms and the length of the chains.

In silicone rubber and resins, there are cross-links between the chains. These silicones repel water and remain elastic, even at low temperatures. They are used in space suits, as gaskets in airplane win-dows, and as sealants that are squeezed into place and left to harden. ■

In polydimethylsiloxane, two methyl groups are bonded to each silicon atom in the chain. Polydimethylsiloxane is used as a lubricant in skin and suntan lotions.

CH3 CH3

CH3

CH3

Si

O

Si Si

O

CH3

CH3

CH3

CH3

n

Did You Know...Did You Know...??Members of the Scott expedition to the South Pole in 1912 may have died because of tin. Their supply of paraffin fuel leaked out through tiny holes in the tin-soldered joints of the storage cans because tin slowly changes to a powder below 13°C.

Group 4A R23

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