268 Chapter 4 NEL

4.64.6 The Structure and Properties of SolidsAll solids, including elements and compounds, have a definite shape and volume, arevirtually incompressible, and do not flow readily. However, there are many specific prop-erties such as hardness, melting point, mechanical characteristics, and conductivity thatvary considerably for different solids. If you hit a piece of copper with a hammer, you caneasily change its shape. If you do the same thing to a lump of sulfur, you crush it. Ablock of paraffin wax when hit with a hammer may break and will deform (Figure 1).Why do these solids behave differently?

In both elements and compounds, the structure and properties of the solid are relatedto the forces between the particles. Although all forces are electrostatic in nature, theforces vary in strength. What we observe are the different properties of substances andwe classify them into different categories (Table 1). To explain the properties of eachcategory, we use our knowledge of chemical bonding.

Ionic CrystalsIonic compounds in their pure solid form are describedas a 3-D arrangement of ions in a crystal structure. Thearrangement of ions within the crystal lattice (Figure2(b)) can be inferred from the crystal shape (Figure 2(a))and from X-ray diffraction experiments. The variation ofcrystalline structures is not a topic here, but the variety ofcrystal shapes suggests that there is an equally wide varietyof internal structures for ionic compounds.

Ionic compounds are relatively hard but brittle solids atSATP, conducting electricity in the liquid state but not inthe solid state, forming conducting solutions in water,

and having high melting points. These properties are interpreted to mean that ionicbonds are strong (evidence of hardness and melting points of the solid) and directional(evidence of brittleness of the solid) and that the lattice is composed of ions (evidenceof electrical conductivity). Ionic bonding is defined theoretically as the simultaneousattraction of an ion by the surrounding ions of opposite charge. The full charge on theions provides a greater force of attraction than do the partial charges (i.e., d� and d�)on polar molecules. In general, ionic bonding is much stronger than all intermolecularforces. For example, calcium phosphate, Ca3(PO4)2(s), in tooth enamel (ionic bonds) ismuch harder than ice, H2O(s), (hydrogen bonding).

Figure 1Different solids behave very differ-ently under mechanical stress.

Table 1 Classifying Solids

Class of substance Elements combined Examples

ionic metal � nonmetal NaCl(s), CaCO3(s)

metallic metal(s) Cu(s), CuZn3(s)

molecular nonmetal(s) I2(s), H2O(s), CO2(s)

covalent network metalloids/carbon C(s), SiC(s), SiO2(s)

Na+

Na+

Na+

Na+

Na+

Na+

Cl–

Cl–

Cl–

Cl–

Cl–

Cl–

Figure 2From a cubic crystal of table salt (a)and from X-ray analysis, scientistsinfer the 3-D arrangement forsodium chloride (b). In this cubiccrystal, each ion is surrounded bysix ions of opposite charge.

(a) (b)

The properties of ionic crystals are explained by a 3-Darrangement of positive and negative ions held together bystrong, directional ionic bonds.

crystal lattice a regular, repeatingpattern of atoms, ions, or moleculesin a crystal

Chemical Bonding 269NEL

Section 4.6

Metallic CrystalsMetals are shiny, silvery, flexible solids with goodelectrical and thermal conductivity. The hardnessvaries from soft to hard (e.g., lead to chromium)and the melting points from low to high (e.g.,mercury to tungsten). Further evidence from theanalysis of X-ray diffraction patterns shows thatall metals have a continuous and very compactcrystalline structure (Figure 3). With few excep-tions, all metals have closely packed structures.

An acceptable theory for metals must explainthe characteristic metallic properties, providetestable predictions, and be as simple as possible.According to current theory, the properties of metals are the result of the bondingbetween fixed, positive nuclei and loosely held, mobile valence electrons. This attrac-tion is not localized or directed between specific atoms, as occurs with ionic crystals.Instead, the electrons act like a negative “glue” surrounding the positive nuclei. As illus-trated in Figure 4, valence electrons are believed to occupy the spaces between the pos-itive centres (nuclei). This simple model, known as the electron sea model, incorporatesthe ideas of

• low ionization energy of metal atoms to explain loosely held electrons

• empty valence orbitals to explain electron mobility

• electrostatic attractions of positive centres and the negatively charged electron“sea” to explain the strong, nondirectional bonding

Figure 5 shows a cross-section of the crystal structure of a metal. Each circled posi-tive charge represents the nucleus and inner electrons of a metal atom. The shaded areasurrounding the circled positive charges represents the mobile sea of electrons. The elec-tron sea model is used to explain the empirical properties of metals (Table 2).

Figure 3Metal crystals are small, and usuallydifficult to see. Zinc-plated or galva-nized metal objects often have largeflat crystals of zinc metal that arevery obvious.

+++

+ +++

+++

+ +++

+ +++

Figure 4In this model of metallic bonding,each positive charge represents thenucleus and inner electrons of ametal atom, surrounded by a mobile“sea” of valence electrons.

Table 2 Explaining the Properties of Metals

Property Explanation

shiny, silvery valence electrons absorb and re-emit the energy from all wavelengths ofvisible and near-visible light

flexible nondirectional bonds mean that the planes of atoms can slide over each other while remaining bonded

electrical valence electrons can freely move throughout the metal; a battery can force conductivity additional electrons onto one end of a metal sample and

remove other electrons from the other end

hard solids electron sea surrounding all positive centres produces strong bonding

crystalline electrons provide the “electrostatic glue” holding the atomic centres togetherproducing structures that are continuous and closely packed

The properties of metallic crystals are explained by a 3-Darrangement of metal cations held together by strong,nondirectional bonds created by a “sea” of mobile electrons.

Metallic Bonding AnalogyThe next time you have a RiceKrispie square, look at it carefullyand play with it. The marshmallowis the glue that binds the ricetogether. If you push on thesquare, you can easily deform it,without breaking it. The marsh-mallow is like the “electron sea” ina metal; the rice represents thenuclei. The mechanical propertiesof a rice krispie square are some-what similar to those of a metal.

DID YOU KNOW ??

270 Chapter 4 NEL

Molecular CrystalsMolecular solids may be elements such as iodine andsulfur or compounds such as ice or carbon dioxide.The molecular substances, other than the waxy solids(large hydrocarbons) and giant polymers (such asplastics), are crystals that have relatively low meltingpoints, are not very hard, and are nonconductors ofelectricity in their pure form as well as in solution.From X-ray analysis, molecular crystals have a crystallattice like ionic compounds, but the arrangementmay be more complicated (Figure 5). In general, themolecules are packed as close together as their sizeand shape allows (Figure 6).

The properties of molecular crystals can be explained by their structure and the inter-molecular forces that hold them together. London, dipole�dipole, and hydrogen bondingforces are not very strong compared with ionic or covalent bonds. This would explainwhy molecular crystals have relatively low melting points and a general lack of hard-ness. Because individual particles are neutral molecules, they cannot conduct an electriccurrent even when the molecules are free to move in the molten state.

Covalent Network CrystalsMost people recognize diamonds in either jewellery or cutting tools, and quartz as gem-stones (Figure 7) and in various grinding materials, including emery sandpapers. Thesesubstances are among the hardest materials on Earth and belong to a group known ascovalent network crystals. These substances are very hard, brittle, have very high meltingpoints, are insoluble, and are nonconductors of electricity. Covalent network crystalsare usually much harder and have much higher melting points than ionic and molecularcrystals. They are described as brittle because they don’t bend under pressure, but they

The properties of molecular crystals are explained by a 3-Darrangement of neutral molecules held together by rela-tively weak intermolecular forces.

Figure 5A model of an iodine crystal basedon X-ray analysis shows a regulararrangement of iodine molecules.

indicates an I2 molecule

Figure 6Solid carbon dioxide, or dry ice, alsohas a crystal structure containingindividual carbon dioxide molecules.

Figure 7Amethyst (a), rose quartz (b), andcitrine (c) are all variations ofquartz, which is SiO2(s).

(a) (b)

(c)

Chemical Bonding 271NEL

are so hard that they seldom break. Diamond (C(s)) is the classic example of a covalentcrystal. It is so hard that it can be used to make drill bits for drilling through the hardestrock on Earth (Figure 8). Another example is silicon carbide (SiC(s))—used for grindingstones to sharpen axes and other metal tools. Carbide-tipped saw blades are steel bladescoated with silicon carbide.

The shape and X-ray diffraction analysis of diamond shows that the carbon atomsare in a large tetrahedral network with each carbon covalently bonded to four othercarbon atoms (Figure 8). Each diamond is a crystal and can be described as a singlemacromolecule with a chemical formula of C(s). The network of covalent bonds leads toa common name for these covalent crystals as covalent network. This name helps todifferentiate between the covalent bonds within molecules and polyatomic ions and thecovalent bonds within covalent network crystals. Most covalent networks involve theelements and compounds of carbon and silicon.

Crystalline quartz is a covalent network of SiO2(s) (Figure 9(a)). Glass shares the samechemical formula as quartz but lacks the long-range, regular crystalline structure ofquartz (Figure 9(b)). Purposely, glass is cooled to a rigid state in such a way that it willnot crystallize.

The properties of hardness and high melting point provide the evidence that theoverall bonding in the large macromolecule of a covalent network is very strong—stronger than most ionic bonding and intermolecular bonding. Although an individualcarbon�carbon bond in diamond is not much different in strength from any othersingle carbon�carbon covalent bond, it is the interlocking structure that is thought tobe responsible for the strength of the material. This is similar to the strength of a steelgirder and the greater strength of a bridge built from a three-dimensional arrangementof many steel girders. The final structure is stronger than any individual component.This means that individual atoms are not easily displaced and that is why the sample isvery hard. In order to melt a covalent network crystal, many covalent bonds need to bebroken, which requires considerable energy, so the melting points are very high. Electronsin covalent network crystals are held either within the atoms or in the covalent bonds.In either case, they are not free to move through the network. This explains why thesesubstances are nonconductors of electricity.

Section 4.6

Figure 8In diamond, each carbon atom hasfour single covalent bonds to eachof four other carbon atoms. As youknow from VSEPR theory, four pairsof electrons lead to a tetrahedralshape around each carbon atom.

Mohs Hardness ScaleA common method used tomeasure hardness is the Mohsscale, based on how well a solidresists scratching by another sub-stance. The scale goes from 1 (talc)to 10 (diamond). Any substance willscratch any other substance loweron the scale. One disadvantage ofthis scale is that it is not linear.Diamond (10) is much harder thancorundum (9), but apatite (5), acalcium phosphate mineral, is onlyslightly harder than fluorite (4), acalcium fluoride mineral.

DID YOU KNOW ??

covalent network a 3-D arrange-ment of covalent bonds betweenatoms that extends throughout thecrystal

(b)(a)

Figure 9(a) Quartz in its crystalline form

has a 3-D network of covalentlybonded silicon and oxygenatoms.

(b) Glass is not crystalline becauseit does not have an extendedorder; it is more disorderedthan ordered.

The properties of network covalent crystals are explained bya 3-D arrangement of atoms held together by strong, direc-tional covalent bonds.

272 Chapter 4 NEL

Other Covalent Networks of CarbonCarbon is an extremely versatile atom in terms of its bonding and structures. More thanany other atom, carbon can bond to itself to form a variety of pure carbon substances.It can form 3-D tetrahedral arrangements (diamond), layers of sheets (graphite), largespherical molecules (buckyballs), and long, thin tubes (carbon nanotubes) (Figure 10).Graphite is unlike covalent crystals in that it conducts electricity, but it is still hard andhas a high melting point. Graphite also acts as a lubricant. All of these properties, plusthe X-ray diffraction of the crystals, indicate that the structure for graphite is hexag-onal sheets of sp2 hybridized carbon atoms. Within these planar sheets the bonding is acovalent network and therefore strong, but between the sheets the bonding is relativelyweak—due to London forces. The lubricating property of graphite arises as the covalentnetwork planes slide over one another while maintaining the weak intermolecular attrac-tions. The electrical conductivity arises through formation of π bonds by the unhy-bridized p orbitals. These π bonds extend over the entire sheet, and electrons withinthem are free to move from one end of the sheet to the other.

SemiconductorsThe last five decades have seen an electronic technological revolution driven by the dis-covery of the transistor—a solid-state “sandwich” of crystalline semiconductors.Semiconductor material used in transistors is usually pure crystalline silicon or germa-nium with a tiny quantity (e.g., 5 ppm) of either a group 13 or 15 element added to thecrystal in a process called doping. The purpose of this doping is to control the electricalproperties of the covalent crystal to produce the conductive properties desired. Transistorsare the working components of almost everything electronic (Figure 11).

In an atom of a semiconductor, the highest energy levels may be thought of as beingfull of electrons that are unable to move from atom to atom. Normally, this would makethe substance a nonconductor, like glass or quartz. In a semiconductor, however, elec-trons require only a small amount of energy to jump to the next higher energy level,which is empty. Once in this level, they may move to another atom easily (Figure 12).Semiconductors can be manipulated chemically by adding small quantities of otheratoms to the crystals to make them behave in specific ways. Semiconductors are anexample of a chemical curiosity where research into atomic structure has turned out tobe amazingly useful and important. Power supplies for many satellites, and for theInternational Space Station (Figure 13), come from solar cells that are semiconductorsarranged to convert sunlight directly to electricity. Other arrangements convert heat toelectricity, or electricity to heat, or electricity to light—all without moving parts in asmall, solid device. Obviously, improving the understanding of semiconductor struc-ture was of great value to our society.

(a) (b) (c) (d)

Figure 10Models of the many forms of purecarbon:(a) diamond(b) graphite(c) buckyball(d) carbon nanotubes

Figure 11Semiconductors in transistors arecovalent crystals that have been pur-posely manipulated by doping themwith atoms that have more or fewerelectrons than the atoms in the maincrystal.

Classifying Mystery Solids (p. 279)Properties of various solids are usedto determine the type of solid.

INVESTIGATION 4.6.1

Chemical Bonding 273NEL

Section 4.6

conduction

valence

conductor(a)

ener

gy

insulator(b)

conduction

forbidden gap

valence

semiconductor(c)

conduction

valence

Figure 12All atoms and molecules haveempty orbitals. In large macromole-cules, partially filled or emptyorbitals extend throughout the solid.

Figure 13The huge solar panels that powerthis space station are multiple solid-state devices that use semiconduc-tors to change light energy toelectric current.

Properties of Ionic, Metallic, Molecular,and Covalent Network CrystalsSUMMARY

Table 3

Crystal Particles Force/Bond Properties Examples

Ionic ions ionic hard; brittle; high NaCl(s),(�, �) melting point; liquid and Na3PO4(s),

solution conducts CuSO4·5H2O(s)

Metallic cations metallic soft to very hard; Pb(s), Fe(s),solid and liquid conducts; Cu(s), Al(s)ductile; malleable; lustrous

Molecular molecules London soft; low melting point; Ne(g), H2O(l),dipole�dipole nonconducting solid, HCl(g), CO2(g),hydrogen liquid, and solution CH4(g), I2(s)

Covalent atoms covalent very hard; very high melting C(s), SiC(s),Network point; nonconducting SiO2(s)

(a) In a conductor, theseorbitals and the valenceorbitals are at or aboutthe same energy andelectrons can be easilytransported throughoutthe solid.

(b) In insulators, there is alarge energy gapbetween empty orbitalsand the valence orbitals.Electrons cannot easilyget to these orbitals andinsulators do not con-duct electricity.

(c) In semiconductors, thereis a relatively smallenergy gap between thevalence orbitals and theempty orbitals thatextend throughout thecrystal. Thermal energycan easily promote someelectrons into the emptyorbitals to provide con-ductivity.

PracticeUnderstanding Concepts

1. In terms of chemical bonds, what are some factors that determine the hardness of asolid?

2. Identify the main type of bonding and the type of solid for each of the following:(a) SiO2 (c) CH4 (e) Cr(b) Na2S (d) C (f) CaO

3. How does the melting point of a solid relate to the type of particles and forces present?

4. Explain why metals are generally malleable, ductile, and flexible.

5. State the similarities and differences in the properties of each of the following pairs ofsubstances. In terms of the particles and forces present, briefly explain each answer.(a) Al(s) and Al2O3(s) (b) CO2(s) and SiC(s)

6. To cleave or split a crystal you tap a sharp knife on the crystal surface with a smallhammer.(a) Why is the angle of the blade on the crystal important to cleanly split the crystal?(b) If you wanted to cleave a sodium chloride crystal, where and at what angle

would you place the knife blade?(c) Speculate about what would happen if you tried to cleave a crystal in the wrong

location or at the wrong angle.(d) State one application of this technique.

7. Match the solids, NaBr(s), V(s), P2O5(s), and SiO2(s), to the property listed below.(a) high melting point, conducts electricity(b) low melting point, soft(c) high melting point, soluble in water(d) very high melting point, nonconductor

Applying Inquiry Skills

8. Metals are generally good conductors of heat and electricity. Is there a relationshipbetween a metal’s ability to conduct heat and its ability to conduct electricity?(a) Predict the answer to this question. Include your reasoning.(b) Design an experiment to test your prediction and reasoning using common

examples of metals.

Making Connections

9. Suggest some reasons why graphite may be better than oil in lubricating movingparts of a machine.

10. Nitinol is known as the “metal with a memory.” It is named after the alloy and placewhere it was accidentally discovered: “Nickel titanium naval ordinance laboratory”This discovery has revolutionized manufacturing and medicine in the form of manyproducts that can “sense” and respond to changes. Research and write a brief reportabout Nitinol including its composition, a brief description of how it works, and someexisting or proposed technological applications.

11. The synthetic material moissanite (silicon carbide) looks like diamond and is used tosimulate diamonds in jewellery.(a) Compare the physical properties of moissanite and diamond.(b) Do these properties suggest a method to distinguish between a real diamond

and a simulated diamond like moissanite? Describe briefly.(c) What test do jewellers use to distinguish between these materials? Describe the

principle used and the distinction made.

Extension

12. If graphite did not conduct electricity, describe how you would change its model toexplain this, but still explain its lubricating properties.

274 Chapter 4 NEL

GO www.science.nelson.com

GO www.science.nelson.com

Traffic ControlSemiconductor “sandwiches” thatconvert electricity directly to light arecalled LEDs (light emitting diodes).LEDs are being used increasingly inbrake and tail lights of automobiles,and in traffic control “walk/don’twalk” signs. Small LEDs are used inlarge groups, so if one fails, thesign/signal is not lost. Althoughmore expensive to produce thanconventional lights, LEDs use lessenergy and last much longer.

DID YOU KNOW ??

Glass, An Ancient TechnologyGlass is one of the oldest, mostuseful and versatile materials usedby humans. It has been produced forat least four thousand years.Ordinary glass is made from sand(silicon dioxide), limestone (calciumcarbonate), and soda ash (sodiumcarbonate), all very common mate-rials. Add a little borax (sodiumborate) and you make a borosilicateglass, commonly known as Pyrexglass. Add a little metal oxide andyou can make coloured glass. Greenglass contains iron(III) oxide orcopper(II) oxide and blue glass con-tains cobalt(II) and copper(II) oxides.

DID YOU KNOW ??