456 Chapter 14 Liquids and Solids

The properties of a solid are determined primarily by the nature of theforces that hold the solid together. For example, although argon, copper, anddiamond are all atomic solids (their components are atoms), they have strik-ingly different properties. Argon has a very low melting point (�189 °C),whereas diamond and copper melt at high temperatures (about 3500 °C and1083 °C, respectively). Copper is an excellent conductor of electricity (it iswidely used for electrical wires), whereas both argon and diamond are insu-lators. The shape of copper can easily be changed; it is both malleable (willform thin sheets) and ductile (can be pulled into a wire). Diamond, on theother hand, is the hardest natural substance known. The marked differencesin properties among these three atomic solids are due to differences in bond-ing. We will explore the bonding in solids in the next section.

We have seen that crystalline solids can be divided into three classes,depending on the fundamental particle or unit of the solid. Ionic

solids consist of oppositely charged ions packed together, molecular solidscontain molecules, and atomic solids have atoms as their fundamental par-ticles. Examples of the various types of solids are given in Table 14.4.

Ionic SolidsIonic solids are stable substances with high meltingpoints that are held together by the strong forces thatexist between oppositely charged ions. The structuresof ionic solids can be visualized best by thinking ofthe ions as spheres packed together as efficiently aspossible. For example, in NaCl the larger Cl� ionsare packed together much like one would pack ballsin a box. The smaller Na� ions occupy the smallspaces (“holes”) left among the spherical Cl� ions,as represented in Figure 14.19.

Bonding in SolidsObjectives: To understand the interparticle forces in crystalline solids.

To learn about how the bonding in metals determines metallic properties.

= Cl– = Na+

TABLE 14.4

Examples of the Various Types of Solids

Type of Solid Examples Fundamental Unit(s)

ionic sodium chloride, NaCl(s) Na�, Cl� ionsionic ammonium nitrate, NH4NO3(s) NH4

�, NO3� ions

molecular dry ice, CO2(s) CO2 moleculesmolecular ice, H2O(s) H2O moleculesatomic diamond, C(s) C atomsatomic iron, Fe(s) Fe atomsatomic argon, Ar(s) Ar atoms

When spheres are packed together, there are many small empty spaces (holes) left among the spheres.

The internal forces in a soliddetermine many of the properties of the solid.

Figure 14.19The packing of Cl� andNa� ions in solid sodiumchloride.

Molecular SolidsIn a molecular solid the fundamental particle is a molecule. Examples of mo-lecular solids include ice (contains H2O molecules), dry ice (contains CO2

molecules), sulfur (contains S8 molecules), and white phosphorus (containsP4 molecules). The latter two substances are shown in Figure 14.20.

Molecular solids tend to melt at relatively low temperatures because theintermolecular forces that exist among the molecules are relatively weak. Ifthe molecule has a dipole moment, dipole–dipole forces hold the solid to-gether. In solids with nonpolar molecules, London dispersion forces hold thesolid together.

Part of the structure of solid phosphorus is represented in Figure 14.21.Note that the distances between P atoms in a given molecule are much shorterthan the distances between the P4 molecules. This is because the covalentbonds between atoms in the molecule are so much stronger than the Londondispersion forces between molecules.

Figure 14.21A representation of part of the structure ofsolid phosphorus, a molecular solid thatcontains P4 molecules.

14.7 Bonding in Solids 457

= P

= London dispersion forces

Covalentbondingforces

Figure 14.20(Left) Sulfur crystals contain S8

molecules. (Right) White phos-phorus contains P4 molecules.It is so reactive with the oxygenin air that it must be stored under water.

458 Chapter 14 Liquids and Solids

Atomic SolidsThe properties of atomic solids vary greatly because of the different ways inwhich the fundamental particles, the atoms, can interact with each other. Forexample, the solids of the Group 8 elements have very low melting points(see Table 14.2), because these atoms, having filled valence orbitals, cannotform covalent bonds with each other. So the forces in these solids are the rel-atively weak London dispersion forces.

On the other hand, diamond, a form of solid carbon, is one of the hard-est substances known and has an extremely high melting point (about3500 °C). The incredible hardness of diamond arises from the very strong co-valent carbon–carbon bonds in the crystal, which lead to a giant molecule.In fact, the entire crystal can be viewed as one huge molecule. A small partof the diamond structure is represented in Figure 14.18. In diamond each car-bon atom is bound covalently to four other carbon atoms to produce a verystable solid. Several other elements also form solids whereby the atoms jointogether covalently to form giant molecules. Silicon and boron are examples.

At this point you might be asking yourself, “Why aren’t solids such as acrystal of diamond, which is a ‘giant molecule,’ classified as molecular solids?”The answer is that, by convention, a solid is classified as a molecular solid onlyif (like ice, dry ice, sulfur, and phosphorus) it contains small molecules. Sub-stances like diamond that contain giant molecules are called network solids.

Science, Technology, and Society

Undoubtedly, you know aboutthe ability of oil to reduce fric-

tion between moving parts. But didyou know that some of the best in-dustrial lubricants are solids? For ex-ample, graphite—you can feel itsslipperiness by rubbing the “lead”in a pencil—is often used to lubri-cate locks, a place where oil woulddraw unwanted dirt. One of thebest lubricants in industry today ispowdered tungsten disulfide (WS2),which exists as flat platelets about500 nm in width. A team of scientists from the Weiz-mann Institute of Science in Rehovot, Israel, led byKeshef Tenne, has produced a new form of WS2 thatexists as spherical particles approximately 120 nmin diameter. The researchers hope that the spheri-cal particles will act like miniature ball bearings andthus provide even more effective lubrication thanthe irregularly shaped particles in the WS2 powders.Although thorough testing of the new lubricant hasbeen hampered by difficulties in making the spher-

ically shaped WS2 particles, initial re-sults indicate that this form providesa modest improvement in lubricatingability relative to the WS2 powders.

Interestingly, other research hasshown that just because something isspherical and rolls does not mean itwill effectively lower friction. For ex-ample, spherical C60 molecules (buck-yballs) are not very good lubricants bythemselves, although they show somepromise as additives to traditional liq-uid lubricants.

Tenne maintains that spherical WS2 might workbetter than the buckyballs, because the much largersize of the WS2 spheres should keep surfaces far-ther apart. Also, because the WS2 particles are con-structed of up to 20 concentric layers, like onions,they tend to maintain their spherical shape even asthey wear away. Only time will tell whether thespherical WS2 can beat the already outstanding lu-bricating properties of WS2 powders.

Reducing Friction

A single nanoparticle of WS2.

Bonding in MetalsMetals represent another type of atomic solid. Metals have familiar phys-ical properties: they can be pulled into wires, they can be hammered intosheets, and they are efficient conductors of heat and electricity. However,although the shapes of most pure metals can be changed relatively easily,metals are also durable and have high melting points. These facts indicatethat it is difficult to separate metal atoms but relatively easy to slide thempast each other. In other words, the bonding in most metals is strong butnondirectional.

The simplest picture that explains these observations is the electronsea model, which pictures a regular array of metal atoms in a “sea” of va-lence electrons that are shared among the atoms in a nondirectional wayand that are quite mobile in the metal crystal. The mobile electrons canconduct heat and electricity, and the atoms can be moved rather easily, as,for example, when the metal is hammered into a sheet or pulled into awire.

Because of the nature of the metallic crystal, other elements can be in-troduced relatively easily to produce substances called alloys. An alloy isbest defined as a substance that contains a mixture of elements and has metal-lic properties. There are two common types of alloys.

In a substitutional alloy some of the host metal atoms are replaced byother metal atoms of similar sizes. For example, in brass approximatelyone-third of the atoms in the host copper metal have been replaced by zincatoms, as shown in Figure 14.22a. Sterling silver (93% silver and 7% cop-per) and pewter (85% tin, 7% copper, 6% bismuth, and 2% antimony) areother examples of substitutional alloys.

An interstitial alloy is formed when some of the interstices (holes)among the closely packed metal atoms are occupied by atoms much smallerthan the host atoms, as shown in Figure 14.22b. Steel, the best-knowninterstitial alloy, contains carbon atoms in the “holes” of an iron crystal.The presence of interstitial atoms changes the properties of the host metal.Pure iron is relatively soft, ductile, and malleable because of the absenceof strong directional bonding. The spherical metal atoms can be movedrather easily with respect to each other. However, when carbon, whichforms strong directional bonds, is introduced into an iron crystal, the pres-ence of the directional carbon–iron bonds makes the resulting alloy harder,stronger, and less ductile than pure iron. The amount of carbon directlyaffects the properties of steel. Mild steels (containing less than 0.2% car-bon) are still ductile and malleable and are used for nails, cables, and chains.Medium steels (containing 0.2–0.6% carbon) are harder than mild steels andare used in rails and structural steel beams. High-carbon steels (containing0.6–1.5% carbon) are tough and hard and are used for springs, tools, andcutlery.

Many types of steel also contain elements in addition to iron and car-bon. Such steels are often called alloy steels and can be viewed as beingmixed interstitial (carbon) and substitutional (other metals) alloys. An ex-ample is stainless steel, which has chromium and nickel atoms substitutedfor some of the iron atoms. The addition of these metals greatly increasesthe steel’s resistance to corrosion.

Figure 14.22Two types of alloys.

(a) Brass is a substitutional alloyin which copper atoms in thehost crystal are replaced by thesimilarly sized zinc atoms.

(b) Steel is an interstitial alloy inwhich carbon atoms occupy interstices (holes) among theclosely packed iron atoms.

14.7 Bonding in Solids 459

copper

zincBrass

ironcarbonSteel

A steel sculpture in Chicago.

460 Chapter 14 Liquids and Solids

Science, Technology, and Society

Adistraught mother walks into the optical shopcarrying her mangled pair of $400 eyeglasses.

Her child had gotten into her purse, found herglasses, and twisted them into a pretzel. She handsthem to the optometrist with little hope that theycan be salvaged. The optometrist says not to worryand drops the glasses into a dish of warm waterwhere the glasses magically spring back to theiroriginal shape. The optometrist hands the restoredglasses to the woman and says there is no chargefor repairing them.

How can the frames “remember” their originalshape when placed in warm water? The answer isa nickel–titanium alloy called Nitinol that was de-veloped in the late 1950s and early 1960s at theNaval Ordnance Laboratory in White Oak, Mary-land, by William J. Buehler. (The name Nitinolcomes from Nickel Titanium Naval Ordnance Lab-oratory.)

Nitinol has the amazing ability to remember ashape originally impressed in it. For example, notethe accompanying photos. What causes Nitinol tobehave this way? Although the details are too com-plicated to describe here, this phenomenon resultsfrom two different forms of solid Nitinol. WhenNitinol is heated to a sufficiently high temperature,the Ni and Ti atoms arrange themselves in a way

that leads to the most compact and regular pat-tern of the atoms—a form called austenite (A).When the alloy is cooled, its atoms rearrangeslightly to a form called martensite (M). The shapedesired (for example, the word ICE ) is set into thealloy at a high temperature (A form), then themetal is cooled, causing it to assume the M form.In this process no visible change is noted. Then, ifthe image is deformed, it will magically return ifthe alloy is heated (hot water works fine) to a tem-perature that changes it back to the A form.

Nitinol has many medical applications, includ-ing hooks used by orthopedic surgeons to attachligaments and tendons to bone and “baskets” tocatch blood clots. In the latter case a length ofNitinol wire is shaped into a tiny basket and thisshape is set at a high temperature. The wires form-ing the basket are then straightened so they canbe inserted as a small bundle through a catheter.When the wires warm up in the blood, the basketshape springs back and acts as a filter to stop bloodclots from moving to the heart.

One of the most promising consumer uses ofNitinol is for eyeglass frames. It’s handy to haveframes that remember their original shape. Nitinolis also now being used for braces to straightencrooked teeth.

Metal with a Memory

The word ICE is formed from Nitinolwire.

The wire is stretched to obliteratethe word ICE.

The wire pops back to ICE whenimmersed in warm water.

Example 14.4

Identifying Types of Crystalline SolidsName the type of crystalline solid formed by each of the following sub-stances:

a. ammonia

b. iron

c. cesium fluoride

d. argon

e. sulfur

Solution

a. Solid ammonia contains NH3 molecules, so it is a molecular solid.

b. Solid iron contains iron atoms as the fundamental particles. It is anatomic solid.

c. Solid cesium fluoride contains the Cs� and F� ions. It is an ionicsolid.

d. Solid argon contains argon atoms, which cannot form covalentbonds to each other. It is an atomic solid.

e. Sulfur contains S8 molecules, so it is a molecular solid.

Self-Check Exercise 14.2

Name the type of crystalline solid formed by each of the following sub-stances:

a. sulfur trioxide

b. barium oxide

c. gold

Sections 14.6–14.7

1. Salt and sugar are both crystalline solids. How are they different?

2. What are the forces holding each type of crystalline solid together? Predict which type would have the lowest boiling point. Explain your choice.

3. How are metals different from an atomic solid such as diamond?

Focus Questions 461

Focus Questions