Chemistry - A Molecular Science

Chapter 8 Solid Materials

8.11

CHAPTER SUMMARY AND OBJECTIVES Atoms in a metal pack like spheres to form the solid state. The basic unit that defines the way in which they pack is called the unit cell.

There are seven types of unit cells, but we

considered only the cubic unit cell, which consists of three types: simple cubic (

sc

), body-

centered cubic (

bcc

) and face-centered cubic (

fcc

). Each type of unit cell is characterized

by the number of atoms present in the unit cell, the coordination number of the atoms in the cell, and the packing efficiency of the cell. The

fcc

is the most tightly packed, with a

packing efficiency of 74%.

Atoms are held in a metal by metallic bonding. Metallic bonding is best understood in

terms of crystal orbitals and bands rather

than atomic orbitals and levels. Metallic

conductors are metals that have

partially filled bands. A band

filled with valence electrons

is called the valence band, while the empty

band above it is called

the conduction band.

The energy separation between the valence a

nd conduction bands is called the band gap.

Materials with small band gaps are semic

onductors because their conductivity increases

with temperature, while those with

large band gaps are insulators.

The sodium chloride structure is a cubic structure in which the sodium ions can be

viewed as occupying the void space in a closest packed arrangement of chloride ions (

fcc

).

It is the structure adopted by the alkali halid

es when the ion sizes are sufficiently different

to allow the smaller ion to fit into the void space without distorting the closest packed arrangement too much. When the anion and cati

on radii are similar, the sodium chloride

structure is a very inefficient way to pack th

e ions, so ionic solids in which the radii are

similar crystallize in the cesium chloride structure, which can be viewed as a

bcc

arrangement in which the cation is in the center and the anions are at the corners.

Network covalent solids are networks of

atoms held together by covalent bonds. The

structure of the solid is responsible for many of its properties. Thus, graphite is a lubricant because it consists of weakly interacting sheets that can slide over one another, while diamond is very hard due to the three-dimensional nature of its covalent bonds. Buckyball, C^60

, has the same shape as a soccer ball and many interesting properties. Nanotubes are

formed from sheets like those in

graphite that have been rolled up. Their conductivity has

led to their use as molecular wires and transistors. Zeolites are porous materials made from Al, Si, and O. The pores are large enough for

other molecules to enter and do chemistry.

Clays have a similar building block to zeolites,

but clays are composed of sheets rather

than pores and channels.

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