The Foundations of Chemistry

(or into which) electrons move to allow electrical conduction is called a conduction band.

The electrical conductivity of a metal decreases as temperature increases. The increase in

temperature causes thermal agitation of the metal ions. This impedes the flow of elec-

trons when an electric field is applied.

Crystalline nonmetals, such as diamond and phosphorus, are insulators—they do not

conduct electricity. The reason for this is that their highest energy electrons occupy filled

bands of molecular orbitals that are separated from the lowest empty band (conduction

band) by an energy difference called the band gap.In an insulator, this band gap is an

energy difference that is too large for electrons to jump to get to the conduction band

(Figure 13-35).

Elements that are semiconductorshave filled bands that are only slightly below, but

do not overlap with, empty bands. They do not conduct electricity at low temperatures,

but a small increase in temperature is sufficient to excite some of the highest energy elec-

trons into the empty conduction band.

Let us now summarize some of the physical properties of metals in terms of the band

theory of metallic bonding.

1.We have just accounted for the ability of metals to conduct electricity.

2.Metals are also conductors of heat.They can absorb heat as electrons become ther-

mally excited to low-lying vacant orbitals in a conduction band. The reverse process

accompanies the release of heat.

3.Metals have a lustrous appearancebecause the mobile electrons can absorb a wide

range of wavelengths of radiant energy as they jump to higher energy levels. Then

they emit photons of visible light and fall back to lower levels within the conduc-

tion band.

Various samples of elemental silicon.
The circle at the lower right is a
disk of ultrapure silicon on which
many electronic circuits have been
etched.

530 CHAPTER 13: Liquids and Solids

Figure 13-34 Overlapping of a filled “3s” band (blue) with an empty “3p” band of MgN

crystal. The higher-energy electrons are able to move into the “3p” band (red) as a result of

this overlap.

Figure 13-35 Distinction among metals, insulators, and semiconductors. In each case an

unshaded area represents a conduction band.

Energy

Metals Insulators Semiconductors

Partially

filled

band

Overlapping

bands

or

Empty band

Filled band

Forbidden zone

(large energy gap)

Small energy gap

Empty band

Energy

3 s

3 p

Mg MgN