Chemistry - A Molecular Science

Chapter 14 Inorganic Chemistry

crystal is very large, the z

2 orbitals in the chain form a band. Subsequently, the z

2 orbitals

are all filled in this ion, so the band is also f

illed, and the crystal is an insulator. However,

this band can be partially oxidized by removing some of the electrons, either electrochemically or by reaction with bromine to form K

[Pt(CN) 2

]Br 4

0.3

·3H

O. Complete 2

oxidation of the platinum(II) to platinum

(IV) would produce a compound with two

bromide ions. The 0.3 subscript on the bromine indicates that only a partial oxidation has occurred. Figure 14.18 shows that the result of

the oxidation is the removal of some of the

electrons from the band formed from the z

2 orbitals. Because the result is a partially filled

band, the material becomes metallic

with a high degree of conductivity.

MAGNETIC MATERIALS Magnetic materials are materials that can act as permanent magnets. The NO molecule has an unpaired electron and is paramagnetic (Section 3.4). Consequently, NO molecules are attracted by a magnetic field. However, they do not form permanent magnets because the spins of the isolated molecules do not align a

nd thus cancel one another. Indeed, there are

many atoms, ions, and molecules with unpaired electrons, but only a very few are magnetic materials. This is because there are two prerequisites for a magnetic material. First, the individual building blocks that ma

ke up the material must have nonzero spins,

and, second, the material must have a three-

dimensional crystal structure in which the

spins of the atoms, ions, or molecules are a

ligned in an ordered fashion. Both of these

requirements are satisfied by solid iron. Each iron atom has four unpaired electrons, and some of them align with their spins oriented

in the same direction to produce a magnetic

material (Figure 14.19a). This type of magnetic material is called a

ferromagnet

.

However, without the three-dimensional crystal structure, the spins cannot stay aligned and a permanent magnet cannot form; con

sequently, molten iron is not magnetic.

The spins of adjacent atoms, ions, or molecu

les in a solid usually interact with one

another so as to pair in much the same way th

at the electrons in a covalent bond pair. This

is the situation depicted in Figure 14.19b, where the spins on neighboring units are opposed, so the material is not a permanent ma

gnet. Materials in which adjacent spins are

opposed are said to be

antiferromagnetic

. This is the most common occurrence and

explains why the solids of most metals are not magnetic even though the individual atoms may be paramagnetic (have unpaired electrons).

The fact that the neighboring spins usually

interact to yield materials with zero net

Energy

partialoxidationwith Br

2

K [Pt(CN) ]^2

4

K [Pt(CN) ]Br240.3

Figure 14.18 Band occupancy changes due to partial oxidation of platinum The material with band structure (a) is an insulator, but partial oxidation produces structure (b), which is a conductor.

© by

North

Carolina

State

University