Chemistry - A Molecular Science

between the solids formed from the Group 4 at

oms (C, Si, and Ge) and those formed from

compounds of Group 3 and Group 5 atoms results because the two sets of solids are isoelectronic;

i.e.,

a pair of atoms in either solid contains a total of eight valence electrons.

Figure 8.21

β-crystobalite

A form of SiO

found at temperatures above 1500 2

oC. Silicon

is shown as grey spheres and oxygen as red spheres. * The word “zeolite” comes from the Greek word for “boiling stone” because steam was observed escaping from the mineral when it was heated.

o4A CH 66

(a)

(b)

Figure 8.22 Zeolite Each line has an Al or Si atom at the end and an O atom in the middle. Openings that are too small to be entered by smaller molecules are closed to emphasize the channel (a) and pore (b) structures. A benzene molecule is shown for size comparison.

Figure 8.21 shows a high-temperature form of SiO

. In this structure, the silicon atoms 2

also adopt the diamond-type structure shown in Figure 8.19; but, in this structure, there is an oxygen atom located between each pair of silicon atoms. This comparison of diamond, common semiconductors, and a high-temperature form of SiO

demonstrates how variation 2

of the composition of materials that adopt similar structures can dramatically change the properties of materials.

Natural

zeolites*

are aluminosilicates, extended ne

tworks built from tetrahedral AlO

(^4)
and SiO
anions that are bridged by oxygen atoms (e 4
ach oxygen atom is part of two units).
The structure of zeolites is filled with channels (Figure 8.22a) and cavities or pores (Figure 8.22b) that are only a few angstroms wide. The aluminosilicate framework is composed of anions, so natural zeolites are commonly found
with sodium cations inside the cavities.
Small molecules can also get into the channels
, where they interact with the oxygen atoms
in the aluminosilicate framework. The interac
tion keeps them from leaving, so zeolites are
used as
molecular sieves

. One common use of molecular sieves is as a drying agent.

Water molecules are small enough to enter th

e cavities where they interact so strongly

with the walls via hydrogen bonding that they do

not exit, so when sieves are added to a

wet liquid, they extract any water that is presen

t. The sieves are filtered out of the liquid to

leave an anhydrous (no water) liquid behind. The sieves can be reactivated by placing them in a hot oven where the thermal energy is

sufficient for the water molecules to break

their interactions with the framework. Th

e ability of zeolites to function as molecular

sieves also makes them excellent blood clotti

ng agents that are used by the military and

rescue professionals. When a zeolite powder is applied to a wound, it draws the water out of the blood, which concentrates the clotting

agents and speeds the clotting process.

The sodium ions, which are only loosely b

ound in the cavities, are easily displaced by

other ions, so zeolites are also used in

ion-exchangers

. For example, when ‘hard water’

(water containing dissolved Ca

2+ and Mg

2+ ions) is passed through a zeolite column, the

Ca

2+ and Mg

2+ ions displace the Na

1+ ions in the zeolite because the more highly charged

ions interact more strongly with the anionic framework. The water that leaves the column contains Na

1+ ions instead of Ca

2+ and Mg

2+ ions, so it is “softened.” Some laundry

detergents contain zeolites that are used to soft

en the wash water during the wash cycle.

Chapter 8 Solid Materials

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