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3.
The properties of ionic crystals
Ionic crystals are crystals formed by the combination of ionic bonds
.
In the CsC1 crystal, 8 CI - are arranged equidistantly in the nearest layer around each Cs + , and 8 Cs + are arranged equidistantly in the nearest layer around each CI - ; but there are other CI -in a slightly further position around Cs + , Similarly , there are other Cs + at a slightly further distance around CI - , but the electrostatic gravitational force decreases rapidly as the distance increases .
The number of ions with opposite signs in the closest layer of ions in the ionic crystal is called the coordination number
.
In CsCl crystals, Cs + and CI - coordination number is 8; in NaCl crystals, of Na + and CI - coordination number was 6; in the ZnS crystal, a Zn 2+ and S 2- feature The number of digits is 4; in the CaF 2 crystal, the coordination number of Ca 2+ is 8, but the coordination number of F- is 4
Ionic crystals have no definite relative molecular mass, and the aqueous solution or molten state conducts electricity
.
Ionic crystals conduct electricity because the ions dissociated in their aqueous or molten state can be electrolyzed
Figure 6-4 Dislocations of ionic crystals
4.
The structure of AB type cubic ionic crystal
The typical structures of AB type cubic ionic crystals are NaCl type, CsCl type, and cubic ZnS type (Figure 6-5)
.
Figure 6-5 The structure of AB type ionic crystal of cubic system
NaCl in the unit cell, four of Na + and CI 4 - , of Na + and CI - coordination number are 6; CsCl unit cell, there is a Cs + and a CI - , Cs + and CI - coordination number is 8; ZnS cubic unit cell, there are four a Zn 2+ and four S 2- , a Zn 2+ and S 2- coordination number was 4
Under normal circumstances, the coordination of AB-type cubic ionic crystals can be judged according to the ratio of positive and negative ions to determine the crystal type (Table 6-3)
Table 6-3 The relationship between the ion radius ratio and coordination number and crystal type of AB type cubic crystal system
