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The effect of ion polarization on the bond type and crystal structure of compounds

 Last Update: 2021-06-18
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Under normal circumstances, the coordination of AB-type cubic ionic crystals can be judged according to the ratio of positive and negative ion radii to determine the crystal type
.

Due to ion polarization, the crystal configuration will deviate from the results predicted in Table 6-3

FeCl ₂ is an ionic compound and FeCl ₃ has obvious covalentness, and the reason is also due to ion polarization
.

6.
4.
1 Ion polarization phenomenon

1.
ion polarization

Under the action of the electric field, the center of gravity of the positive charge and the center of gravity of the negative charge of the ion no longer overlap, resulting in an induced dipole
.

The phenomenon that ions induce dipoles in an electric field is called ion polarization (Figure 6-41)

As charged particles, ions can act as an electric field to cause other ions to produce dipoles and deform, that is, ions have polarizing ability; ions produce dipoles and deform under the action of other ions, that is, ions have deformability
.

So ions have duality: polarization ability and deformability

Figure 6-41 Polarization of ions in an electric field

Factors affecting ion polarization and deformability

The essence of ion polarization is the effect of ions as an electric field on other ions.
The polarization ability of ions is reflected in the strength of the electric field; the deformability of ions is the deformation of the electron cloud under the action of the electric field, and the positive charge center and negative The center of gravity of the charge no longer overlaps
.

1) Ionic radius

The ion charge is the same, the smaller the radius, the stronger the ion polarization ability, and the smaller the ion deformability
.

Polarization ability H+>Li+>Na+>K+>Rb+>Cs+

Deformability H+<Li+<Na+<K+<Rb+<Cs+

The H+ radius is the smallest, the polarization ability is strong, and the deformability is small
.

2) Ion charge

The higher the charge number of the ion, the stronger its polarization ability and the smaller its deformability
.

Polarization ability Si ⁴⁺ >Al ³⁺ >Mg ²⁺ >Na ⁺

Deformability Sin ⁴⁺ <Al ³⁺ <Mg ²⁺ <Na ⁺

3) Electronic configuration of ions

When the half meridians are similar and the charges are the same, the more electrons in the outer layer of the ion, the stronger the polarization ability and the greater the deformability
.

The more electrons in the outer layer, the easier it is to deform the electron cloud; at the same time, the more electrons in the outer layer, the higher the effective nuclear charge and the stronger the ion polarization ability
.

4) The influence of the symmetry of complex anions

Anionic high complexity symmetry small deformation, such as SO _.

4
^2- (tetrahedral), ClO _{.

3.
Mutual polarization
In general, the polarization ability of cations is mainly considered, and the deformability of anions is mainly considered
.

But for cations with large radius and more electrons in the outer layer, the deformability should also be considered, such as Ag ⁺ , Hg ²⁺ , Pb ²⁺ and so on

Considering both the polarization of cations to anions and the polarization of anions to cations, the overall result is called mutual polarization, or additional polarization
.

The cations with high polarizability and deformability and the anions with high deformability have obvious mutual polarization, such as Hgl ₂ , Agl and so on

6.
4.
2 The effect of ion polarization on the bond type and crystal structure of compounds
1.
Ion polarization affects the bond type of compounds
The polarization of the cation is that the positive ion attracts the electron cloud of the negative ion, and the negative ion is deformed.
The electron cloud overlaps between the positive and negative ions in the compound.
As a result, the covalent bond component of the compound increases, and the bond type of the compound changes from ionic bond to covalent bond Conversion
.
AgF between the Ag + and F- are mainly ionic bond, whereas in the AgI of Ag ⁺ and the I ^- between the main is a covalent bond; in SnCl2 ₂ are of Sn ²⁺ and CI ^- between mostly ionic bond, whereas in SnCl2 _{.
4} in Sn ⁴⁺ and Cl ^- are mainly covalent bonds
.
2.
Ion polarization affects the type of crystal structure
Ion polarization increases the electron cloud overlap between the positive and negative ions in the compound, and the crystal structure transitions from a high-coordination structure to a low-coordination structure
.

For example, CuCl radius ratio r+/r=0.
53 (greater than 0.
414), which should belong to NaCl type.
Due to ion polarization, its crystal configuration becomes ZnS type structure
.

Related Links: The formation of hydrogen bonds and its influence on the properties of compounds}

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