Fajans' rules
Encyclopedia
In inorganic chemistry
, Fajans' Rules, formulated by Kazimierz Fajans
in 1923, are used to predict whether a chemical bond
will be covalent
or ionic
, and depend on the charge on the cation and the relative sizes of the cation and anion. They can be summarized in the following table:
Thus sodium chloride
(with a low positive charge (+1), a fairly large cation (~1 Å) and relatively small anion (2Å) is ionic; but aluminium iodide
(AlI3) (with a high positive charge (+3) and a large anion) is covalent.
Polarization will be increased by:
Considering an ionic bond, in this the electron(s) is(are) not shared but transferred between the atoms conveying definite charges to each participant in the bond. The "size" of the charge depends on the number of electrons transferred so an aluminum atom with a +3 charge has a relatively large positive charge. That positive charge then exerts an attractive force on the electron cloud of the other ion which has accepted the electrons from the aluminium (or other) positive ion.
Two contrasting examples can illustrate the variation in effects. In the case of aluminum iodide an ionic bond with much covalent character is present.
In the AlI3 bonding, the Aluminum gains a +3 charge. The large charge pulls on the electron cloud of the iodines. Now, if we consider the iodine atom, we see that it is relatively large and thus the outer shell electrons are relatively well shielded from the nuclear charge. In this case, the aluminum ion's charge will "tug" on the electron cloud of iodine, drawing it closer to itself. As the electron cloud of the iodine nears the aluminum atom, the negative charge of the electron cloud "cancels" out the positive charge of the aluminum cation. This produces an ionic bond with covalent character.
Now, if we take a different example, for example AlF3. We see that a similar situation occurs, but instead of Iodine we now have fluorine, a relatively small highly electronegative atom. The Fluorine's electron cloud is definitely less shielded from the nuclear charge and will thus be less polarizable. Thus, we get an ionic compound (metal bonded to a nonmetal) with slight covalent character.
Inorganic chemistry
Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry...
, Fajans' Rules, formulated by Kazimierz Fajans
Kazimierz Fajans
-External links:*...
in 1923, are used to predict whether a chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
will be covalent
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
or ionic
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
, and depend on the charge on the cation and the relative sizes of the cation and anion. They can be summarized in the following table:
| Ionic | Covalent |
|---|---|
| Low positive charge | High positive charge |
| Large cation | Small cation |
| Small anion | Large anion |
Thus sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
(with a low positive charge (+1), a fairly large cation (~1 Å) and relatively small anion (2Å) is ionic; but aluminium iodide
Aluminium iodide
Aluminium iodide is any chemical compound containing only aluminium and iodine. Invariably, the name refers to a compound of the composition AlI3, formed by the reaction of aluminium and iodine or the action of HI on Al metal. The hexahydrate is obtained from a reaction between metallic aluminum...
(AlI3) (with a high positive charge (+3) and a large anion) is covalent.
Polarization will be increased by:
- High charge and small size of the cation
- Ionic potential Å Z+/r+ (= polarizing power)
- High charge and large size of the anion
- The polarizability of an anion is related to the deformability of its electron cloud (i.e. its "softness")
- An incomplete valence shell electron configuration
- Noble gas configuration of the cation produces better shielding and less polarizing power
- e.g. Hg2+ (r+ = 102 pm) is more polarizing than Ca2+ (r+ = 100 pm)
- Noble gas configuration of the cation produces better shielding and less polarizing power
Considering an ionic bond, in this the electron(s) is(are) not shared but transferred between the atoms conveying definite charges to each participant in the bond. The "size" of the charge depends on the number of electrons transferred so an aluminum atom with a +3 charge has a relatively large positive charge. That positive charge then exerts an attractive force on the electron cloud of the other ion which has accepted the electrons from the aluminium (or other) positive ion.
Two contrasting examples can illustrate the variation in effects. In the case of aluminum iodide an ionic bond with much covalent character is present.
In the AlI3 bonding, the Aluminum gains a +3 charge. The large charge pulls on the electron cloud of the iodines. Now, if we consider the iodine atom, we see that it is relatively large and thus the outer shell electrons are relatively well shielded from the nuclear charge. In this case, the aluminum ion's charge will "tug" on the electron cloud of iodine, drawing it closer to itself. As the electron cloud of the iodine nears the aluminum atom, the negative charge of the electron cloud "cancels" out the positive charge of the aluminum cation. This produces an ionic bond with covalent character.
Now, if we take a different example, for example AlF3. We see that a similar situation occurs, but instead of Iodine we now have fluorine, a relatively small highly electronegative atom. The Fluorine's electron cloud is definitely less shielded from the nuclear charge and will thus be less polarizable. Thus, we get an ionic compound (metal bonded to a nonmetal) with slight covalent character.