inorganic chemistry Inorganic chemistry deals with synthesis and behavior of inorganic and organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subjects of organic chemistry. The distinction between the two disci ...

, Fajans' rules, formulated by Kazimierz Fajans in 1923, are used to predict whether a

chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...

will be

covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

or ionic, and depend on the charge on the

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

and the relative sizes of the cation and

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

. They can be summarized in the following table: : Thus

sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...

(with a low positive charge (+1), a fairly large cation (~1 Å) and relatively small anion (0.2 Å) is ionic; but aluminium iodide (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) The "size" of the charge in an ionic bond depends on the number of electrons transferred. An aluminum atom, for example, 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 aluminum (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 iodine. 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. A cation having inert gas like configuration has less polarizing power in comparison to cation having pseudo-inert gas like configuration. The situation is different in the case of

aluminum fluoride Aluminium fluoride refers to inorganic compounds with the formula AlF3·''x''H2O. They are all colorless solids. Anhydrous AlF3 is used in the production of aluminium metal. Several occur as minerals. Occurrence and production Aside from anhydr ...

, AlF₃. In this case, iodine is replaced by fluorine, a relatively small highly electronegative atom. The fluorine's electron cloud is less shielded from the nuclear charge and will thus be less polarizable. Thus, we get an ionic compound (metal bonded to a nonmetal) with a slight covalent character.

References

External links

* {{cite web , author = Adrian Faiers , publisher = chembook.co.uk , title = Chapter 4: Chemical Bonding , work = Chemistry in Perspective , url = http://www.chembook.co.uk/chap4.htm Inorganic chemistry