inorganic chemistry Inorganic chemistry deals with chemical synthesis, synthesis and behavior of inorganic compound, inorganic and organometallic chemistry, organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subj ...

, Fajans' rules, formulated by

Kazimierz Fajans Kazimierz Fajans (Kasimir Fajans in many American publications; 27 May 1887 – 18 May 1975) was a Polish-Jewish physical chemist, a pioneer in the science of radioactivity and the co-discoverer of chemical element protactinium. Education and ca ...

in 1923, are used to predict whether a

chemical bond A chemical bond is the association of atoms or ions to form molecules, crystals, and other structures. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds or through the sharing of electrons a ...

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 convent ...

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 conven ...

. They can be summarized in the following table: : Although the bond in a compound like X+Y- may be considered to be 100% ionic, it will always have some degree of covalent character. When two oppositely charged ions (X+ and Y-) approach each other, the cation attracts electrons in the outermost shell of the anion but repels the positively charged nucleus. This results in a distortion, deformation or polarization of the anion. If the degree of polarization is quite small, an ionic bond is formed, while if the degree of polarization is large, a covalent bond results. Covalent and ionic bonds

Thus

sodium chloride Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...

(with a low positive charge (+1), a fairly large cation (~1 Å) and relatively small anion (~2 Å) is ionic; but

aluminium iodide Aluminium iodide is a chemical compound containing aluminium and iodine. Invariably, the name refers to a compound of the composition , formed by the reaction of aluminium and iodine or the action of on metal. The hexahydrate is obtained fr ...

(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, due to

ionic potential Ionic potential is the ratio of the electrical charge (''z'') to the radius (''r'') of an ion. \text = \frac = \frac As such, this ratio is a measure of the charge density at the surface of the ion; usually the denser the charge, the stronger t ...

Å Z+/r+ (= polarizing power) *High charge and large size of the anion, due to polarizability which is related to the deformability of its electron cloud (i.e. its "softness") *An incomplete valence shell electron configuration, due to the noble gas configuration of the cation producing better shielding and less polarizing power, for example 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. Ionic character vs electronegativity 01

The situation is different in the case of

aluminum fluoride Aluminium fluoride is an inorganic compound with the formula . It forms hydrates . Anhydrous and its hydrates are all colorless solids. Anhydrous is used in the production of aluminium. Several occur as minerals. Occurrence and production Aside ...

, 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 Eponymous chemical rules Inorganic chemistry