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Fajans' Rules for Chemical Bonds

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  • 0:04 Fajans' Rules
  • 1:11 Discussion of Fajans' Rules
  • 6:04 Polarization Consequences
  • 7:10 Lesson Summary
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Lesson Transcript
Instructor: Saranya Chatterjee

Saranya has a masters degree in Chemistry and in Secondary Education. She has taught high school, AP chemistry for 2 years and is teaching undergraduate college chemistry for 3 years.

This lesson will talk about polarization of ionic compounds and Fajans' rules. Fajans' rules discuss introduction of covalent character in ionic compounds and how some properties of ionic compounds change because of polarization.

Fajans' Rules

Before going into discussion of Fajans' rules, we need to first talk a little bit about polarization and deformation of ions. Ionic compounds are compounds of metals and non-metals in which we have a cation, which is a positively charged ion, and an anion, which is a negatively charged ion, making an ionic bond. An ionic bond is usually an electrovalent bond formed from transfer of electrons. When cations and anions come to their equilibrium distance to make a bond, the electron cloud of the anion is attracted by the positively charged cation. The electron cloud of the anion is thereby slightly deformed and said to be polarized. A departure from an ideal ionic setup is thus initiated, and some covalent character is incorporated in the bond. So polarization develops covalent character.

Fajans' rules state that polarization is enhanced by the following three things:

  1. High charge and small size of the cation
  2. High charge and large size of the anion
  3. Cations with 18 electron structures such as Cu+, Hg2+, etc.

Discussion of Fajans' Rules

Ionic compounds traditionally have the following properties:

  • Hard and brittle with high melting points
  • Conduct electricity in solution, and
  • Dissolve in polar solvents

Polarization in an ionic bond will lead to covalency and thus deviate from these three typical properties. Let's look at specific examples and discuss.

Our first example can be seen in the development of covalent character with an increase in cation charge. The melting point, which is the temperature at which a solid substance changes states into a liquid substance, of the compound sodium bromide (NaBr) is 755°C, that of magnesium bromide is 700°C, and that of aluminium bromide is 97.5°C. We can rationalize this trend by Fajans' rules. In these three compounds, bromide is the common anion. The cationic charge increases from +1 in Na to +2 in Mg and +3 in Al. According to Fajans' rules, higher charge leads to polarization and hence increased covalency in a molecule. So covalency is highest in AlBr3 and lowest in NaBr. Hence the melting point of AlBr3 is the lowest melting point.

Now, let's take a look at the development of covalent character with increase in its cation radius.

Please look at the following trend in melting points. The melting point of the compound beryllium chloride (BeCl2) is 405°C, that of magnesium chloride (MgCl2) is 712°C, that of calcium chloride (CaCl2) is 772°C, that of strontium chloride (SrCl2) is 872°C, and melting point of barium chloride (BaCl2) is 960°C. In all these compounds, chloride is the common anion. The size of the cations increases from Be2+ (0.031 nm) to Ba2+ (0.135 nm) in the following manner. The size of Be2+ is 0.031 nm, that of Mg2+ is 0.065 nm, that of Ca2+ is 0.099 nm, that of Sr2+ is 0.133 nm, and that of Ba2+ is 0.135 nm. According to Fajans' rules, small size cation leads to polarization and hence increased covalency in a molecule. So covalency is highest in BeCl2 and least in BaCl2. So, BaCl2 is the most ionic among the given compounds, and it has the highest melting point. Hence, the prior trend in melting points is easily explained by Fajans' rules.

Now, let's look at the development of covalent character with increase in anion radius. The following trend in melting points can be explained by Fajans' rules again. If we look at the halides of calcium, the melting points decrease from calcium fluoride(CaF2) to calcium iodide(CaI2). The melting point of calcium fluoride (CaF2) is 1392°C, that of calcium chloride (CaCl2) is 772°C, that of calcium bromide is 730°C, and that of calcium iodide is 375°C. If we look at the compounds, we find that calcium is the common cation here. The radius of the anions increases from fluoride to iodide in the following manner. The anionic radius of fluoride is 0.136 nm, that of chloride is 0.181 nm, that of bromide is 0.195 nm, and that of iodide is 0.216 nm. According to Fajans' rules, a large size anion leads to polarization and hence increased covalency in a molecule. So, covalency is highest in CaI2 and least in CaF2. So, CaF2 is the most ionic among the given compounds, and it has the highest melting point. Hence the prior trend in melting points is easily explained by Fajans' rules.

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