Fajans Rule :
The covalent or ionic nature of a chemical bond may be predicted using the Fajans rule. Partial covalent properties of certain ionic bonds were initially reported by Kazimierz Fajans in 1923. Using X-ray crystallography, he was able to anticipate ionic or covalent bonding based on characteristics such as ionic and atomic radius.
The prediction of whether a bond is mostly ionic or covalent was first proposed by Kazimierz Fajans in 1923. Understanding chemical bonding and its many forms can help you understand Fajans rule. The basic outline of Fajans rule and its principles is provided in this article.
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What is Fajans Rule?
Fajans’ rule states that a compound with a low positive charge, a big cation, and a small anion forms an ionic bond, whereas a compound with a high positive charge, a small cation, and a large anion forms a covalent bond. Small cations have more polarizing power when they are highly charged.
Fajan's Rule And Polarizability
Fajan's Rule developed by Polish chemist Kazimierz Fajans in the early 20th century, Fajan's Rule provides valuable insights into the prediction of ionic character and covalent character in chemical bonds. Fajan's Rule is particularly relevant when dealing with polar covalent compounds and ionic compounds that possess covalent characteristics. The rule is based on the idea that the polarization of an ionic bond increases with the size and charge of the ions involved.
Positive ions attract the outermost electron of an anion and repels its nucleus, causing distortion or polarization of the anion. And anion is called Polarized.
Polarization Power
It is generally represented by ϕ. The ability of a cation to polarize the anion is referred to as polarizing power. It is directly proportional to the charge density, which is directly related to the charge on cation, while inversely related to the size of anion.
The polarizing power increases with increase in the size of cation i.e. smaller cations are very effective in the polarization of anion. However, the polarizing power increases with increase in the charge on cation.
Polarizability of Anion
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It is the tendency of an anion to undergo polarization. It indicates the easiness with which an anion undergoes distortion in the presence of a cation.
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It is directly proportional to the size as well as the negative charge on the anion.
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The larger anions can undergo distortion very easily than the smaller ones.
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It is also important to note that the anions with greater negative charge also undergo polarization easily.
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Oxyanions are generally less polarizable because charge is present on O atom which is very small, and we need to consider only the element which acquires the charge (and not the complete anion)
Important Points To Note
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Smaller size and increase in charge density on cation, increases its polarizing power.
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Larger anions hold its valence electron loosely — more polarization occurs.
Fajan’s Rule For covalent character
Fajan’s Rule For covalent character :
This rule is for covalent character of an ionic bond. Covalent character of an ionic bond is favored by:
i)
Small positive ion
ii)
Large negative ion
iii)
large charge on ions (Cations & Anions).
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Thus, for a fixed cation, larger the size of anion, more the magnitude of the charge, more is covalent character.
Fajan’s Rule Example
Covalent character of sodium halides follows the order: NaI > NaBr > NaCl > NaF.
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For fixed anion, the smaller the size of cation, more is the magnitude of the charge, more is the covalent character.
Example:
BeCl
2
> MgCl
2
> CaCl
2
> SrCl
2
> BaCl
2
-
Charge on cation increases.
-
Covalent character increases
-
Ionic character decreases (M.P. decreases)
AlN > Al
2
O
3
> AlF
3
Determine the covalent And ionic character of molecule
From left (larger size) to right (smaller size) in a period ϕ increases so covalent character increases.
From top to bottom in a group ϕ decreases so covalent character decreases.
Applications of Fajans’ Rule
(a)
To compare the covalent and ionic character of molecule.
(b)
To compare the nature of oxide.
(c)
To compare the electrical conductivity of ionic compounds.
(d)
Tendency of the formation of complex compounds.
(e)
To compare the thermal stability of metal salts.
(f)
To compare the intensity of colour of compounds.
(g)
To compare the solubility of heavier metal halide in water.
3 Important Factor About Main Postulates of Fajans Rule
Three factors, which are as follows, may be used to formulate the rule:
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Ion size:
Anion size increases with covalent bond strength; smaller cations have a higher covalent character.
-
The cation charge:
The covalent nature of the ionic bond increases with the cation's charge.
-
Electronic configuration:
Among cations of the same charge and size, the one with the electronic configuration of (n-1)dn nso, found in transition elements, is more covalently bonded than the one with the configuration of ns2 np6, found in alkali or alkaline earth metals.
What is Polarising Power?
It is the degree to which an anion may get polarized by a cation. In relation to charge density, it is proportional. The ratio of charge to volume is known as charge density. Charge density α Polarization power. The polarizing power of that cation increases with increasing charge density.
Explanation of Fajans Rule
Fajans First Rule:
The first guideline discusses the cation's power to polarize. We may state that the ion's volume is lower if the cation is smaller. We can determine that if the volume is lower, the ion's charge density would be higher.
The polarizing power of the ion would be considerable due to the high charge density. The compound becomes more covalent as a result.
Fajans Second Rule:
The anion's polarizability is covered in the second rule. The effective nuclear charge holding the valence electron of the ion in place decreases with increasing anion size. Large anions have a loosely attached last electron, which makes it easy for a cation to polarize and increase the covalent nature of the molecule.
Fajans Third Rule:
There is an exception to the third rule. To explain this concept, let's look at an example.
For example, we cannot use size as a determining criterion for deciding which of the compounds, calcium chloride and hgCl2, is more covalent. This is due to the almost identical sizes of Ca2+ and Hg2+. We use the third rule to explain this.
Hg2+ has an electron arrangement of 6s0 5d10. Because the d-orbital is completely filled yet the element lacks an octet or eight electrons, this arrangement is known as a pseudo-octet.
Given the poor shielding properties of d orbitals, we could assume that the anion (Cl–) would be more polarized, making HgCl2 more covalent than CaCl2 due to the noble gas configuration of the Ca2+ ion.
In answer to our first query, which alkali chloride is the most covalent is as follows:
We must compare the cations since the anion is the same. The greater the covalency, the smaller the cation, according to Fajans' rules. LiCl is the most covalent as a result.
Main Factors in Fajans Rule
The three key factors in this rule are as follows:
-
Charge of the Cation:
Consider a multi-armed strongman attempting to grab a feather. The feather, or electrons, are more likely to be drawn towards the strongman, or cation, and form a stronger bond, the greater the pull (charge).
-
Size of the Cation:
Now imagine the strongman curling the feather in his fist. Stronger cations draw electrons more forcefully than weaker ones because their concentrated positive charge is more tightly bound, like a stronger grip on a feather.
-
Anion Size:
Imagine that an anion is the same size as a feather. A bigger anion gives the cation more "electron real estate" to pull on, which helps the strongman's grip. Smaller anions hence often result in greater covalent character.
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Fajans Rule Examples
Example 1:
Of all the metal halides, which compound should be, in theory, the most ionic and covalent?
In theory, the most covalent ions should be the biggest anion and the smallest metalion.
LiI is the most covalent as a result.
The most ionic substances should be the largest cation and the smallest anion. CsF should be the most ionic as a result.
Example 2:
Sort the following in ascending order of covalency:
NaF, NaCl, NaBr, NaI
LiF, NaF, KF, RbF, and CsF
-
Compare the anions because the cation is the same. The greater the size among the anions, the higher the covalency. As a result, the sequence is NaF < NaCl < NaBr < NaI.
-
Since the anion is the same in this case, we may compare it to cations. The covalency increases with the size of the cation. As a result, the sequence is as follows: CsF < RbF < KF < NaF < LiF.
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Summary of the Discussion
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Ionic
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Covalent
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Low positive charge
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High positive charge
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Large cation
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Small cation
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Small anion
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Large anion
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