Fajan rule
Change of Ionic Character to Covalent Character
When two oppositely charged ions of unequal size approach
each
other closely (during formation of an electrovalent bond), the ion
smaller
in size attracts outermost electrons of the other ion and
repel its nuclear
charge. The net result is distortion or polarisation
of the bigger ion. This
distortion is usually done by the cation as
its size is smaller than the anion.
The electron cloud of anion no longer remains symmetrical but
is elongated towards the cation. The ability of a cation to polarise
the nearby
anion is called its Polarising power and the tendency
of an anion to get distorted or
deformed or polarised by the cation
is called its Polarisability.
Due to polarisation, sharing of electrons
occur between two ions to some extent
and the bond shows some
covalent character.
This is
shown in figure.
The magnitude of polarisation depends upon a number of
factors
or the increased covalent character is favoured by a number of factors.
These factors were suggested by Fajan and are known as Fajan’s rule:
A) Small Positive Ion
(Cation): Due to
greater concentration of
positive charge on a small area, the smaller cation
has high polarising
power. This explains why LiCl is more covalent than KCl.
B) Large Negative Ion
(Anion): The larger
the anion, the greater
is its Polarisability, i.e. susceptibility to get
polarised. It is due to the
fact that the outer electrons of a large anion are
loosely held and hence
can be more easily pulled out by the cation. This
explains why
iodides, among halides, are most covalent in nature.
C) Large Charge on
Either of the Two Ions: As the charge on
the ion increases, the electrostatic attraction of the
cation for
the outer electrons of the anion also increases, with the result its
ability for forming the covalent bond increases.
D) Electronic
Configuration of the Cation: For the two ions of
the same size and charge, one with a pseudo noble gas configuration
(i.e., 18
electrons in outer-most shell) than a cation with
noble gas configuration (i.e.
8 electrons in outermost shell) will
be more polarising. Thus copper (I)
chloride is more covalent than
sodium chloride although Cu+ ion
(0.96A°) and Na+ ion (0.95A°)
have same size and charge.
LiF = 870°C
LiCl = 613°C
LiBr =
547°C LiI = 446°C
From the above discussion, we find that greater the
possibility of
polarisation, lower is the melting point and heat of sublimation
and
greater is the solubility in non-polar solvents.
Percentage of Ionic Character
Every ionic compound having some percentage of covalent
character according to Fajan’s rule. The percentage of ionic character in a
compound having some covalent character can be calculated by the following
equation.
% ionic
character = Obs. dipole moment/ Calc. dipole moment × 100
Example
1:
Dipole moment of KCl is
3.336 × 10–29 coulomb metre which indicates that it is highly polar
molecule. The interatomic distance between k+ and Cl– is
2.6 ×10–10 m. Calculate the dipole moment of KCl molecule if there
were opposite charges of one fundamental unit located at each nucleus.
Calculate the % ionic character of KCl.
Solution:
Dipole
moment μ = e
× d coulomb metre
For KCl d =
2.6 × 10–10 m
For complete
separation of unit charge
e = 1.602 × 10–19 C
Hence μ = 1.602 × 10–19 × 2.6 × 10–10 =
4.1652 × 10–29 Cm
μ KCl = 3.336 × 10–29 Cm
∴ % ionic character of KCl = 3.336×10–29/4.165×10–29
x 100= 80.09%
Example 2. Calculate the % of ionic character of
a bond having length = 0.83 Ã… and 1.82 D as it’s observed dipole moment.
Solution: To
calculate μ considering 100% ionic bond
= 4.8 × 10–10 × 0.83 × 10–8 esu
cm
= 4.8 × 0.83 × 10–18
esu cm = 3.984 D
∴ % ionic character = 1.82/3.984 × 100
= 45.68 %
The % ionic character is nearly 43.25%, so the
% covalent
character is (100 – 43.25) = 56.75%.
From the octet rule HF should have been a purely covalent
compound but actually it has some amount of ionic character in it, which is due
to the electronegativity difference of H and F. Similarly knowing the bond
length and observed dipole moment of HCl, the % ionic character can be known.
It was found that HCl has 17% ionic character. Thus it can be clearly seen that
although we call HCl and HF as covalent compounds but it has got appreciable
amount of ionic character. So from now onwards we should call a compound having
more of ionic less of covalent and vice versa rather than fully ionic or
covalent.