Covalent radius: It is defined as one half of the
distance between the nuclei (inter nuclear distance) of two covalently bonded
like atoms in a homo nuclear diatomic molecule is called the covalent radius of
that.
(A) For Homo atomic Molecules: The
covalent radius (rA) of atom A in a molecule A2 may be
given as:
The distance
between nuclei of two single covalently bonded atoms in homo diatomic molecules
is equal to the sum of covalent radii of both the atoms.
Illustrative
example (1): A given compound A2 whose total dA-A is 1.4
A0. The atomic (covalent) radius of an atom is.
(B)
For Heteroatomic Molecules: In a hetero diatomic molecules AB where the electro negativity of
atoms A and B are different, the experimental values of inter nuclear distance
dA-B is less than the theoretical value (rA +rB).
(1) Stevenson
& Schoemaker Equation (1941):
Covalent
radius of heterogeneous molecule like A-B etc determine by Stevenson &
Schromaker Equation, if atoms are formed different type of covalent bond i.e.
on atom is more electronegative than the other combined atom. Then the covalent
radius is calculated by the relation given by Stevenson & Schoemaker, given
as:
For a
diatomic Hetero molecule:
Bond
Length (lA-B) = rA + rB- 0.09(XA-XB)
Where XA=
Electronegativity of more electronegative atom
Where XB=
Electronegativity of less electronegative atom
Illustrative examples
(2) Puling
equation: If the
electro negativities of the two atoms A and B are XA and XB
respectively then
Bond Length (dA-B) =( rA
+rB)-(C1XA-C2XB)
Where C1
and C2 are the Stevenson’s coefficients for atoms A and B
respectively
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