Atomic
radii may be defined as the distance between the nucleus and the
outermost electronic level of the atom. Since electrons are considered
as the negatively charged electronic cloud there is no well defined
boundary for an atom.The diffused nature of the electron cloud makes
it difficult to give exact definition of atomic size or atomic radii.
Introduction: Atomic radii
Thus the atomic radii is an arbitrary concept and is influenced by the nature of neighbouring atoms.
Types of atomic radii
As
there is no exact definition for the atomic radius, a number of
radii have been defined for an atom. They are Covalent radius, Crystal
radius (otherwise called as metallic radius) Vander Waal radius
(otherwise called Collision radius). Let us learn one by one.
Covalent radius
Covalent Radius:
Covalent radius is used to measure the atomic radii of non- metals.
The atomic radius of a non- metal is calculated from the covalent
bond length. In case of homonuclear diatomic molecules ( type AA) ,
like F
2, Cl
2,Br
2 ....etc half of the covalent bond length is taken as atomic radius. For example the value of Cl - Cl bond idstance is 1.98 A
o half of the distance 0.99 A
o is taken as the atomic radius of chlorine
Another example: measuring the atomic radius of carbon in diamond.
The value of C- C bond distance in the diamond is 1.54 A
o half of the distance 0.77 A
o is considered as the atomic radius of carbon atom.
Heteronuclear diatomic molecule:
In the calse of heteronuclear diatomic molecule of AB type (example CCl
4 , SiC ..etc) bond length distance d
(A-B) is given by
d
(A -B) = r
(A) + r
(B)
r
(A) and r
(B) are the covalent radii of A and B respectively.
Example: The experimental value of d
(C-Cl) in CCl
4 molecule is 1.76 A
o
d
(C-Cl) = r
(C) + r
(Cl)
r
(C) = d
(C-Cl) - r
(Cl)
r
(C) = 1.76 A
o - r
(cl)
if r
(cl) is given, then the covalent radius of carbon atom can be calculated by subtracting the
covalent radius of Cl from the d
(C-Cl) bond length.The covalent radius of Cl atom can also be obtained, provided that covalent radius of C atom is known
Crystal Radius:
It is otherwise called as Atomic or Metallic radius, and defined as
one half of the distance between the nuclei of two adjacent metal atoms
in the metallic close-packed crystal lattice. For example the
internuclear distance between two adjacent Na atoms in a crystal of
sodium metal is 3.80 A
oand hence the atomic radius of a Na metal is half of the distance, that is 3.80 A
o/ 2 = 1.90 A
o
since there are weak bonding forces between the metal atoms, the
metallic radii are higher than the single bond covalent radii and at
the same time the metallic radii are smaller than the vander Waal
radii since the bonding forces in the metallic crystal lattice are
much staonger than the vaner waals forces
Vander Waal Radius:
The
name is derived from theVander Waal forces which is found in noble
gases.This type of atomic radii is other wise called Collision Radius.
Tthe distance between the two non-bonded isolated atoms or the
distacnce between two non-bonded atoms belonging to two adjacent
molecules of an element in the solid state is called Vander Waals
distance while half of this is called vader Waals Radius.
Example : The vander Waals distance of Cl
2 molecule = 3.6 A
o half of this value is 1.8 A
o and 1.8 A
o is the Vander Waal radius of chlorine atom.
It is to be noted that the vander Waal radius of an element is
higher than its covalent radius. Example the measured Vander Waal radius
of chlorine is 1.8 A
o and the covalent radius is 0.99 A
o
The variation in the atomic radii can be explained as follows.
When two chlorine atoms are just in contact with each other and
there is no bond between them, now the distance between nuclei of those
two chlorine atoms is called the vander Waals distance (3.6 A
o) and half of it ( 1.8 A
o) is called vander Waals radius.
where as when the electron clouds of the two chlorine atoms merge with
each other to form chlorine molecule by forming covalent bond between
them, the distance (covalent bond length) between them further decreases
and the distance become 1.8 A
o and half of it 0.99 A
o is the covalent radius.
Thus while describing the atomic radii of various atoms, any of the radii described above can be used.
