Monday, June 20, 2011

Atomic Radii

Assuming atoms to be spheres, the atomic radii may be defined as, “The distance from the centre of nucleus of the atom to the outermost shell of electrons.” However, it is not possible to find precisely the radius of the atoms because of the following reasons;
(i).        Atom is too small to be isolated.
(ii).       Due to wave nature of electron, it is not possible to measure the exact distance from nucleus.
(iii).      The probability distribution of an atom is affected by other atoms present in its neighbourhood.
(iv).      Size of an atom also changes from one bonding state to another.
Thus, it is not possible to measures the absolute value of the atomic radius of an element. However, it   may expressed in three different forms depending upon the nature of bonding in the atoms. These are:
¤    Covalent radii
¤    Vanderwaal’s radii
¤    Metallic radii
 “The covalent radius of an atom is equal to half the internuclear distance between two identical atoms that are joined by a covalent bond”.
The covalent bond should essentially be a single covalent bond. If it is double or triple bond, the value of covalent radius changes.
The distance between the centres of the two bonded atoms (inter nuclear distance) can be determined by X-ray diffraction methods, Thus,
Covalent radius = Inter Nuclear distance in bonded atoms/2
Covalent radius is always shorter than the actual radius because overlapping of orbitals involved in Covalent bond formation decreases the internuclear distance.
Vanderwaal’s radius may be defined as, “one half of the inter–nuclear distance between two similar adjacent atoms belonging to two neighbouring molecules of the same substance in the solid state.
The Vanderwaal’s forces of attraction are quite weak forces. Their magnitude is small in the gaseous as well as in liquid state of substance. Therefore, Vanderwaal’s radius is determined in solid state when their magnitude is maximum. The Vanderwaal’s radius is always more than the corresponding value of the covalent radius e.g. Vanderwaal’s radius of chlorine is 180 pm while its covalent radius is 99 pm.
Metallic radius may be defined as “the distance between the centres of the nuclei of two adjacent atoms in the metallic crystal.”
Metallic bond is weaker than Covalent bond, therefore, inter-nuclear distance between two atoms in Metallic bond is more than as in the covalent bond. But metallic bond is stronger than Vanderwaal’s radius Thus, Vanderwaal’s radius > Metallic radius > Covalent radius. Out of these, covalent radius is easier to determine. Therefore the atomic radius of an element is normally expressed as its covalent radius except in noble gases which do not form covalent bonds in their atoms. In noble gases, Vanderwaal’s radius represents the atomic radius.

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