VAN DER WAALS ’ RADIUS : Half the inter-nuclear separation between two similar adjacent atoms belonging to the two neighbouring molecules of the same substance in the solid state is called the van der waals ’ radius of that atom .
METALLIC RADIUS : Half the distance between the nuclei of the two adjacent metal atoms in a close packed lattice of the metal is called its metallic radius .
Van der Waals ’ radius > Metallic radius > Covalent radius
IONIC RADIUS : The effective distance from the centre of the nucleus of an ion upto which it has an influence on its electron cloud is called its ionic radius .
A cation is smaller but the anion is larger than the parent atom . In case of iso-electronic species , the cation with greater positive charge has smaller radius but anion with greater negative charge has the larger radii .
IONISATION ENTHALPY : The ionisation enthalpy is the molar enthalpy change accompanying the removal of an electron from a gaseous phase atom or ion in its ground state . Thus enthalpy change for the reaction ; M ( g ) → M + ( g ) + e -
Is the ionisation enthalpy of the element M . Like ionisation energies for successive ionisation , the successive ionisation enthalpy may also be termed as 2 nd ionisation enthalpy ( ∆rH2 ), third ionisation enthalpy ( ∆rH3 ) etc . The term ionisation enthalpy is taken for the first ionisation enthalpy , ( ∆rH1 ) is expressed in kg mol - or in eV .
Periodicity :
i ) Generally the ionisation enthalpies follow the order ( there are few exceptions ):
( ∆rH1 ) < ( ∆rH2 ) < ( ∆rH3 )