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)