periodicity and different atomic radius and ionic radius.

What is periodicity ?

In periodic table, it has been found that elements withsimilar properties fall in the same group.

The repetition of similar properties of elements after certain regular intervals when they are arranged in the order of increasing atomic numbers is called periodicity.

What is the cause of periodicity ?

According to modern periodic table, all the physical and chemical properties of elements  depends on their outer electronic configuration or outer valence electrons.

For this,the elements  belonging  to each group have similar properties but they differ from the elements belonging to the other groups.

For example , all the alkali metals of group-I having outer electronic configuration ns1, are strongly metallic and electropositive . 

The elements of group zero  with electronic configuration ns2 np6, are inert and exhibit high ionization potential.

Hence, we can define that the cause of periodicity in properties of elements  appears due to the repetition of similar outer electronic configuration of their atoms at certain regular intervals.

What is covalent radius ?

One half of the distance between the nuclei of twocovalently bonded atoms of the same element in a molecule, is taken as the covalent radius of the atom of that element.

While defining the covalent radius, it is assumed that atoms are hard, incompressible spheres which are touching each other in normal covalent formation.

The covalent bond  distance is measured by  X-ray  diffraction or spectroscopic studies.

What is Vander waals’ radius ?

The van der waals’ radius is defined as one half of the distance between the nuclei  of two adjacent identical atoms belonging to two neighbouring  molecules of an element in the solid state.

It is essentially the distance between two non bonded atoms of two adjacent molecules.

The van der waals’ radii are obtained from X-ray studies of various elements in the solid state.

What is ionic  radius ?

The ionic radius may be defined as the distance from the nucleus of an ion upto which it has influence on its electron charge cloud.

The inter nuclear distance in any ionic compound is determined from X-ray measurements.

This distance is taken as the sum of the radii of the two ions involved. Knowing the radius of one , that of the other can be calculated.

What  is metallic radius ?

Metallic radius is defined as one half of the distance between the two adjacent metal atoms in a metallic cluster.

Summary: 


What is periodicity ?
What is the cause of the periodicity ?
What is covalent radius ?
What is ionic radius ?
What is Vander waal's radius ?
What is metallic radius ?
  

Nucleophilic substitution reaction