Atomic and ionic radii

Introduction :

Atomic radii are also known as covalent radius.  Atomic radii describes the size of the atom of an element in its elementary state or in covalently linked molecule. It is measured in terms of picometers or Angstroms. It is the distance from the nucleus to the boundary of the surrounding cluster of electrons.

The bond length of a covalent linkage is the distance between the nuclei of the two bonded atoms. The bond length can be measured by X-diffraction method.

The internuclear distance between two unlike atoms in a covalent bond is the sum of the atomic radii of the two. For example, the covalent radii of hydrogen and chlorine are 0.037 nm and 0.099 nm respectively. Hence the internuclear distance in HCl molecule is 0.037 nm + 0.099 nm = 0.136 nm.

The atomic size will generally decrease from left to right in a period. This is because, when we proceed from one element another in a period, this results in a greater pull on electrons towards the nucleus. In a given period, the alkali metal atom of group 1A is the largest and the halogen atom in 7A is the smallest.

Ionic radii:

The ionic radii may be defined as the distance between the nucleus of an ion and the point up to which the nucleus has influence on its electron cloud.
The concept of ionic radii was developed independently by Victor Goldschmidt and Linus Pauling in 1920.
If ions in a crystal are regarded as spheres, the internuclear distance between two ions is equal to the sum of the radii of the ions. The internuclear distance is measured by X-ray analysis of ionic compounds. Knowing the radius of one ion, that of the other is calculated.
The interionic distance in potassium chloride crystal is 0.314 nm= r K⁺ + r Cl^- = 0.314 nm where r K⁺ and r Cl^- are the radii of potassium ion and chloride ion respectively. It is found that r K⁺ = 0.314 nm   Therefore r Cl^- = 0.314-0.133 = 0.181 nm.