Atomic radii refers to a consistent set of radii for the elements based on the size of their atoms in single bonds. Atomic radii are found by measuring the distance between bonded atoms and assigning part of the distance to each atom.

Atomic radius of A:-

A - A bond length = 2r_A

Ionic radii are the radii of the anions and cations in crystalline ionic compounds. The radius of a positive ion is always smaller than the radius of the atom from which it was formed.

The loss of an electron leaves the same nuclear charge acting on a smaller number of electrons, and as a result, the electrons are pulled in toward the nucleus a bit more.

Also, in many of the common positive ions, the atom has lost all of its valence electrons and is smaller because it has one less occupied energy level.

And so, the radius of a negative ion is always larger due to the added electron repelling other electrons causing the electron cloud to spread out.

 

• Radii generally decrease across the periods
• Radii generally increase down the families
• Cations are smaller and anions are larger than neutral atoms of the elements from which they are formed
• Cations are generally smaller than anions
• Effective nuclear charge increases across each period
• In p-block families, the second and third members are similar in size and properties, because of the intervention of the d-transition elements
• In d-transition element families, the two heaviest members are similar in size and properties because of the intervention of the lanthanides
• The first element in each representative element family differs from other elements in the family because of the much smaller size of its atoms

Information provided by: http://www.bpx.cov.ac.uk