We know that cations form when atom loses its electrons and anions form when atom gains electrons.Trends we discussed in other tutorial of atomic radius are clearly visible with ionic radii as well.So,
Ionic radius increases moving down a group in the periodic table.radii of both cation and anion decreases from left to right across period.
If we compare the radii of cations and anions with those of the atoms from which they are derived-
- Positive ions are smaller than the metal atoms from which they are formed.For example: Na+ ion has a radius of 0.095 nm,a little more than half that of Na atom,0.186 nm.
- Negative ions are larger than the non metals atoms from which they are formed.Cl- ion radius is 0.181 nm,is nearly twice that of Cl atom,0.099 nm.
Let see an example:
Sample problem 1: Using only the periodic table ,arrange each of the following sets of atoms and ions in order of increasing size.
Fe , Fe2+ , Fe3+
Step 1: Fe atom with no charge must be larger than Fe2+ when compared , as in Fe2+ Fe has lost 2 electrons and as cation is smaller as compared to the metal atoms from which they are formed Fe2+ is smaller than Fe.
Step 2 : Fe3+ with a 3+ charge must be smaller than Fe2+ as it is carrying +3 charge so has lost 3 electrons.
The predicted order must be:
Fe3+ < Fe2+<Fe