This is the which arise due to the inability of inner electrons to take part in the chemical reactions.
For example in case of group 15 we have Phosphorus, Arsenic , Tin and Bismuth. They all show the oxidation states of +3 and +5, but as we move down the group we will see that the +3 oxidation state becomes more stable. This is due to the inability of inner electrons to take part in reaction.
These are more closely held by the nucleus.