The reluctance of the valence s- electrons of atom with high atomic number to take part in a chemical reactions is called inert pair effect because of ineffective shielding of these electrons by inner orbital electrons. As a result these valence s - electrons are more exposed to the nucleus than p- electrons. Therefore these s-electrons are not available for bond formation, however the p electrons are available for the bond formation.
Inert pair effect is more as we go down the group. As a result the stability of highest oxidation state decreases and the stability of lowest oxidation state increases.
For example, Aluminum forms stable compounds by losing 3 electrons to form stable compounds in +3 oxidation state but Thallium (Tl) of same group element prefers to lose one electrons and forms compounds in +1 oxidation state.
