The 6s orbital electron pair is strongly attracted by the nucleus due to the poor shielding effect of 4f and 5d orbitals. However there are other filled orbitals that shield the nucleus. In case of 5s orbital, it is nearer to the nucleus and has the other filled orbitals preceding it, in common with 6s orbital, except the 5p, 4f and 5d orbitals. For 5s orbital there are fewer orbitals shielding the nucleus and it is also nearer to the nucleus, in comparision to 6s orbital. So, why Bi shows inert pair effect and not Sb?