There is small increase in size from As to Bi. why?rn
Both As (Arsenic ) and Bi (Bismuth) belong to group 15 of the periodic table. As we proceed down a group, the size of the atom increases because a new shell of electrons is added and the electrons lie farther from the nucleus. However as we move from As to Bi, only a small increase in size is observed because of the presence of completely filled d and f orbitals. The d and f orbitals are diffused because of which the electrons present in these orbitals do not screen the nuclear charge effectively. Hence because of poor shielding effect of inner d and f electrons, the valence electrons are attracted more strongly to the nucleus and are pulled closer to the nucleus. Therefore, the increase in size from As to Bi is small.