Electronic configuration of thallium is `[Xe]4f^(14) 5d^(10) 6s^(2) 5p^(1)`. Due to the presence of fully filled lesser shieling `4f^(14)` and `5d^(10)` orbitals between the valence shell and noble gas core, the valence shell electrons are more tightly bound of the nucleus , hence, it is difficult to unpair `6s^(2)` effect. This gives rise to inert pair effect. Therefore, `6s^(2)` electrons are less reactive and do not participate in the chemical bond formation. Hence, in thallium, `+1` oxidation state in more stable than `+3` oxidation state.
