Thallous chloride is more stable than thallic chloride because of

TlCl -Thallous chloride →Tl+1TlCl3-Thallic chloride →Tl+3For "Tl" +1 oxidation state is more stable,due to inert pair effect.NOTE: For IIIA group higher oxidation state is due to involvement of ns and np electrons.                IIIA group ns2  np1         +3 oxidation stateLower oxidation state is due to involvement of only valency "np" electrons.              IIIA group ns2 np1      +1 oxidation stateHigher oxidation state stability decreases from top to bottom.Lower oxidation state stability increases from top to bottom.Stability order of +3 oxidation state: B+3 >Al+3 > Ga+3  > In+3  >Tl+3Stability order of +1 oxidation state: B+ > Al+ >Ga+1  > In+1 >Tl+1Therefore, Tl+3 is acting as strong oxidising agent.Due to poor shielding effect by the innershell f- and d-orbitals electrons ,the valency s-orbial electrons are strongly held by the nucleus,therefore valency s-orbital electron pair does not involve in bonding [inert pair] and hence only one p-orbital electron involves in bonding and shows mainly +1 oxidation state.  Hence +1 oxidation state is stable for Tl is due to inert pair effect.