Which of the following is not correct ?

→ Monomeric trihalide [ AlCl₃ ] is lewis acid . AlCl₃ is electron deficient and hence in order to get octet configuration the central atom "Al" accepts the electron pair and shows lewis acidic character.

→ Sizes of Al and Cl are large, therefore in between  Al and Cl "π" bond formation is not possible.In order to get octet configuration around the "Al" AlCl₃ undergoes dimerisation and forms Al₂Cl₆  dimer.

$\therefore$ AlCl₃ achieves stability by forming a dimer.

→ Anhydrous AlCl₃ fumes in air.

White fumes appear around the bottle of anhydrous aluminium chloride.
 Anhydrous aluminium chloride is partially hydrolysed with atmospheric moisture to liberate HCl gas. Moist HCl appears white in colour.

                           OR

→TlI₃ cannot exist in nature and it is highly unstable,

Reason :Due to inert pair effect Tl forms only Tl (+I) compounds and cannot forms Tl(+III)compounds.

For "Tl" +1 oxidation state is more stable,due to inert pair effect.

NOTE: For III_A group higher oxidation state is due to involvement of ns and np electrons.

                IIIA group ns²  np¹         +3 oxidation state

Lower oxidation state is due to involvement of only valency "np" electrons.

              III_A group ns² np¹      +1 oxidation state

Higher 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⁺³

Stability order of +1 oxidation state: B⁺ > Al⁺ >Ga⁺¹  > In⁺¹ >Tl⁺¹

Therefore, 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.