• # question_answer 10) How can you explain higher stability of $BC{{l}_{3}}$ as compared to $TlC{{l}_{3}}$?

Inert pair effect is maximum in thallium. Due to this, only $6{{p}^{1}}$ electron participates in bond formation and thus, the stable oxidation state of thallium is +1 and not +3. Therefore, $TlCl$ is stable but $TlC{{l}_{3}}$ is not stable. $BC{{l}_{3}}$is a covalent compound. Boron shows an oxidation state +3 as the inert pair effect does not exist. All the three valence electrons take part in bond formation. However, it is an electron deficient compound but due to back donation by chlorine atom, it gets extra stability. Thus, $BC{{l}_{3}}$ is more stable than$TlC{{l}_{3}}$.