p Block Elements Part 2

Assertion (A) is correct. ${TII}_{3}$ and ${CsI}_{3}$ are the triiodide, ${I}_{3}^{-}$ (polyhalide) compounds of ${TI}^{+}$and ${Cs}^{+}$ions. Due to inert pair effect of group $-13$ elements, ${Tl}^{+}$is more stable than ${TH}^{3+}$. Cs and Tl belong to the same period (6th). Sizes of ${Cs}^{+}$and ${Tl}^{+}$are nearly same. So, geometrical network and lattice pattern of both ${CsI}_{3}$ and ${TII}_{3}$ are same (bcc lattice). Csl_3 and ${TII}_{3}$ are also able to form mixed crystals when crystallisation is carried out from a mixture of saturated solutions of ${CsI}_{3}$ and ${TII}_{3}$. These properties enable ${TII}_{3}$ and ${Csl}_{3}$ to be isomorphous crystals. Reason $(R)$ is also correct. Electronic configuration of $C$ s $(Z=55)$ and $T I(Z=81)$ as follows ${ }_{55} {Cs}:[{ }_{54} {Xe}] 6 s^{1}$ ${ }_{81} {Tl}:[{ }_{54} {Xe}] 4 f^{14}{ }_{5 d}{ }^{10} 6 s^{2} 6 p^{1}$ So, Tl has fourteen $f$-electrons in the anti-penultimate (4th) shell. ${A}$ and ${R}$ are individually correct and ${R}$ is the correct explanation of ${A}$.