Suggest reasons why the $B-F$ bond lengths in $BF_3\, (130\, pm)$ and $BF_4^- \,(143\, pm)$ differ. 

Why does boron trifluoride behave as a Lewis acid ?

The element which exists in liquid state for a wide range of temperature and can be used for measuring high temperature is :- 

Three pairs of compounds are given below. Identify that compound in each of the pairs which has group $-13$ element in more stable oxidation state. Give reason for your choice. State the nature of bonding also. $(A)$ $TlCl_3, TlCl$ $(B)$ $AlCl, AlCl$ $(C)$ $InCl_3, InCl$

In $IIIA$ group, $Tl$ (thalium) shows $+1$ oxidation state while other members show $+3$ oxidation state. Why