1. Discuss the pattern of vibration in the oxidation states of ( i ) B to Tl and (ii) C to Pb. 

( i )    B to Tl

The electronic  configuration of group 13 elements is ns² np¹. Therefore, the most common oxidation state exhibited by them should be +3. However, it is only boron and aluminium which particularly show the +3 oxidation state. The remaining elements,that is Ga, In, Tl show both the +1 and +3 oxidation states. On moving down the group, the +1 state becomes more stable. For example, Tl ( +1 ) is more stable than Tl ( +3 ). This is because of the inert pair effect. The two electrons present in the s - shell are strongly attached by the nucleus and do not participate in bonding. This inert pair effect becomes more and more prominent on moving down the group. Hence, Ga ( +1 ) is unstable, in ( +1 ) is fairly stable , and Tl ( +1 ) is very stable.

The stability of the +3 oxidation state decreases on moving down the group.

(ii) C to Pb

      The electronic configuration of group 14 elements is ns² np².  Therefore, the most  common oxidation state exhibited by them should be + 4 .  However, the +2 oxidation state becomes more  and more common on moving down the group, C  and Si mostly show the + 4 state.  On moving down the group, the higher oxidation  state becomes less stable.  This is because of the inert pair effect.  Thus, although Ge, Sn, and Pb show both the +2  and +4  states, the oxidation stability of the lower oxidation state increases  and that of the higher oxidation state decrease on moving down the group. 

2.  How can you explain  higher stability of BCl ₃ as compared to TlCl₃ ?

Boron and thallium belong to group 13 of the periodic table, In this group, the +1 oxidation state becomes more stable  on moving down the group.  BCl₃ is more stable than TlCl₃, because the +3 oxidation state of B is more stable than the +3 oxidation state  of Tl.  In Tl, the +3 is highly oxidising and it reverts back to the more stable +1 state.

3.  Why does boron trifluoride behave as a Lewis acid?

The electronic configuration of boron is ns² np¹.  It has three electrons in its valence shell.  Thus it can form only three covalent bonds.  This means that there are only six electrons around boron and its octet remains incomplete.  When  one atom of boron combines with  three fluorine atoms,  its octet remains incomplete.  Hence, boron trifluoride remains electron - deficient and acts as a Lewis acid.  
                 

4. Consider the compounds, BCI₃ and CCl₄.  How will they behave with water? Justify.

Being a Lewis acid, BCI₃ readily undergoes hydrolysis.  Boric acid is formed as a result.
                                           BCI₃ + 3H₂O    →   3HCI  + B(OH)₃
CCI₄ completely resists hydrolysis.  Carbon does not have any vacant orbital.  Hence, it cannot accept electrons from water  to form an intermediate.  When CCI₄  and water are mixed, they form separate layers. 
                                           CCI₄ + H₂O   →  No reaction

5. Is boric acid a protic acid? Explain.

Boric acid is not a protic acid.   It is a weak monobasic acid, behaving as a lewis acid.
                                   B(OH)₃ + 2HOH   → [ B(OH)₄]^- + H₃O⁺
It behaves as an acid by accepting a pair of electrons from ^-OH ion.
  
6. Explain what happens when boric acid  is heated.

On heating orthoboric acid ( H₃BO₃)   at 370 K or above, it changes to metaboric acid (HBO₂).  On further heating, this yields boric oxide B₂O₃. 
                         

7.  Describe  the shapes of BF₃  and BH₄^-.  Assign  the hybridisation  of boron in these species.

( i )  BF₃

As a result of its small size  and high electro negativity, boron tends to form monomeric covalent halides.  These halides  have a planar triangular geometry.  This triangular shape  is formed  by the overlap of three sp² hybridised orbitals of boron with the sp orbitals of three halogen atoms.  Boron is sp²  hybridised in BF₃.

(ii) BH₄^-

Boron - hydride ion (BH₄^-)  is formed by the sp³ hybridisation of boron orbitals.  Therefore, it is tetrahedral in structure.
              

8. Write reactions to justify amphoteric nature of aluminium.

A substance is called amphoteric if it displays characteristics of both acids and  bases.  Aluminium dissolves in both acids and bases, showing  amphoteric behaviour.

                                     

9.  What are electron deficient compounds ?  Are BCl₃ and SiCl₄  electron deficient species?  Explain.

In an electron - deficient compound, the octet  of electrons is not complete,  i.e, the central metal atom has an incomplete octet.  Therefore, it needs electrons to complete its octet.

(i) BCl₃

BCl₃  is an appropriate example of an electron - deficient compound.  B has 3 valence electrons.  After forming three covalent bonds with chlorine, the  number of electrons around it increases to 6.  However, it is still short of two electrons to complete its octet. 

(ii) SiCl₄

The electronic configuration of silicon is ns² np².  This indicates that it has four valence electrons.  After it forms four covalent bonds with four chlorine atoms, its electron count increases to eight.  Thus SiCl ₄ is not an electron - deficient compound.