The p -Block Elements

In BF₃, boron is sp² hybridized.
? shape of BF₃ = planar.
In [BH₄]^–, boron is sp3 hybridized, thus the shape is tetrahedral.

On heating boric acid above 370 K, it forms metaboric acid, HBO₂ which on further heating yields boric oxide B₂O₃.

H₃B₂O₃ → HBO₂ → B₂O₃

Boric acid is a Lewis acid since it accepts electrons from hydroxyl ion of H₂O molecule. It is not a protic acid.
B (OH)₃ + 2HOH → [B (OH)₄]^– + H₃O⁺

In BCl₃, there is only six electrons in the valence shell of B atom. Thus, the octet is incomplete and it can accept a pair of electrons from water and hence BCl₃ undergoes hydrolysis. Whereas, in CCl₄, C atom has 8 electrons and its octet is complete. That's why it has no tendency to react with water.

In BF₃, central atom has only six electrons after sharing with the electrons of the F
atoms. It is an electron-deficient compound and thus behaves as a Lewis acid.

BCl₃ is quite stable. Because there is absence of d- and f-electrons in boron three valence electrons (2s² 2p_x1) are there for bonding with chlorine atom. In Tl the valence s-electron (6s²) are experiencing maximum inert pair effect. Thus, only 6p¹ electron is available for bonding. Therefore, BCl₃ is stable but TlCl₃ is comparatively unstable.

(i) B to Tl
Common oxidation states are +1 and +3. The stability of +3 oxidation state decreases from B to Tl while +1 oxidation state increases from B to Tl.

(ii) C to Pb
The common oxidation states are +4 and +2. Stability of +4 oxidation state decreases from C to Pb.