Diagonal relationship between Boron and Silicon   

         Due to its small size and similar charge/mass ratio, boron varies from other group 13 members, but it resembles to the silicon, the second element of the group 14 to exhibit diagonal relationship. Some significant similarities between boron and silicon are given below,

         (1) Both boron and silicon are typical non-metals, having high melting point and boiling point nearly same densities (B=2.35gml^-1 S=2.34 g//ml). low atomic volumes and bad conductor of current. Though both are used as semiconductors.

         (2) Both of these do not form cation and form only covalent compounds.

         (3) Both exist in amorphous and crystalline state and exhibit allotropy.

         (4) Both possess closer electronegativity values  (B=2.0; Si=1.8).

         (5) Both form numerous volatile hydrides which spontaneously catch fire on exposure to air and are easily hydrolysed.

   (6) The chlorides of both these are liquid, fume in most air and readily hydrolysed by water.

         BCl₃ + 3H₂O → B(OH)₃ + 3HCl 

         SiCl₄ + H₂O → Si(OH)₄ + 4HCl

(7) Both form weak acids like H₃BO₃ and H₂SiO₃.

         (8) Both form binary compounds with several metals to give borides and silicide. These borides and silicide react with H₃PO₄ to give mixture of boranes and silanes.

            3Mg+2B → Mg₃B₂; Mg₃B₂+H₃PO₄ → Mixture of boranes

                                ^{(Magnesium boride)}

         2Mg  + Si → Mg₂Si  ;  Mg₂Si + H₃PO₄ → Mixture of silanes

                                         ^{(magnesium silicide)}   

   (9) The carbides of Boron and silicon (B₄C and SiC) are very hard and used as abrasive.

         (10) Oxides of both are acidic and can be reduced by limited amount of Mg In excess of Mg boride and silicide are formed.

         B₂O₃+3Mg → 3MgO+2B  ;  SiO₂+2Mg →2MgO+Si

   (11) The both metals and their oxides are readily soluble in alkalies.

         2B + 6NaOH → 2Na₃BO₃ + 3H₂ ­

                                        ^(borate)

         Si + 2NaOH + H₂O → Na₂SiO₃ + 2H₂ ­

                                      ^{(Formula for silicate)}

         B₂O₃ + 6NaOH → 2Na₃BO₃ + 3H₂O 

         SiO₂ + 2NaOH → Na₂SiO₃ + H₂O

   Both borates and silicates have tetrahedral structural units and respectively. The boro silicates are known in which the boron replaces silicon in the three dimensional lattice. Boron can however form planar BO₃ units.

         (12) Acids of both these elements form volatile esters on heating with alcohol in presence of concentrated H₂SO₄.

         B(OH)₃ + 3ROH→B(OR)₃ + 3H₂O

            Si(OH)₄ + 4ROH→ Si(OR)₄ + 4H₂O

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