Do boron and silicon have a diagonal relationship?

Answer 1

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

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

(3) Both exists 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 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 both 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) Both the metals and their oxides are readily soluble in alkalies.

2B + 6NaOH → 2Na3BO3 + 3H2

(borate)

Si + 2NaOH + H2O → Na2SiO3 + 2H2

(silicate)

B2O3 + 6NaOH → 2Na3BO3 + 3H2O

SiO2 + 2NaOH → Na2SiO3 + H2O

Both borates and silicates have tetrahedral structural units and respectively. Boro silicates are known in which 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 conc. H2SO4.

B(OH)3 + 3ROH → B(OR)3 + 3H2O

Si(OH)4 + 4ROH → Si(OR)4 + 4H2O