Chemical properties of Boron

From the chemical point of view, boron is decidedly a non-metallic element. Thus, the halides are volatile compounds which are readily and irreversibly hydrolysed by water. The sulphide of boron is similarly hydrolysed by water; several hydrides are volatile. On the other hand, boron sesqui-oxide appears to be amphoteric. From the physical standpoint, boron exhibits a slight resemblance to the metallic elements; the binary systems boron-metal, for example, are to some extent similar to the binary systems composed of two metals.

From the reactions of boron it is clear that the binary compounds of boron are very stable substances. The heats of formation (H.F.), given below, fully bear out this conclusion: -

(BF₃) H.F. = 234.9 Cals.; BCl₃ H.F. = 93.3 Cals.; BBr₃ H.F. = 43.2 Cals.; [B₂S₃] H.F. = 82.6 Cals.; [B₂O₃] H.F. = 272.6 Cals.;

Boron is tervalent in its halogen derivatives, and appears to be usually tervalent in its compounds. However, it seems at times to exhibit a valency of five. Thus, Moissan has prepared boron pentasulphide; and the compounds of the formulse C₆H₅BCl₄, (CH₃)₃BNH₃, (C₂H₅)₃B.NH₃, and (C₂H₅.O)₃B.C₂H₅.ONa may perhaps contain pentad boron and be formulated as follows: -



The compound B(CH₃)₃.NH₃, for instance, is fairly stable; it melts at 51° and boils at 110°. The existence of certain additive products of boron trichloride and tribromide also suggests that boron is pentad. The same hypothesis appears necessary to account for the nature of tbe borohydrates and the lower oxides of boron.

The existence of a fairly stable hydride, B₂H₆, clearly shows that on occasion boron may be at least quadrivalent. According to Stock, boron is quadrivalent in its hydrides, and also in the hypoborates, e.g. KOBH₃, which would accordingly be written



It is of some interest in this connection to note that since boron is occasionally at least quadrivalent, its maximum valency is greater than its group number in the periodic table.