Boron Oxide and Boric Acid

Boron's principal oxide, boron oxide ($\text{B}_2\text{O}_3$), is formed by strongly heating boric acid or boron itself in air/oxygen. Structurally it is a covalent network solid built from planar $\text{BO}_3$ triangular units (boron is $sp^2$ hybridised, three-coordinate) linked together through shared bridging oxygen atoms into a disordered, glass-forming network; this is why $\text{B}_2\text{O}_3$ is the key glass-former in borosilicate glass. Like most oxides of the lighter p-block elements, $\text{B}_2\text{O}_3$ is acidic — it dissolves in water to regenerate boric acid, and reacts with basic oxides/hydroxides to form borates.

Orthoboric acid, $\text{B(OH)}_3$ (commonly written $\text{H}_3\text{BO}_3$), is the hydrated form obtained when borax is treated with a mineral acid. Its structure is planar: each boron atom is $sp^2$ hybridised and surrounded by three $-\text{OH}$ groups at the corners of a triangle, and these planar $\text{B(OH)}_3$ units stack into two-dimensional sheets held together by extensive hydrogen bonding between the $-\text{OH}$ groups of neighbouring molecules, giving boric acid its characteristic soft, flaky, layered crystalline habit — broadly analogous to graphite's layered structure, though held by hydrogen bonds rather than delocalised $\pi$ bonding.

Boric acid is described as a weak, monobasic acid, but — unusually — it does not ionise by donating a proton the way a typical Brønsted acid does. Boron in $\text{B(OH)}_3$ has only six electrons around it (three $\sigma$ bonds, no lone pair), leaving an empty $2p$ orbital that makes the boron atom electron-deficient and hence a good Lewis acid (electron-pair acceptor). In water, $\text{B(OH)}_3$ acts on this Lewis acidity: rather than releasing $\text{H}^+$ itself, it accepts a hydroxide ion (the electron pair) from a water molecule, releasing a proton from that water molecule in the process:

$\text{B(OH)}_3 + 2\text{H}_2\text{O} \rightleftharpoons [\text{B(OH)}_4]^{-} + \text{H}_3\text{O}^{+}$ …