We encounter the borderline between metals and nonmetals for the first time in Group IIIA; boron at the top of the group is nonmetallic. Crystalline boron has atoms arranged at the twelve vertices of an icosahedron, and the various boron hydrides with formulas , , , , and so on, have boron frameworks that are fragments of icosahedra. They all have high heats of combustion with oxygen per unit of weight, and at one time were considered seriously as possible rocket fuels.

Boron makes very strong covalent bonds to C and N in crystals. Boron carbide, , and boron nitride, BN, are nearly as hard and as good abrasives as diamond. Boron nitride has the diamond structure (bottom left) and is isoelectronic with diamond. Every boron atom has one less electron than carbon, but every nitrogen atom has one more, so there are just enough electrons to hold the diamondlike structure together. Boric oxide, , is obtained from natural borax, , which was made famous by the Death Valley twentymule teams that hauled it out of desert deposits.