4.2 Main group elements of 2nd and 3rd periods and their compounds
(a) Boron
Refined elemental boron is a black solid with a metallic luster. The unit cell of crystalline boron contains 12, 50, or 105 boron atoms, and the B12 icosahedral structural units join together by 2 center 2 electron (2c-2e) bonds and 3 center 2 electron (3c-2e) bonds (electron deficient bonds) between boron atoms (Fig. 4.1). Boron is very hard and shows semiconductivity.