(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