Properties and modifications of Carbon, Phosphorus, Sulfur and Selenium
Carbon: 2 natural modifications diamond and graphite; synthetic fullerenes
Diamond:
each C is tetrahedrally coordinated by 4 other C
bondings due to overlapping sp3-orbitals => no free valence electrons
=> not conductive, no color (color due to impurities)
high C-C bonding energy with 348 kJmol-1 => causes very high stiffness of diamond
=> use as material like glass cutter etc.
diamond is metastable: diamond at 1500˚C  graphite (without O2)
Graphite:
modification with layer structure => only Van-der-Waals forces between single layers
=> use as lubricant, lead pencil etc.
each atom is neighbored by 3 more atoms (triangle) => fused six-rings
atoms are bonded due to sp2 – hybrid orbitals
 1 electron occupies p-orbital perpendicular to layer plane
 delocalised π bondings over total layer plane cause black color, metallic glance,
high conductivity in plane direction
more reactive than diamond: graphite at 700˚C  CO2 (with O2)
Fullerenes:
C60 consists of 12 five-rings and 20 six-rings => bonding due to overlapping sp2- orbitals
=> localized π bondings over complete cage
rule: each five-ring is neighbored by six-rings => no fullerenes between C60 and C70
Sulfur: several modifications
at room temperature thermodynamically stable S8 – rings ( rhombic α-sulfur, crystalline )
 95.6˚C: rhombic α-sulfur => monocline β-sulfur
 119.6˚C: sulfur gets liquid (S8 – rings ; with higher temperatur Sn – rings with
n = 6 – 26 [yellow] plus Sx – chains with x = 103 – 106 [brown])
synthetic modifications of Sn ring molecules with n = 6,7,9,10,11,12,13,15,18,20 well known.
Selenium: 6 modifications
=> 3 crystalline monocline modifications with red color constituted of Se8 – rings;
=> upper 100ºC transition into thermodynamic stable metallic, grey selenium constituted of
infinite spiral, parallel chains with van der Waals and covalent forces between chains;
=> red amorphous selenium with same chains, but slightly deformed;
=> black, glassy selenium constituted of Sen – rings (n ≤ 1000)
Phosphorus: many modifications
White phosphorus:
- can ignite by itself => storage under water;
- its build up by tetrahedral P4 – molecules with low valence angle of 60º
=> molecules are stressed, bulk material is instable and reactive
Red phosphorus:
- white phosphorus at 180 - 400ºC  polymeric, amorphous red phosphorus (without O2)
- build up by 3 dimensional network, not toxic, doesn´t ignite by itself (< 300ºC)
- each P is linked to three other P
Black phosphorus:
- thermodynamic stable modification under normal pressure (≤ 550ºC)
- bulk material shows metallic glance and behaves like a semiconductor, not poisonous
- black phosphor crystallizes in a rhombohedral layer structure (parallel layers of zigzag
chains; one of the three bond of P links the layers)
Violet phosphorus:
- Hittorf´s (German Chemist) phosphorus
- monoclinic crystal structure (consisting of parallel oriented pentagonal P-tubes; the tubes of
neighboured layers are perpendicular orientated)
Tasks:
(1) Explain the lubricant effect of graphite.
(2) Explain the high reactivity of white phosphor.