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.