Molecular Solid - Structure and Composition

Structure and Composition

Melting points of some molecular solids
Formula Tm °C
H2 −259.1
F2 −219.6
O2 −218.8
N2 −210.0
CH4 −182.4
C2H6 −181.8
C3H8 −165.0
C4H10 −138.3
C5H12 −129.8
Cl2 −101.6
C6H14 −95.3
HBr −86.8
HF −80.0
NH3 −80.0
HI −50.8
C10H22 −29.7
HCl −27.3
Br2 −7.2
H2O 0.0
C6H6 5.5
I2 113.7
S8 119.0
C6Cl6 220.0
  • See also
    higher alkanes

The term "molecular solid" may refer not to a certain chemical composition, but to a specific form of a material. For example, solid phosphorus can crystallize in different allotropes called "white", "red" and "black" phosphorus. White phosphorus forms molecular crystals composed of tetrahedral P4 molecules. Heating at ambient pressure to 250 °C or exposing to sunlight converts white phosphorus to red phosphorus where the P4 tetrahedra are no longer isolated, but are connected by covalent bonds into polymer-like chains. Heating white phosphorus under high (GPa) pressures converts it to black phosphorus which has a layered, graphite-like structure.

The structural transitions in phosphorus are reversible: upon releasing high pressure, black phosphorus gradually converts into the red allotrope, and by vaporizing red phosphorus at 490 °C in inert atmosphere and condensing the vapor, covalent red phosphorus can be transformed back into the white molecular solid.

White, red, violet and black phosphorus samples Structure unit
of white phosphorus		 Structures of red violet and black phosphorus

Similarly, yellow arsenic is a molecular solid composed of As4 units; it is metastable and gradually transforms into gray arsenic upon heating or illumination. Some forms of sulfur and selenium are composed of S8 (or Se8) units and are molecular solids at ambient conditions, but they can convert into covalent allotropes having atomic chains extending all through the crystal.

Changes in the chemical composition can have even stronger effects on the bonding in solids. For example, whereas both hydrogen and lithium belong to the first group of the periodic table, LiCl is ionic and HCl is a molecular solid.

Read more about this topic:  Molecular Solid

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