Structure and Composition
| 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
Famous quotes containing the words structure and, structure and/or composition:
“One theme links together these new proposals for family policy—the idea that the family is exceedingly durable. Changes in structure and function and individual roles are not to be confused with the collapse of the family. Families remain more important in the lives of children than other institutions. Family ties are stronger and more vital than many of us imagine in the perennial atmosphere of crisis surrounding the subject.”
—Joseph Featherstone (20th century)
“For the structure that we raise,
Time is with materials filled;
Our to-days and yesterdays
Are the blocks with which we build.”
—Henry Wadsworth Longfellow (1809–1882)
“When I think of God, when I think of him as existent, and when I believe him to be existent, my idea of him neither increases nor diminishes. But as it is certain there is a great difference betwixt the simple conception of the existence of an object, and the belief of it, and as this difference lies not in the parts or composition of the idea which we conceive; it follows, that it must lie in the manner in which we conceive it.”
—David Hume (1711–1776)