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:
“Science is intimately integrated with the whole social structure and cultural tradition. They mutually support one other—only in certain types of society can science flourish, and conversely without a continuous and healthy development and application of science such a society cannot function properly.”
—Talcott Parsons (1902–1979)
“What is the most rigorous law of our being? Growth. No smallest atom of our moral, mental, or physical structure can stand still a year. It grows—it must grow; nothing can prevent it.”
—Mark Twain [Samuel Langhorne Clemens] (1835–1910)
“Every thing in his composition was little; and he had all the weaknesses of a little mind, without any of the virtues, or even the vices, of a great one.”
—Philip Dormer Stanhope, 4th Earl Chesterfield (1694–1773)