Describe what is meant by molecular solids giving an example, do this solids have a high or low melting points.?

A molecular solid is a type of solid in which molecules are held together by van der Waals forces rather than the ionic or covalent bonds. The relatively wear intermolecular forces causes molecular solids to have relatively low melting point, typically less than 300 degree Celsius. They tend to dissolve in organic solvents.

Eg: sulfur and water ice

Molecular solids ....

Matter in the solid state can consist of networks (lattices) of atoms and have bonds from 100% covalent to those with high ionic character, or a solid may consist of discrete molecules which are held together by relatively weak intermolecular attractions.

Because the forces (*) holding together the discrete molecules as a "molecular solid" are relatively weak, they also have relatively low melting points. Some examples are water ice, dry ice (CO2), iodine (I2), P4O10, sucrose, naphthalene, paraffin, and many other organic molecules.

* The forces holding one discrete molecule to another are called intermolecular forces and consist of hydrogen bonding and the other three van der Waals forces.

1. London dispersion forces .... forces between all molecules. The strength of LDF's is proportional to the polarizability of the molecule, which in turn, depends on the number of electrons and the surface area of the molecule. In the past London dispersion forces were characterized as the weakest of the three van der Waals forces. Contrary to what some teachers and even some authors say, London dispersion forces are often stronger than Keesom or Debye forces and are second only to hydrogen bonding. Some teachers are guilty of telling students that the strength of London dispersion forces depends on the molar mass. That is only coincidental since "heavier" molecules often contain more electrons and are larger. But please understand that the molar mass is not the reason for variations in the strength of London dispersion forces.

2. Keesom forces (dipole-dipole attraction) ... the attraction of one polar molecule for another. Oppositely charged ends of the molecules undergo electrostatic attraction.

3. Debye forces (induced attraction) .... the attraction between a polar molecule and one which is nonpolar, where the polar molecule induces a charge separation in the nonpolar molecule. Debye forces can also exist between two polar molecules, even the same kind of molecule. In such a case the induced charge make momentarily increase or decrease the dipole moment of the substance.

4. Hydrogen bonding .... The weakly covalent bonds found between the hydrogen atom of one molecule where it is bonded to N, O or F, and the N, O, or F of an adjacent molecule. The hydrogen atom functions as a bridge, forming a bond between two molecules. Hydrogen bonding is more than an electrostatic attraction between molecules, instead there is evidence of orbital overlap and covalent bond formation.

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