molecular crystals

Crystals

A crystalline substance is one in which the atoms or ions that make up the structure are arranged in a very ordered and uniform way. A structure where the atoms or ions are arranged in a random or disordered way is called a non-crystalline or amorphous structure. As a rather simple example we could say a brick wall is crystalline, the brick are laid in a very ordered and regular way, if you knock the wall down then you end up with a large pile of brick, this would be a disordered or amorphous arrangement.

Molecular crystals

Most covalent substances have a molecular structure . This will consist of a small group of atoms with strong covalent intramolecular bonding and weak intermolecular bonding between different molecule. The force between the molecules will be Van der Waals (London dispersion forces), or if the molecules have a permanent dipole then dipole-dipole interacts will be possible, and for those molecules such as the alcohols, ammonia, hydrogen fluoride and water which contain a H atom bonded to either a N, O or F atom the hydrogen bonding will be possible. As a consequence of this most covalent molecular substances will be gases or volatile liquids with relatively low melting and boiling points.

Melting and boiling points

Since molecular substance have only weak intermolecular bonding it will not take much energy to separate the molecules by breaking these bonds; so the melting and boiling points will be low when compared to macromolecular structures with strong covalent bonds in all directions and between all the atoms involved in the structure.
Van der Waals forces exist between all atoms and molecules. This form of intermolecular bonding is very weak, it is caused by the formation of temporary induced dipoles within the atoms and molecules as they approach each other. The size of the attraction between the molecules and atoms increases with increasing molecular size and as the number of electrons increases.

In the case of the noble gases the boiling and melting points will increase from helium at the top of group 0 to radon at the bottom. The reason for this is due to an increase in the amount and strength of the Van der Waals bonding present as the atoms get larger (larger atoms means more electrons are present).

As another example consider the hydrogen halides, hydrogen fluoride, hydrogen chloride, hydrogen bromide and hydrogen iodide.

hydrogen halide melting point/0C boiling point/0C dipole moment
H-F -84 19 large
H-Cl -114 -85
H-Br -86 -67
H-I -56 -35 small

Hydrogen fluoride despite being the smallest molecule has the highest boiling point. Remember that when a covalent substance changes state we DO NOT break the covalent bonds, it is the intermolecular bonds that break. In the case of hydrogen fluoride this will be hydrogen bonding, the strongest type of intermolecular bond. This hydrogen bond will outweigh any effects from the weak Van der Waals bonding present in this small molecule. As we descend the table above the size of the molecule increases, so the amount of Van der Waals bonding will increase; this will more than compensate for the reduction in the strength of the dipole-dipole bonding present.

Bonding in a crystal of iodine

Iodine consists of a small diatomic molecule held together by a strong covalent bond. Being a large atom there with lots of electrons there is strong Van der Waals bonding between the iodine molecules, in fact the Van der Waals bonding is sufficiently strong that iodine is a solid at room temperature. The crystal structure of iodine is shown below:

structure of an iodine crystal

When solid iodine is heated it does not melt, in fact it changes directly from a solid to a gas, this state change is called sublimation. The reason for this is that as the crystal is heated and the iodine molecules start to vibrate more and more the weak intermolecular Van der Waals bonds are broken leaving the small individual diatomic iodine molecules, which then enter the gas phase. The strong covalent bond between the iodine atoms in the small iodine molecules are not broken when it is heated and sublimes.

images shows what happens to an iodine crystal when it is heated and the iodine sublimes

Solubility

iodine dissolves very well in organic solvents but poorly in water When considering if a solute will dissolve in a solvent there are a number of factors to consider. In order to dissolve the solvent and solute must interact in some way, this is most likely to be some sort of intermolecular bonding. However you also need to consider the interactions between the solvent molecules themselves and the solute molecules.

In order for a solute to dissolve the solvent molecules will have to separate the solute particles from each other, that is the solvent needs to be able to overcome the intermolecular bonding holding the solute particles together. Similarly solvent-solvent intermolecular bonding will need to be overcome before the solvent molecules can interact with the solute.

Finally the solvent and solute molecules need to be able to interact with each other. The type of interaction will depend largely on the bonding present in the solvent and solute molecules. The image below shows some of the possible interacts between the solvent and solute molecules:



Key Points