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 molecules. The forces between the molecules could 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 then hydrogen bonding will be possible. As a consequence of this small molecular size and weak intermolecular bonding 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 present by simply breaking these weak intermolecular 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 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 and mass of the molecules increase 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 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 solid iodine is shown below:

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 unusual state change is that as the iodine 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 bonds between the iodine atoms in the small iodine molecules are not broken when it is heated and sublimes.

Solubility

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:

• If both the solute and the solvent consist of non-polar molecules then the only way for them to interact with each other is through Van der Waals (London dispersion) forces. In the example opposite iodine; a covalent molecule dissolves readily in cyclohexane, another covalent molecule. Both iodine and cyclohexane molecules will only have Van der Waals forces between them before they are mixed and when they do mix they will also have Van der Waals forces between them, it is these forces which enable the iodine molecules to dissolve in the cyclohexane.
  
  As another example consider the one shown below, here two liquid alkanes, octane and pentane are mixed to form a solution. These two substances will readily mix to form a solution as there is only Van der Waals bonding between the solvent and solute molecules, so when they are mixed we are simply replacing one set of Van der Waals interacts for another.


• A similar argument can be made when polar molecules are mixed. In the example below methanol and chloromethane (chloroform) are mixed. There are dipole-dipole interacts between the solvent and solute molecules before mixing and once these 2 liquids are mixed there will be new dipole-dipole interactions between them.

• Perhaps the most commonly used solvent is water. If a substance can form hydrogen bonds with water then it is likely it will dissolve. In the example above ethanol and water molecules both form hydrogen bonds between themselves so when they are mixed to form a solution ethanol and water will readily form hydrogen bonding between them. Alcohols and short chain carboxylic acids such as ethanol and ethanoic are both soluble in water since the O-H group in both these molecules can form a hydrogen bond to a water molecule.


• However as the chain length in alcohols and carboxylic acids increases the organic portion of the molecule increases and its solubility decreases. Even organic molecules which contain no hydrogen atoms can form hydrogen bonds with water. Propanone (acetone) is a ketone. Propanone molecules cannot form hydrogen bonds between themselves since they have no hydrogen atom attached to a oxygen but the carbonyl oxygen atom can form hydrogen to a water molecule making propanone soluble in water, as shown opposite:


• Ionic compounds can form ion-dipole interacts with water molecules. The water molecules can form ion-dipole interactions and coordinate bonds with metal ions. In the example shown above the water molecules surround the metal ion and form 6 coordinate or dative covalent bonds with the metal ion.