Covalent Molecular - small molecules

This section deals with 'small' covalent molecules. Click here for 'big' covalent molecules.

A molecule is a discrete group of two or more atoms that are held together by covalent bonding. There are many familiar simple molecules (small molecules).

Ions with covently bonded atoms...

Groups of covalently bonded atoms that carry an overall positive or negative charge are cations or anions respectively, e.g. NH₄⁺ (ammonium ion) and SO₄^2- (sulphate(VI) ion).

The many different molecules have widely varying numbers of atoms. In these pages, big molecules refers to structures such as DNA (deoxyribonucleic acid), proteins, and polymers, for example. Some texts also refer to these as macromolecules. Unlike small molecules these may not have a fixed molecular formula. For example, the number of monomer units that make up a polymer chain may vary.

Molecules have shapes...

Some molecules are linear in shape, others are non-linear (bent or V-shaped), pyramidal or tetrahdral, for example. Others have more complicated shapes. Use Chemis3D to examine the shapes of some simple molecules:

Molecules attract each other...

Even helium atoms (sometimes referred to as molecules) attract each other. Helium can be liquefied as the pressure of the gas is increased (pushing the atoms closer together) and the temperature decreased (lowering their kinetic energy). Polar molecules, for example, attract each other more strongly. Under room conditions of temperature and pressure, the greater the intermolecular forces of attraction the more likely the substance is to be a liquid, or greater still, a solid. In a liquid, the magnitide of the intermolecular bonding is related to boiling point; in a solid it is related to melting point. There is more about this in the section on intermolecular bonding.

In the solid state, the molecules of an element or compound have a highly regular arrangement. This type of chemical structure is called a simple molecular lattice.

Iodine provides an example of a simple molecular lattice...

Iodine, I₂, is a bluish-black lustrous solid which evaporates (sublimes) to a purple vapour. In its crystalline structure, the covalent I₂ molecules are held together by weak intermolecular bonding. Gently warming a small sample of iodine readily produces the purple vapour. It should be appreciated that as the purple vapour is formed only the weak intermolecular bonds are being overcome. The covalent bonds between the iodine atoms in I₂ molecules are not broken.

I₂(s)    ®    I₂(g)

The diagram on the left shows the arrangement of I₂ molecules in crystalline iodine. The arrangement is face-centred cubic because there is a molecule at each corner of a cube and one at the centre of each face.

It is necessary to consider the particular characteristics of each molecular substance separately in order to assess its numerous properties...

Typically, the properties of molecular substances can be explained in terms of their structures. Here are some points:

• Whilst the atoms of a single molecule are joined by strong covalent bonds, the separate molecules of that substance are held together by weak intermolecular bonding. These weak intermolecular forces of attraction allow the molecules to separate from each other quite easily. Their crystals are therefore usually soft with low melting points and low boiling points.
• Molecules do not carry an overall charge and so they do not conduct electricity.
• Some molecular substances may, however, react with water to form solutions containing ions which do allow the flow of an electric current.
• Molecules with an overall dipole may be soluble to varying degrees in the polar water solvent. Where hydrogen bonding with water molecules is possible this might favour solubility.
• Non-polar molecues like iodine have very little solubility in water. The strong intermolecular bonding between water molecules is stronger than that between iodine molecules or between iodine and water molecules. Iodine molecules can't easily get between water molecules, which have little tendency to solvate uncharged iodine molecules.
• However, iodine is soluble in non-polar solvents such as benzene and tetrachloromethane. The strength of the intermolecular bonding is similar, for example, between benzene molecules, benzene and iodine molecules, and iodine molecules. Benzene molecules can penetrate the iodine crystal and solvate the I₂ to form a solution of iodine in benzene.

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