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).

Ammonia 

Carbon dioxide 

Carbon monoxide 

Hydrogen chloride 

Hydrogen sulphide 

Methane 

Nitrogen dioxide 

Nitrogen monoxide 

Sulphur dioxide 

Sulphur trioxide 

Water 

Bromine 

Hydrogen 

Nitrogen 

Oxygen 

Br2

CH4

CO 

CO2

H2

H2O

HCl 

H2S

NH3

NO 

NO2

N2

O2

SO2

SO3

Comment:

Select the corresponding radio buttons to relate the name of a molecule to its formula.

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 SO42- (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:

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  1. Carbon Dioxide, CO2
  2. Water, H2O
  3. Ammonia, NH3
  4. Tetrachloromethane, CCl4
  5. Sulphur Hexafluoride, SF6
  6. Buckminsterfullerene, C60
Use the mouse pointer to rotate the model.
Carbon Dioxide, CO2, is Linear
Chemis3D is made available by Didier Collomb.
Thanks also to those who make their 'molecules' available on the Internet for download.

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:



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