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).
Select the corresponding radio buttons to relate the name of a molecule to its formula.
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).
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:
|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.
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, 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.
Typically, the properties of molecular substances can be explained in terms of their structures. Here are some points:
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