THE STRUCTURES OF THE PERIOD 3 ELEMENTS
This page describes the structures of the Period 3 elements from sodium to argon, and shows how these structures can be used to explain the physical properties of the elements.
Variation in physical properties in period 3
Melting and boiling points
In a moment we shall explain all the ups and downs in this graph.
Sodium, magnesium and aluminium are all good conductors of electricity. Silicon is a semiconductor. None of the rest conduct electricity.
Explaining the trends
Warning! To understand this section you must be familiar with metallic bonding and the structure of metals, giant covalent structures, simple molecular structures and van der Waals forces. If you are uncertain of any of this, now is the time to revise it.
Three metallic structures
Sodium, magnesium and aluminium all have metallic structures, which accounts for their electrical conductivity and relatively high melting and boiling points.
Melting and boiling points rise across the three metals because of the increasing number of electrons which each atom can contribute to the delocalised "sea of electrons". The atoms also get smaller and have more protons as you go from sodium to magnesium to aluminium.
The attractions and therefore the melting and boiling points increase because:
Silicon - a giant covalent structure
Silicon is a non-metal, and has a giant covalent structure exactly the same as carbon in diamond - hence the high melting point. You have to break strong covalent bonds in order to melt it.
There are no obviously free electrons in the structure, and although it conducts electricity, it doesn't do so in the same way as metals. Silicon is a semiconductor.
Note: Explaining how semiconductors conduct electricity is beyond the scope of A'level chemistry syllabuses.
Four molecular elements
Phosphorus, sulphur, chlorine and argon are simple molecular substances with only van der Waals attractions between the molecules. Their melting or boiling points will be lower than those of the first four members of the period which have giant structures. The presence of individual molecules prevents any possibility of electrons flowing, and so none of them conduct electricity.
The sizes of the melting and boiling points are governed entirely by the sizes of the molecules:
Argon molecules consist of single argon atoms.
There are several forms of phosphorus. The data in the graph at the top of the page applies to white phosphorus which contains P4 molecules. To melt phosphorus you don't have to break any covalent bonds - just the much weaker van der Waals forces between the molecules.
Sulphur consists of S8 rings of atoms. The molecules are bigger than phosphorus molecules, and so the van der Waals attractions will be stronger, leading to a higher melting and boiling point.
Chlorine, Cl2, is a much smaller molecule with comparatively weak van der Waals attractions, and so chlorine will have a lower melting and boiling point than sulphur or phosphorus.
Argon molecules are just single argon atoms, Ar. The scope for van der Waals attractions between these is very limited and so the melting and boiling points of argon are lower again.
Note: You might also be interested in the trends in ionisation energy, atomic radius and electronegativity in this period. You will find relevant descriptions and explanations if you follow these links - or they are available via the menus below.
If you are exploring Period 3 in detail, this link will take you to a major section covering all the aspects of Period 3 chemistry needed for the UK A level syllabuses - including reactions of the elements, their oxides, chlorides and hydroxides.
© Jim Clark 2000 (modified October 2012)