Melting points generally increase going from sodium to silicon, then decrease going to argon.Boiling points generally increase going from sodium to aluminium, then decrease to argon .


Sodium, magnesium and aluminium are all metals. They have metallic bonding, in which positive metal ions are attracted to delocalised electrons. Going from sodium to aluminium:

* the charge on the metal ions increases from +1 to +3 (with magnesium at +2) ...
* the number of delocalised electrons increases ...
* so the strength of the metallic bonding increases and ...
* the melting points and boiling points increase.


Silicon is a metalloid (an element with some of the properties of metals and some of the properties of non-metals). Silicon has giant covalent bonding. It has a giant lattice structure similar to that of diamond, in which each silicon atom is covalently-bonded to four other silicon atoms in a tetrahedral arrangement. This extends in three dimensions to form a giant molecule or macromolecule.

Silicon has a very high melting point and boiling point because:

* all the silicon atoms are held together by strong covalent bonds ...
* which need a very large amount of energy to be broken.


These are all non-metals, and they exist as small, separate molecules. Phosphorus, sulphur and chlorine exist as simple molecules, with strong covalent bonds between their atoms. Argon exists as separate atoms (it is monatomic).

Their melting and boiling points are very low because:

* when these four substances melt or boil, it is the van der Waals’ forces between the molecules which are broken ...
* which are very weak bonds ...
* so little energy is needed to overcome them.

Sulphur has a higher melting point and boiling point than the other three because:

* phosphorus exists as P4 molecules ...
* sulphur exists as S8 molecules ...
* chlorine exists as Cl2 molecules ...
* argon exists individual Ar atoms ...
* the strength of the van der Waals’ forces decreases as the size of the molecule decreases ...