Melting and Boiling Points

 

* The melting points and boiling points depend upon the bond strength and structure of the elements.

* From Na to Al, the elements are metals.

o       The melting points and boiling points increase because:

1.      the atoms are smaller

2.      they have increasing effective nuclear charge

3.      so the strengths of the metal-metal bonds increase.

* Silicon has high melting and boiling points because

o       It is macromolecular with a diamond structure.

1.      Strong covalent bonds link all the atoms in three dimensions

2.      A great deal of energy is required to break these bonds

* P, S and Cl are all molecular substances

o       Melting point is determined by the strengths of van der Waals’ forces.

o       These are determined by the size of the molecules.

o       These elements have low melting points because the van der Waals’ forces are weak and easily broken.

Molecular formula

P4

S8

Cl2

Melting point/K

317

392

172

Boiling point/K

553

718

238

 * The sulphur molecule is the biggest and most polarisable – sulphur has a melting point which is higher than that of phosphorus or chlorine.