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