Clearly the direction of change—increase or decrease—down the
group depends on the kind of bonding. Between the free atoms of the
noble gases there are weak forces of attraction which increase with
the size of the atom (Chapter 12) and similar forces operate between
the molecules of the hydrogen halides HC1, HBr and HI. The forces
between the atoms in a metal and the ions in a salt, for example
sodium chloride, are very strong and result in high melting and boiling
points. These forces decrease with increasing size of atom and ion
and hence the fall in melting and boiling points.