Answer to Question #180215 in General Chemistry for Amy

Question

Why do group 1 elements boiling points decrease down the group? I heard it was to do with atomic radius but I am not sure why? Similarly, why do group 7 elements boiling points increase down the group?

Thanks

Answer 1

The group 1 elements are the so-called alkali-metals. The bonding between the atoms is caused by the interaction of the nuclei with the delocalized electrons. With increasing number of electrons and protons, the atomic radii get bigger and hence this interaction becomes weaker as the average distance between nuclei and electrons increases as you go down in group 1.

The group 7 elements are the so-called halogens. They exist under normal circumstances in their molecular form (F₂FX₂, CL₂CLX₂ and so on). In contrast to the group 1 elements, the dominant intramolecular force is here London dispersion (or van der Waals forces if you prefer). This attraction is caused by the correlated motion of electrons. With increasing amount of electrons, there can be more correlated motion and hence there is a stronger interaction between the molecules and an increasing melting / boiling point when you go down in group 7.