How do the melting and boiling points change down group 17?

The change in melting and boiling points down group 17, also known as the halogens (which include fluorine, chlorine, bromine, iodine, and astatine), is observed to increase as one moves down the group. This trend can be attributed to the increasing molecular sizes and the associated increase in the number of electrons. As the size of the atoms increases, the van der Waals (dispersion) forces, which are weak intermolecular forces, also strengthen due to more significant electron cloud interactions.

As the number of electrons increases, the polarizability of the molecules becomes greater, which enhances these attractive forces between molecules. Consequently, more energy is required to overcome these forces when transitioning from solid to liquid (melting) and from liquid to gas (boiling). Therefore, the melting and boiling points of halogens increase down the group due to these intermolecular forces becoming stronger as the size and electron cloud density of the atoms increase.

Overall, understanding the molecular structure and the role of intermolecular forces is essential in explaining the trends in physical properties such as melting and boiling points in various groups of the periodic table.