Intermolecular forces

In terms of intermolecular forces why does Hydrogen iodide requires more heat energy for melting than hydrogen chloride does ?

The lower down group 7, the lower the electronegativity and the greater the masses/sizes. They are therefore more stable and form more fleeting dipole-dipole interactions/Van der Waals forces. It takes more thermal energy to separate their molecules.

The lower down group 7, the lower the electronegativity and the greater the masses/sizes. They are therefore more stable and form more fleeting dipole-dipole interactions/Van der Waals forces. It takes more thermal energy to separate their molecules.

Don't talk about electronegativity because H-Cl bonds are more polar than H-I and so the dipole is larger and you might argue that there are dipole-dipole interactions that would imply the opposite. in fact the strong dipole outweighed by the instantaneous interactions due to having more electrons so it can be ignored.

Both molecules are polar, so the strongest intermolecular bonds they form are permanent dipole - permanent dipole bonds. Chlorine is more electronegative that Iodine, so the H-Cl bond is more polar than the H-I bond. Therefore, the strength of the intermolecular forces between HCl molecules is greater, and so they require more energy (i.e. a higher temperature) to break.

If you can't ignore it until you've take it into account, are you really ignoring it? The greater permanent dipole interactions in HCl are important and should be noted in any discussion of the relative strength of internuclear forces.