Explain the trend in the boiling temperatures of the elements on descending group 7, from fluorine to iodine.

Understanding the question:

The process of boiling is heating the mixture until the liquid turns into gas. This process requires breaking intermolecular forces (interactions between molecules (eg: hydrogen bonds, london forces/Van der Waal and permanent dipole dipole). NOTE: not breaking the bonds within molecules - intramolecular forces

Group 7 elements tend to exist as diatomic molecules (X₂). Therefore, when you are boiling the molecules you are breaking the intermolecular reactions between these X₂ molecules.

Group 7 elements cannot hydrogen bond (as there is no hydrogens involved). They cannot form permanent dipole-dipole interactions either as there is no permanent dipoles in the X₂ molecules. Hence, their only interaction is london forces.

The strength of london forces increases when the number of electrons in the molecules increase.

You can obtain the number of electrons in a molecule by multiplying the atomic number by the number of atoms of that element present in the molecule.

Answering the question:

As you progress down group 7 boiling temperature increases(the first mark):

Flourine - F₂ = 18 electrons

Chlorine - Cl₂ = 34 electrons

Bromine - Br₂ = 70 electrons 

Iodine - I₂ = 106 electrons

This is because there are more electrons per X₂ molecule going from fluorine to iodine (second mark).

Therefore, the strength of london forces increases from fluorine to iodine (third mark).

Therefore, more energy is required to break the intermolecular forces (fourth mark)