At 25 C, #F_2# is a gas but #I_2# is a solid. What is most likely the case of this?

Question

At 25 C, #F_2# is a gas but #I_2# is a solid.  What is most likely the case of this?

Serenity Allen · Accepted Answer

The most likely cause is that the attractive forces between the molecules of #"I"_2# are greater than those in #"F"_2#.

The molecules of each substance attract each other through London dispersion forces.

Whether a substance is a solid, liquid, or gas depends on the balance between the kinetic energies of the molecules and their intermolecular attractions.

Molecules at the same temp have the same average kinetic energy (#"KE" = 1/2mv^2#).

In fluorine, the electrons are tightly held to the nuclei.

The electrons have little chance to wander to one side of the molecule, so the London dispersion forces are relatively weak.

At room temperature, the fluorine molecules have enough energy to escape these attractive forces.

The attractions are not strong enough to make fluorine condense or solidify.

In iodine, the electrons are so far from the nuclei that the electron clouds can easily distort.

The London dispersion forces are strong.

The molecules no longer have enough kinetic energy to escape the attractive forces.

The attractions are strong enough to cause the iodine to be a solid at room temperature.

Henry Antrim · Answer

undefined This is due to differences in molecular size and intermolecular forces. Fluorine (F2) molecules are smaller and experience weaker van der Waals forces, so they remain a gas at room temperature. Iodine (I2) molecules are larger and experience stronger van der Waals forces, causing them to form a solid at room temperature.