Arrange the following in order of decreasing boiling point. (a) \(\mathrm{I}_{2}\) (b) \(\mathrm{F}_{2}\) (c) \(\mathrm{Cl}_{2}\) (d) \(\mathrm{Br}_{2}\)

The concept of intermolecular forces is crucial when understanding why different substances have different physical properties, like boiling points. Intermolecular forces are the forces that mediate interaction between molecules, including attractions or repulsions that affect the physical properties of compounds. There are several types of intermolecular forces, such as dipole-dipole interactions, hydrogen bonding, and dispersion forces. These forces are weaker than the chemical bonds within molecules (intramolecular forces) but are vital in the study of chemistry.

For halogens, as encountered in the given exercise, dispersion forces are the primary type of intermolecular forces present because these molecules are non-polar. Understanding these forces helps explain why molecules stay together in the liquid phase and the energy required to separate them into a gas, which we observe as the boiling point.

Dispersion forces, also known as London dispersion forces, are the weakest form of intermolecular attractions, yet they govern the boiling points of non-polar molecules such as the halogens. These forces arise from fluctuations in the electron distribution within molecules or atoms, which create temporary dipoles. When molecules are close together, the electron cloud of one molecule can induce a temporary dipole in another molecule, leading to an attraction between them.

The strength of dispersion forces increases with the size and number of electrons in the atoms or molecules. Larger atoms have more diffuse electron clouds that are more polarizable, meaning their electrons can more easily shift to create temporary dipoles. This is why, as shown in the exercise, heavier halogens like iodine have stronger dispersion forces and hence higher boiling points than lighter halogens like fluorine.

The periodic table organizes elements by increasing atomic number and groups them with similar chemical properties. One such group is the halogens, found in Group 17. As you move down this group, the atoms increase in size due to the addition of energy levels; they also gain more electrons, which contributes to stronger dispersion forces as explained earlier.

The trend that boiling points increase as you move down the group in the periodic table is consistent across the halogens. This occurs because heavier halogens have larger, more polarizable electron clouds. Consequently, in the context of the exercise provided, the physical property of boiling point can be anticipated from the element's position in the periodic table, making it a valuable tool for predicting and rationalizing the differences in physical properties among elements with similar chemical characteristics.