Question

Rank the following molecules in order of increasing London dispersion forces: \(\mathrm{CCl}_{4}, \mathrm{CI}_{4}\), and \(\mathrm{CBr}_{4}\).

Short answer

CCl_4, CBr_4, CI_4

Step-by-step solution

- Understand London Dispersion Forces

London dispersion forces are a type of van der Waals force and are the weakest of intermolecular forces. They are dependent on the size of the molecule and the number of electrons.

- Determine the Molecular Size and Weight

Compare the sizes and weights of the molecules: - \(\text{CCl}_4\): Carbon tetrachloride- \(\text{CBr}_4\): Carbon tetrabromide- \(\text{CI}_4\): Carbon tetraiodideIodine (I) has the largest atomic radius, followed by Bromine (Br), and then Chlorine (Cl).

- Compare the Number of Electrons

Check the number of electrons for each halogen: - Chlorine (Cl) has 17 electrons each.- Bromine (Br) has 35 electrons each.- Iodine (I) has 53 electrons each.The larger the number of electrons, the stronger the London dispersion forces.

- Rank the Molecules

Based on the information, rank the molecules from smallest to largest London dispersion forces: - \(\text{CCl}_4\) (smallest)- \(\text{CBr}_4\)- \(\text{CI}_4\) (largest)

Key concepts

intermolecular forces

Intermolecular forces are the attractions between molecules. These forces are crucial because they determine many physical properties such as boiling points and melting points. One of these forces is the London dispersion force, which is a type of van der Waals force. It is the weakest among intermolecular forces, but it is present in all molecules, regardless of whether they are polar or nonpolar. London dispersion forces occur due to temporary fluctuations in electron distribution within molecules. These fluctuations create brief, temporary dipoles that attract neighboring molecules.

molecular size

The size of a molecule significantly influences its London dispersion forces. Larger molecules have more surface area, allowing more temporary dipoles to form, which in turn increases the attraction between molecules. In our exercise example, iodine (I) in \(\text{CI_4}\) has the largest atomic radius compared to bromine (Br) in \(\text{CBr_4}\) and chlorine (Cl) in \(\text{CCl_4}\). This means that \(\text{CI_4}\) has the largest molecule size, and therefore stronger London dispersion forces.

electron count

The number of electrons in a molecule also plays a vital role in determining the strength of London dispersion forces. More electrons mean greater electron cloud distortion possibilities, which enhance the temporary dipoles in the molecule. In the provided molecules, chlorine (Cl) has 17 electrons, bromine (Br) has 35 electrons, and iodine (I) has 53 electrons. Consequently, \(\text{CCl_4}\) will have the weakest London dispersion forces, followed by \(\text{CBr_4}\), and \(\text{CI_4}\) will have the strongest. This relationship between electron count and London dispersion forces directly explains why the molecules are ranked as \(\text{CCl_4}\) < \(\text{CBr_4}\) < \(\text{CI_4}\) in terms of increasing London dispersion forces.