Problem:  Rank the following compounds in order of decreasing boiling point: potassium fluoride (KF), methane (CH4 ), and bromoform (CHBr3) Rank from highest to lowest boiling point.

Recall that there are several types of intermolecular forces (IMF):

1. Ion-dipole interaction – occurs between an ion and a polar covalent compound; strongest IMF

2. Hydrogen bonding – occurs in compounds where hydrogen is directly connected to an electronegative element such as N, O, or F; 2^nd strongest IMF

3. Dipole-dipole interaction – occurs between two polar covalent compounds; 3^rd strongest IMF

4. Dispersion forces – occurs in all compounds and is the primary IMF exhibited by nonpolar compounds; weakest IMF

Compounds with strong intermolecular forces have high boiling points. This is because they require more energy to be able to break the bonds during the phase transition.

So, we need to identify which among the given compounds has the strongest intermolecular forces.

For potassium fluoride, KF: 

Potassium is made up of a cation (K⁺) and an anion (F^-). Therefore, KF is an ionic compound and it exhibits ion-dipole interaction.

For methane, CH₄: Since hydrogen cannot be the central atom, carbon goes in the center. The total number of valence electrons present in CH₄ is

            Group            Valence Electrons

C         4A                   1 × 4 e^– = 4 e^–

H         1A                   4 × 1 e^– = 4 e^–

Total:  8 valence e^–

The Lewis structure of CH₄ is: