Given that electronegativity increases across a period and decreases down a group in the periodic table, in which of the following compounds will the molecu...

Hydrogen bonding is a type of intermolecular force that occurs between a hydrogen atom bonded to a highly electronegative atom (such as nitrogen, oxygen or fluorine) and another highly electronegative atom in a separate molecule. The strength of a hydrogen bond depends on the electronegativity of the atoms involved and the distance between them. In the given compounds, HF has the strongest hydrogen bond because it has the highest electronegativity difference between the hydrogen and fluorine atoms. Fluorine is the most electronegative element in the periodic table, so the bond between hydrogen and fluorine is highly polarized. This results in a strong partial positive charge on the hydrogen atom, which attracts the highly electronegative fluorine atom of another molecule, forming a strong hydrogen bond. In contrast, NH3 and CH4 have weaker hydrogen bonds because nitrogen and carbon are less electronegative than fluorine. HCl has the weakest hydrogen bond because hydrogen and chlorine have a smaller electronegativity difference than hydrogen and fluorine.