Question

Which type of intermolecular attractive force operates between (a) all molecules, (b) polar molecules, (c) the hydrogen atom of a polar bond and a nearby small electronegative atom?

Short answer

The intermolecular attractive forces are as follows: (a) London dispersion forces or van der Waals forces operate between all molecules, (b) dipole-dipole interactions occur between polar molecules, and (c) hydrogen bonds form between the hydrogen atom of a polar bond and a nearby small electronegative atom, such as oxygen, nitrogen, or fluorine.

Step-by-step solution

Case (a) - Attractive Force for All Molecules

The force that operates universally, i.e., between all types of molecules (whether they are polar or nonpolar) is known as London dispersion forces or van der Waals forces. These forces can be described as the weak attractive force resulting from the temporary uneven distribution of electrons around the molecules, creating an instantaneous dipole. This dipole interacts with the electron cloud of neighboring molecules, inducing more dipoles, leading to attractive forces.

Case (b) - Attractive Force for Polar Molecules

Polar molecules have a permanent dipole moment due to the presence of polar bonds. Polar bonds result from the difference in electronegativity between the atoms. The attractive force that operates between polar molecules is called dipole-dipole interaction. These forces are stronger than dispersion forces as they result from the attraction between the permanent dipoles of two polar molecules.

Case (c) - Attractive Force between Hydrogen Atom of a Polar Bond and Nearby Electronegative Atom

In this special case, an even stronger force of attraction operates between the hydrogen atom present in a polar bond and the nearby small electronegative atom, such as oxygen, nitrogen, or fluorine. This type of intermolecular attractive force is called a hydrogen bond. Hydrogen bonding occurs when the positive hydrogen atom in a highly polar bond is attracted to the electronegative atom, thus forming a sort of "bridge" between two molecules. Hydrogen bonds are stronger than dipole-dipole interactions and London dispersion forces.