Why do some substances form hydrogen bonds while others don't?

Some substances form hydrogen bonds due to the presence of highly electronegative atoms and a hydrogen atom bonded to them.

Hydrogen bonding is a special type of dipole-dipole interaction that occurs between an electronegative atom and a hydrogen atom bonded to another electronegative atom. It is a particularly strong type of dipole-dipole interaction because of the small size and high electronegativity of the atoms involved, which typically include nitrogen, oxygen, and fluorine.

The ability of a substance to form hydrogen bonds depends on its molecular structure. For a hydrogen bond to form, there must be a hydrogen atom bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine. This is because these atoms pull the bonding electrons towards themselves, creating a partial negative charge on the electronegative atom and a partial positive charge on the hydrogen atom. This charge separation creates a dipole, which can interact with the negative end of another dipole to form a hydrogen bond.

However, not all substances have this molecular structure. For example, hydrocarbons, which are compounds made up of hydrogen and carbon atoms, cannot form hydrogen bonds because carbon is not highly electronegative. Similarly, noble gases and many metals do not form hydrogen bonds because they do not typically form covalent bonds with hydrogen.

In addition to the presence of a hydrogen atom bonded to a highly electronegative atom, the ability of a substance to form hydrogen bonds also depends on the spatial arrangement of its atoms. For a hydrogen bond to form, the hydrogen atom and the two electronegative atoms must be nearly linear. This is because hydrogen bonds are directional, meaning they have a preferred orientation in space. If the atoms are not arranged in the right way, a hydrogen bond cannot form.

In summary, whether a substance can form hydrogen bonds depends on its molecular structure, including the presence of a hydrogen atom bonded to a highly electronegative atom and the spatial arrangement of its atoms.