Van der Waals interactions can occur, though

Difference Between Van der Waals and Hydrogen Bonds

Van der Waals vs hydrogen bonds

Van der Waals forces and hydrogen bonds are intermolecular attractions between molecules. Some intermolecular forces are stronger and some are weak. These bonds determine the behavior of molecules.

Van der Waals forces

Charge separation should take place for intermolecular attraction. There are some symmetrical molecules like H2, Cl2 where there are no charge separations. However, electrons are constantly moving in these molecules. Hence, an instant charge separation can take place within the molecule as the electron moves to one end of the molecule. The end of the electron will temporarily have a negative charge while the other end will have a positive charge. These temporary dipoles can induce a dipole in the neighboring molecule and then an interaction between opposite poles can occur. This type of interaction is called an induced dipole-induced dipole interaction. Furthermore, there can be interactions between a permanent dipole and an induced dipole or between two permanent dipoles. All of these intermolecular interactions are known as van der Waals forces.

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Hydrogen bonds

When hydrogen is bonded to an electronegative atom such as fluorine, oxygen, or nitrogen, a polar bond is created. Because of the electronegativity, the electrons in the bond are more attracted to the electronegative atom than to the hydrogen atom. Therefore, the hydrogen atom is partially positively charged while the more electronegative atom receives partially a negative charge. When two molecules with this charge separation are close to each other, there will be an attraction between hydrogen and the negatively charged atom. This attraction is known as hydrogen bonds. Hydrogen bonds are relatively stronger than other dipole interactions and determine molecular behavior. For example, water molecules have an intermolecular hydrogen bond. A water molecule can form four hydrogen bonds with another water molecule. Since oxygen has two lonely pairs, it can form two hydrogen bonds with positively charged hydrogen. Then the two water molecules can be known as dimers. Each water molecule can be linked to four other molecules due to its hydrogen bonding ability. This results in a higher boiling point for water even though a water molecule has a low molecular weight. Therefore, the energy required to break the hydrogen bonds when they go into the gas phase is high. In addition, hydrogen bonds determine the crystal structure of ice. The unique arrangement of the ice lattice helps to float on the water and thus protects aquatic life in winter. Furthermore, hydrogen bonds play an important role in biological systems. The three-dimensional structure of proteins and DNA is based exclusively on hydrogen bonds. Hydrogen bonds can be destroyed by heating and mechanical forces.

What is the difference between Van der Waals forces and hydrogen bonds?

• Hydrogen bonds between hydrogen bound to an electronegative atom and an electronegative atom of another molecule. This electronegative atom could be fluorine, oxygen, or nitrogen.

• Van der Waals forces can occur between two permanent dipoles, dipole-induced dipoles or two induced dipoles.

• For Van der Waals forces to take place, the molecule should not necessarily have a dipole, but rather hydrogen bonds between two permanent dipoles.

• Hydrogen bonds are much stronger than van der Waals forces.