Intermolecular Attractions or van der Waals Forces

�Before You Begin:

To master this material you will need to know the types of bonding, molecular geometry, and Kinetic Molecular Theory.

Atoms and molecules are never truly ideal because they all interact with other gas particles; weak attractions between separate gas particles are known as intermolecular attractions or van der Waals forces after the chemist who proposed a correction to the ideal gas law to calculate pressure of a real gas. Van der Waals forces aren’t just important when dealing with gases; these forces are important in understanding the bulk properties of the other states of matter, too. Although these attractions may be weak, they become significant at low temperatures and high pressures. There are three types of van der Waals forces that may occur in pure substances: induced dipole attractions, dipole-dipole attractions, and hydrogen bonds. Another type of intermolecular attraction, ion-dipole attraction, is an important factor in mixtures of ionic and molecular substances.

Induced Dipole Forces

The weakest of the intermolecular attractions are induced dipole forces, also called London dispersion forces after Fritz London, who proposed these weak forces in 1930.

Any atom or molecule has a surrounding electron cloud. The electron cloud is roughly spherical, but, due to the Heisenberg Uncertainty Principle and the quantum nature of the atom, it is statistically likely that the electrons will spend part of the time unevenly distributed throughout the volume. For a very short time, the atom will have a partial negative charge, δ-, and a partial positive charge, δ+. This “lopsided” state is called an instantaneous dipole.

The electron cloud of one gas particle is repelled by the electron cloud of another neighboring particle or attracted to a partial positive charge. These forces are very weak over very short distances. If the atoms are close together, the attractive and repulsive forces of an instantaneous dipole can distort the electron cloud of a neighboring atom. This distortion is known as an induced dipole. The opposite partial charges are attracted to one another. This short term, very weak attraction is enough to cause slight variations in the actual pressure and volume of a gas compared to the ideal gas law predictions.

It is important to note that all gases are “real” gases; all atoms and molecules have induced dipole attractions. For gases at high temperature and low pressure, these attractions do not have a significant impact on the properties of the gas; however, the attractions are still happening.

Some particles are able to sustain a stronger partial charge for longer periods of time. The larger the atom or molecule the better its electron cloud can distort. This is because the electrons are farther from the positive nucleus and so are held less strongly (this is like the concept of ionization energy from periodic law). The London forces are also stronger if the particle is a molecule and that molecule is already somewhat lopsided.

4Concept Check: Which of the noble gases is the most ideal?

Answer: Helium obeys the ideal gas law better than the other noble gases. It has the lowest molar mass and the shortest atomic radius, so it has the weakest induced dipole attractions.

Dipole-dipole Forces

All atoms and molecules exhibit induced dipole forces. Some molecules also have a permanent dipole. Polar molecules have an uneven distribution of electron density because they have non-bonding pairs of electrons and/or polar covalent bonds which are unevenly distributed in space. The molecule HCl has both a polar covalent bond and three non-bonding electron pairs. Partial opposite charges attract one another, as they do in induced dipole attractions. In the case of polar molecules, the partial charges are permanent and the dipoles stronger, which make the attractions stronger than induced dipole forces. Note that all molecules have induced dipole forces; polar molecules have induced dipole forces and dipole-dipole forces.

diagram of the difference between a polar covalent bond (shared pair of electrons inside a molecule) and a dipole-dipole force (attraction of opposite partial charges on two different molecules).

Dipole-dipole attractions occur over longer distances than induced dipole attractions, so they take place among larger groups of molecules. The opposite partial charges attract one another, while the like partial charges repel one another. Molecules will tend to move so as to maximize attractions and minimize repulsions.

diagram of several polar covalent molecules showing attractions between opposite partial charges and repulsions between like partial charges

Dipole-dipole attractions are stronger if the molecule is highly polar. To determine if a molecule is polar, draw its dot structure, assign electronegativity values to the atoms, and determine the molecular geometry. The molecule is polar if it has non-bonding electron pairs and/or polar bonds unevenly distributed in space.

4Concept Check: Rank these molecules as to increasing strength of dipole-dipole attractions: water, hydrogen sulfide, and hydrogen selenide. What is the difference between this ranking and the rank of increasing strength of induced dipole force for the same molecules?

Answer: The rank of increasing dipole-dipole force is hydrogen selenide < hydrogen sulfide < water. All three molecules have the same basic dot structure and geometry:

dot structure and molecular model of hydrogen sulfide

These bonds are polar and the three dimensional shape is tetrahedral with two non-bonding pairs. The molecules are all polar. Oxygen has the highest electronegativity and selenium has the lowest, so water has the highest dipole-dipole force and hydrogen selenide has the lowest. The rank for increasing induced dipole force is water < hydrogen sulfide < hydrogen selenide, because induced dipole force increases as the size of the molecule increases. The induced dipole force is very weak compared to the dipole-dipole forces, however.

Hydrogen Bonding

hydrogen bonds in DNA

Hydrogen bonds are a special type of dipole-dipole attraction. The bonds between the hydrogen atom and nitrogen, oxygen or fluorine are exceptionally short and polar. Dipole-dipole attractions between these specific bonds are particularly strong. Because water is able to hydrogen bond, it has very unusual properties which we will explore in the next section.

Hydrogen bonds are important in biological molecules such as proteins and DNA. Attractions between a hydrogen (bonded to an oxygen or nitrogen) and oxygen or nitrogen on a different part of the molecule twist the three dimensional shape and hold it in place. Note that these attractions are NOT bonds; they are particularly strong van der Waals forces.

To determine if a molecule can hydrogen bond, draw its dot structure and look for H-F, H-O, and H-N covalent bonds. These specific bonds are so polar that they form particularly strong dipole-dipole forces.

4Concept Check: Can this molecule hydrogen bond?

adenine

Answer: Sure. It contains H-N bonds. This is adenine, one of the base pairs of DNA.

Ion-dipole Forces

The partial charges on a polar molecule are attracted to ions with the opposite charge. In an aqueous solution of an ionic compound, such as sodium chloride, the sodium ion is attracted to the partial negative charge on neighboring water molecules and the chloride ion is attracted to the partial positive charge on neighboring water molecules. The solvent forms a shell around the ions giving them enough stability to counter the lattice energy.

diagram of ion-dipole attractions--positive ions are attracted to the partial negative charge on polar molecules and negative ions are attracted to the partial positive charge.

This site was last updated 05/13/05