Following is a series of topics that compare bonding with
intermolecular forces.
One of the biggest sources of difficulty for a chemistry student is the distinction between
chemical bonds and intermolecular forces. While both are used to hold chemical systems
together, they each introduce their own specific qualities into structures. This presentation
is designed to draw basic comparisons between the two very different mechanisms.
Bonds and intermolecular forces have one very fundamental thing in common. Both mechanisms are electrostatic forces of attraction (Coulombic forces) between areas of charge. The primary difference between bonds and intermolecular forces is the locations of the areas of charge and the magnitudes of the areas of charge. As a result of these differences, there are significant differences in the strengths of the resulting attractions.
Bonds
Chemists tend to consider three fundamental types of bonding.
Metallic bonding is a unique situation that is not as well understood as the more common forms of bonding, ionic and covalent. In this presentation, attention is focused primarily on ionic and covalent bonding.
Because ionic and covalent bonding uses electrostatic attractions between areas of full charge, the resulting force of attraction is strong. Ionic bonds are held together by attractions between cations and anions. Covalent bonds are held together by attractions between positive nuclei and the negative electron clouds that reside between them.
The results of these bonding process are the strongest commonly used mechanisms for attaching atoms to one another. When a solid state chemical system is held together by bonds, ONLY, then the system will have qualities that are associated with this strong method of attachment. In other words, anything that only uses ionic or covalent bonding will have high melting points, high boiling points and be relatively hard and rigid.
There are two common examples of such systems. Quartz, or SiO2, is composed exclusively of covalent bonds. Table salt, or NaCl, is composed exclusively of ionic bonds.
In both cases, the substances tend to be quite hard. In addition, they both exist as solids at room temperature because of their high melting points and boiling points. Some of the hardest substances known exist using bonding, exclusively, in their structures.
Intermolecular Forces
Intermolecular forces are a secondary method of holding a solid state structure together. As the name implies, these are forces that exist BETWEEN molecules. Bonds exist WITHIN molecules. For reasons that will not be discussed here, intermolecular forces are only associated with systems that use covalent bonding within the molecules. Intermolecular forces are not encountered in systems that employ ionic bonding. Some elements, such as the Noble Gases, exist with intermolecular forces and no bonding at all.
Intermolecular forces exist in three different levels of strength. The differing strengths are a function of the magnitude of the areas of charge that hold them together. These three different forces are
Two of the intermolecular forces are associated with POLAR structures.
One of the intermolecular forces is associated with NONPOLAR structures.
Imagine a system composed of polar molecules. By definition, the polar molecules will have a partially positive side and a partially negative side, or a dipole. The partial positive on one molecule will be attracted to the partial negative on a second molecule. This attraction is an intermolecular force. Because the molecules are polar, the force is either a dipole-dipole attraction or a Hydrogen bond.
Because these attractions are between areas of partial charge, they will produce weak forces
of attraction. A system that has this mechanism holding the structure together will break
up relatively easily. It will always break at the weak links--the dipole-dipole forces or
Hydrogen bonds. The covalent bonds will remain intact. The boiling point, melting point
and hardness will be less than if the system used bonding exclusively.
The difference between dipole-dipole forces and Hydrogen bonding is subtle. When a Hydrogen is directly bonded to a Nitrogen, Oxygen or Fluorine, then the system will be capable of Hydrogen bonding. In these systems, the difference between the electronegativity values of the bonded atoms will produce fairly large partial charges. As a result, the resulting intermolecular forces will be strong. They will still not be as strong as a true bond, however. An example of a molecule that undergoes Hydrogen bonding is water. The Hydrogen-Oxygen combinations will produce some very large partial charges. Consequently, systems such as water will have stronger structures than systems that use regular dipole-dipole forces. In molecules that undergo Hydrogen bonding, the H-O, H-N and H-F combinations are known as Hydrogen bonding sites. Many biological systems--DNA, RNA, enzymes--have lots of Hydrogen bonding sites. The behaviors of these systems are greatly affected by the presence of these areas of strong partial charge.
Nonpolar systems lack partial charges. Yet, they are also held together by the electrostatic forces such as are seen in dipole-dipole systems. In these cases, the partial charges are produced temporarily. The partial charges are a result of the electrons in a nonpolar system existing briefly at one end of the structure, as a result of random motion. This temporary condition creates a temporary dipole. As long as the electrons exist at the end of a structure the system will have a partial negative charge in that area. When the electrons leave that area, then the system loses the partial charge and the dipole is gone.
The temporary dipole created briefly in one system will stimulate electrons in neighboring
structures to move to the ends of those structures. As a result, those structures will also
develop dipoles. In these cases, the dipoles are referred to as induced dipoles. They are
named as such because the dipoles are created as a response to a dipole somewhere else in
the environment. The combination of temporary and induced dipoles creates an attractive
force between the various structures in the system. This attractive force is referred to as a
London Dispersion Force.
Once the temporary dipole is lost when the electrons return to the center of the structure, the stimulus for the induced dipoles in the environment is lost. All electrons return to their preferred locations in the structures. The dipoles are all gone and any electrostatic attractions between the structures disappears. The London Dispersion Forces disappear. Consequently, systems that use this type of electrostatic attraction have a very poor mechanism holding them together. These systems are characterized by having qualities associated with a very weak attractive force. Such systems will have very low boiling points, melting points and be very soft. These types of systems are frequently liquids or gases at room temperature.
Some elements, such as Noble Gases, will use a modified version of the London Dispersion Force. They exist as single atoms and do not have any bonding at all. Instead, the normal even distribution of electrons on the atoms becomes temporarily unbalanced. When these occurs, the atoms will develop brief and weak partial charges. These partial charges will attract other comparable partial charges on the other atoms in the environment. The resulting electrostatic force is one of the weakest known in chemical systems. The resulting melting points and boiling points of these substances are some of the lowest known.
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Last updated March 4, 1997