Fundamentals of Chemistry
Molecules having polar covalent bonds are called dipolar or polar molecules. Water is an
example of a polar molecule. When two atoms of the same element share one or more pairs
of electrons (such as H or N), each atom exerts the same attraction for the shared electron pair
or pairs. When the electron pairs are distributed or shared equally between the two like atoms,
the bond is called a nonpolar covalent bond. If all the bonds in a molecule are of this kind, the
molecule is called a nonpolar covalent molecule.
Another chemical bonding mechanism is the metallic bond. In the metallic bond, an atom
achieves a more stable configuration by sharing the electrons in its outer shell with many other
atoms. Metallic bonds prevail in elements in which the valence electrons are not tightly bound
with the nucleus, namely metals, thus the name metallic bonding. In this type of bond, each
atom in a metal crystal contributes all the electrons in its valence shell to all other atoms in the
Another way of looking at this mechanism is to imagine that the valence electrons are not
closely associated with individual atoms, but instead move around amongst the atoms within
the crystal. Therefore, the individual atoms can "slip" over one another yet remain firmly held
together by the electrostatic forces exerted by the electrons. This is why most metals can be
hammered into thin sheets (malleable) or drawn into thin wires (ductile). When an electrical
potential difference is applied, the electrons move freely between atoms, and a current flows.
Van der Waals Forces
In addition to chemical bonding between atoms, there is another type of attractive force that
exists between atoms, ions, or molecules known as van der Waals forces.
These forces occur between the molecules of nonpolar covalent substances such as H , Cl , and
He. These forces are generally believed to be caused by a temporary dipole, or unequal charge
distribution, as electrons constantly move about in an atom, ion, or molecule. At a given
instant, more electrons may be in one region than in another region, as illustrated in Figure 10.
The temporary dipole induces a similar temporary dipole on a nearby atom, ion, or molecule.
Every instant, billions of these temporary dipoles form, break apart, and reform to act as a weak
electrostatic force of attraction known as van der Waals forces.
It is important to note that van der Waals forces exist between all kinds of molecules. Some
molecules may have these forces, as well as dipole or other intermolecular forces. Van der
Waals forces, however, are the only intermolecular bonds between nonpolar covalent molecules
such as H , Cl , and CH . The number of electrons in a substance increases as the gram
molecular mass (mass in grams of one mole of compound) increases. Therefore, the strength
of the van der Waals forces between substances increases with increasing gram molecular mass.