Custom Search
|
|
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 crystal. 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 H2, Cl2, 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 H2, Cl2, and CH4,.
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.
Figure 10 Van der Waals Forces Van der Waals forces are
small compared to the forces of chemical bonding and are significant only when
the molecules are very close together. Organic chemistry is defined as the chemistry of carbon compounds.
The compounds of carbon are in forms of living things and in synthetic fabrics
and plastics. Organic chemistry is so broad a topic that it is usually
subdivided into smaller areas. How the carbon is combined and what it is
combined with determines the subdivision for a particular compound. These
subdivisions are referred to as families or classes. The carbon atoms can
combine to form straight chains, rings, or branched chains. The bonds between
carbon atoms can be single, double, triple or a combination of these. Other
atoms (H, O, N, S, P) and the halogens can be attached to the carbon atoms to
yield derivatives. The large family of
organic compounds containing only carbon and hydrogen are called hydrocarbons.
These can be further divided into two broad classes, aliphatic (fatty) and
aromatic (fragrant). Aliphatic hydrocarbons are
divided into two categories, saturated and unsaturated and into subdivisions
alkanes, alkenes, and alkynes. These subdivisions reflect the type of bond
between the carbon atoms.
|
||