Chemical Formulas and Structures
Author: Lectromec
Published: 12-18-2006
Chemical Formulas
and Structures
Elemental
abbreviations
C =
carbon
H = hydrogen
Cl
=chlorine
N = nitrogen
F =
fluorine
O = oxygen
The concept of
chemical formulas and structures is the basis for a beginning
understanding of all materials. Most everyone has heard of
chemical formulas, whether they realize it or not-often from the
media. For example, H2O is the “shorthand”
for a water molecule. The empirical chemical formula often does
not give us an indication of how the atoms are bonded (or arranged)
in a molecule. (A brief description about ATOMIC
STRUCTURE
is at the end of this page.) A very simple molecule is H2,
hydrogen, and it is easy to visualize, i.e., HH. The chemical
method for depicting the structure of this molecule is H-H. The
line shows the bonding between the two atoms, and it is used to
indicate two electrons in the bond. With H2
these are the only electrons for the molecule.
If we have three
atoms, there are more atomic and spatial arrangements possible.
It is conceivable to write CO2 (carbon dioxide) as COO or
OCO as the atomic arrangement. In fact, OCO is the correct
configuration. The correct bond designation for CO2
is O=C=O. Again, each line indicates two electrons from the
outer shells of these atoms; the two lines between the carbon atom
and an oxygen atom are called a double bond. Four of the
electrons are from the carbon atom and each oxygen atom supplies two
electrons. (Note: for neutral species there should be four
bonds to each carbon atom.) This representation also indicates
that CO2 is linear; this is the normal structure.
For another three atom molecule, H2O, the structure H-O-H
shows the correct number of bonds, but not the proper spatial
configuration. For water this is
I
with the HOH
angle being just under 110o. The structure I is a
visual description of the atomic bonding and spatial orientation for
water. Depending on the context, either the simple linear
representation or the spatial representation is used. More
complex molecules will follow a similar approach.
Simple
Organic Molecules
Carbon is the
basis of organic chemicals and the following summary is sufficient
for the materials encountered at Lectromec. (Note: there are
some completely inorganic molecules and ions that contain carbon;
but, the number is relatively small). Perhaps the simplest
organic carbon molecule is methane, CH4, the main
component in natural gas. The structural formula for CH4
is
II
where again each
line represents two electrons. The carbon atom is at the center
of a tetrahedron. Going to a higher compound of this series,
iso-butane (i-C4H10), the structural
formula is more complex as shown in structure III. With these
examples it is easy to see that with most organic compounds it is
difficult to represent completely a three dimensional structure in
two dimensions, since most of them have more than just a few atoms.
III
Often this is not
tried. The large number of hydrogen atoms makes complete
figures very complex when there are more than a few central carbon
atoms. There are two ways to overcome this problem. The
first is to just indicate the bonds between the carbon atoms, as
shown in structure IV for iso-butane.
IV
If there are
atoms other than carbon or hydrogen, e.g., oxygen, nitrogen, etc.,
they will be shown in the structure even if hydrogen and carbon are
not. The second approach is not to show the carbon-hydrogen
bonds; see structure V for iso-butane.
V
The organic
compounds used as examples above are hydrocarbons, i.e., they contain
only carbon and hydrogen. They also are in the class of
compounds called aliphatics.
Aromatic
Molecules
The other major
category for organic molecules is aromatic and is based on
benzene. The structural formula for benzene is
VI
but it is equally
correct to draw the benzene molecule in this fashion.
VII
These structures
(VI and VII) are the same, since a simple rotation can change one
representation to the other. (Actually, they are in resonance
with each other; a more realistic representation is shown in
structure VIII where we are going between structures VI and VII.)
VIII
Again when
depicting benzene based molecules, often the structural formula will
omit the carbon-hydrogen bonds, and in some instances the carbon and
hydrogen atoms are omitted as shown in IX.
IX
Polymers
The word Polymer
comes from the Greek "poly" meaning many, and "meros",
parts or units.
With
polymers the same conventions are used for structural formulas.
For example, Poly (tetrafluorethylene) (the duPont trade name for
this material is Teflon® and often abbreviated as
PTFE) is shown in structure X.
X
Since a polymer
is just the repeat of a simple molecule, the structural formula
follows the same conventions with the brackets enclosing the
representation of the basic molecular unit. This is not an
exact spatial representation, but it requires less space.
Another structural representation for this polymer is [CF2-CF2]n.
As with the hydrocarbon examples, the carbon-fluorine bonds are
omitted. “n” will range from under ten to thousands
depending on the polymer.
In some instances
the letter R is used. This indicates that many different groups
of atoms or an element can be bound to the other atom, and is used to
show a class of polymers or compounds. An example is shown in
structure XI.
XI
Polymer
Naming
A brief overview
about polymer naming is in the Chemical
Names
page.
Atomic
Structure
This is a very
brief description of atomic structure that is not completely correct,
but will serve to give a simple picture. The two main
stable components of an atom are the nucleus (composed of protons and
neutrons) and the orbiting electrons. The structure of an
element is a “solar system” (XII) with the electrons
orbiting the central nucleus. Each element has a set number of
electrons and protons. The electrons that have discussed above
are at the very edge of the “solar system” and are known
as valence electrons.
XII