Catenation - Chemical Bonding
Catenation - Chemical Bonding
of chemical bonds leads to the formation of inorganic polymers.
However, inorganic polymers are mostly solids in the form of crystals.
Typical inorganic polymers are diamond, graphite, silicates, and other
solids in which all atoms are connected by covalent bonds.
Atoms and their nuclei
In order to understand the covalent bond, we must take a look at the
structure of atoms.
During the 20th century, the investigation of the material world
turned to the very heart of material world - the structure of atoms.
The discovery of electrons in 1897 by J.J. Thomson showed that there
were more fundamental particles present in the atoms.
Fourteen years later, Rutherford discovered that most of the
mass of an atom resides in a tiny nucleus whose radius is 100,000 times
smaller than that of an atom. In the mean time, light beams were discovered
to be made of photons which are equivalent to particles of
wave motion. These discoveries created new concepts.
When these concepts and discoveries are integrated, new ideas
emerge. The result is quantum theory. This theory
gives good interpretations of the phenomena of the
atomic and subatomic world. In this microscopic world,
distances are measured in nanometers (10-9 or 1e-9 meter) and
fantometers (1e-15 meter, also called fermi, in honour of Fermi who
built the first nuclear reactor).
The electrons in an atom are confined by the electromagnetic force of the
atomic nuclei. At this level, we need a
quantum mechanical approach to
understand the energy states of the electrons in the atom. However,
we do not have the time to discuss this in details.
Quantum number and atomic orbitals
Quantum mechanics on atomic structures is a mathematical approach to
describe the behavior of electrons in atoms. The electrons are represented
by wavefunctions, and each of them are characterized by a set of numbers.
Each set of numbers represent a state, which is often called an orbital.
Quantum Numbers and
Atomic Orbitals are pages that give a
bit more details on this subject, but a summary of the atomic orbitals is
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f 5g
6s 6p 6d 6f 6g 7h
7s 7p 7d 7f 7g 7h 8i
A film has shown how these are related to the Period Tables of Elements,
and it also shows the shape and concepts of atomic orbitals. These concepts
are vital for the understanding of bonding, such as the bonds formed between
carbon atoms of diamond, silicon, graphite etc.
In the assignment, you have been asked to apply these concept to describe the
bonding for carbon. The same argument also applies to the bonding of
The electronic configuration of an element or atom shows the energy states
of electrons in it. Pauli exclusion principle and Hund's rules are some
of the theories involved in assigning
electronic configurations. For the
discussion of bondings in some light elements, please note the following:
Li: 1s2*2s1 after * are valence electrons
Electrons in an atom may have properties of several orbitals, and they share
each other's characters. In other words, atomic orbitals may be combined
hybrid orbitals. These hybrid orbitals are
particularly useful in the discussion of
For carbon, the hybrid orbitals are made up of
2s, 2po, 2p+ and 2p-
orbitals. Since 1 s and 3 p orbitals are used, the 4 orbitals sharing s and p
characters are called sp3 hybrid orbitals.
The shapes and directions of these orbitals should have been demonstrated
in lectures, and diagrams are needed here.
The bonding of diamond is beautifully described using the sp3
Bonding in benzene and graphite
The bonding of benzene should have been fully discussed in the organic
chemistry course you have taken. Simply, the orbitals used to form the
sigma bond are sp2 hybrid orbitals resulting from combining
2s, 2p+ and 2p-
orbitals. Furthermore, the overlap of the 2po orbitals leads
to the formation of the pi bond.
Resonance, benzene and graphite
Again, the structure of benzene serves an excellent example for the concept
called resonance. If one insisted on the fact that the 3 double bonds
and 3 single bonds in benzene alternate along the ring, one can start with a
single or a double bond. Nether structure represent the structure of benzene,
beccause all 6 bond lengths are about the same. Thus a combination of the
two structures is used to represent the structure of benzene, and such
an approach is called resonance. In other words, electrons in the
double bonds delocalize over the entire ring.
The bonding description for benzene can be applied to that of a sheet of
graphite. The electrons are delocalized on two planes in graphite. Thus,
it no surprize that graphite is a good conductor along the sheet.
The graphite structure is the result of expanding the pi electrons into
planes. Since all rings in graphite consist of 6 carbon atoms, the sheets
are flat. If the hybrid orbitals are somewhat flexible, it is easy to
understand that the 5-member rings are also possibilities. However, formation
of 5-member rings reults in buckling of a flat structure, and we usually
do not think this will happen.
The discoverer of the buckminsterfullerenes spent a long time figuring
out the structure of a cluster of carbon consisting of 60 carbon
atoms, which is represented by C60. However, once they
have deducted the structure, its shape is very common. The carbon atoms
are at the junction of lines on a soccer ball (other parts of the world call
it foot ball). A geometric description is a truncated icosahedron. The
buckyballs have become the talk of the news media since the award of
Nobel Prize to its discoverers.
The fullerenes actually are common, and their discovery adds a nice touch
to theories of bonding and electronic structures. The electrons on the
surface of the ball perhaps think they are on a very large
atom. The discoverers are still very active in the study of
Boron nitride, BN
A compound with equal number of boron and nitrogen atoms, BN, has on average
4 valence electrons per atom, same as that of diamond or graphite in carbon.
Thus, we anticipate BN to form solids with simlar bonding and structures
as diamond and graphite.
In fact, the bonding of boron and boron compounds is also very interesting.
The following items are mentioned here for future development.
Try any first year chemistry text book for additional information on sulfur,
phosphorus, and other compounds.
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