What
is it that makes the objects you see in your surroundings
different from each other? What is it that discriminates
their colours, shapes, smells, and tastes? Why
is one substance soft, another hard, and yet another
fluid? From what you have read so far, you may
answer these questions saying, "The differences
between their atoms do this". Yet, this answer
is not sufficient, because if the atoms were the
cause for these differences, then there would
have to be billions of atoms bearing different
properties from each other. In practice, this
is not so. Many materials look different and bear
different properties although they contain the
same atoms. The reason for this is the different
chemical bonds the atoms form among them to become
molecules.
On
the way to matter, molecules are the second step
after atoms. Molecules are the smallest units
determining the chemical properties of matter.
These small bodies are made up of two or more
atoms and some, of thousands of groups of atoms.
Atoms are held together inside molecules by chemical
bonds determined by the electromagnetic force
of attraction, which means that these bonds are
formed on the basis of the electrical charges
of the atoms. The electrical charges of atoms,
in turn, are determined by the electrons on their
outermost shell. The arrangement of molecules
in different combinations give rise to the diversity
of matter we see around us. The importance of
the chemical bonds that lie at the heart of the
diversity of matter come forward at this very
point.
Chemical
Bonds
As explained
above, chemical bonds are formed through the motion
of electrons in the outermost electron shells
of the atoms. Each atom has a tendency to fill
up its outermost shell with the maximum number
of electrons it may shelter. The maximum number
of electrons the atoms can hold in their outermost
shells is 8. To do this, atoms either receive
electrons from other atoms to complete the electrons
in their outermost shells to eight, or if they
have lesser electrons in their outermost shells,
then they give these to another atom, making a
sub-shell that had previously been completed in
their outermost orbits. The tendency of the atoms
to exchange electrons constitutes the basic inciting
force of the chemical bonds they form between
each other.
This driving
force, that is, the objective of the atoms to
raise the number of electrons in their outermost
shells to maximum, causes an atom to form three
types of bonds with other atoms. These are the
ionic bond, covalent bond and metallic bond.
Commonly, special
bonds categorised under the general title of "weak
bonds" act between molecules. These bonds
are weaker than the bonds formed by atoms to constitute
molecules because molecules need more flexible
structures to form matter.
Let us now, in
brief, see the properties of these bonds and how
they are formed.
Ionic
Bonds
Atoms combined
by this bond swap electrons to complete the number
of electrons in their outermost shells to eight.
Atoms having up to four electrons in their outermost
shells give these electrons to the atom with which
they are going to combine, that is, with which
they will bond. Atoms having more than four electrons
in their outermost shells receive electrons from
the atoms with which they will bond. Molecules
formed by this type of bond have crystal (cubic)
structures. Familiar table salt (NaCl) molecules
are among substances formed by this bond. Why
do atoms have such a tendency? What would happen
if they did not have it?
Until today,
the bonds formed by atoms could be defined only
in very general terms. It has not yet been understood
why atoms adhere to this principle. Could it be
that atoms decide by themselves that the number
of electrons in their outermost shells should
be eight? Definitely not. This is such decisive
behaviour that it goes beyond the atom, because
it has no intellect, will, or consciousness. This
number is the key in the combination of atoms
as molecules that constitute the first step in
the creation of the matter, and eventually, the
universe. If atoms did not have such a tendency
based on this principle, molecules, and in turn,
matter would not exist. Yet, from the first moment
they were created, atoms have been serving in
the formation of molecules and matter in a perfect
manner thanks to this tendency.
Covalent
Bonds
Scientists who
studied the bonds between atoms faced an interesting
situation. While some atoms swap electrons for
bonding, some of them share the electrons in their
outermost shells. Further research revealed that
many molecules that are of critical importance
for life owe their existence to these 'covalent'
bonds.
Let us give a
simple example to explain covalent bonds better.
As we mentioned previously on the subject of electron
shells, atoms can carry a maximum of two electrons
in their innermost electron shells. The hydrogen
atom has a single electron and it has the tendency
to increase the number of its electrons to two
to become a stable atom. Therefore, the hydrogen
atom forms a covalent bond with a second hydrogen
atom. That is, the two hydrogen atoms share each
other's single electron as a second electron.
Thus, the H2 molecule is formed.
Metallic
Bonds
If a large number
of atoms come together by sharing each others'
electrons, this is called a "metallic bond".
Metals like iron, copper, zinc, aluminium, etc.,
that form the raw material of many tools and instruments
we see around us or use in daily life, have acquired
a substantial and tangible body as a result of
the metallic bonds formed by the atoms constituting
them.
Scientists are not able to answer the question
as to why electrons in the electron shells of
the atoms have such a tendency. Living organisms,
most interestingly, owe their existence to this
tendency.
The
Next Step:Compounds
Do you wonder
how many different compounds these bonds can form?
In laboratories,
new compounds are produced everyday. Currently,
it is possible to talk about almost two million
compounds. The simplest chemical compound can
be as small as the hydrogen molecule, while there
are also compounds made up of millions of atoms.1
How many different compounds can an element form
at most? The answer to this question is quite
interesting because, on the one hand, there are
certain elements that do not interact with any
others (inert gases), while, on the other hand,
there is the carbon atom that is able to form
1,700,000 compounds. As stated above, the total
number of compounds is about two million. 108
elements out of the total of 109 form 300,000
compounds. Carbon, however, forms 1,700,000 compounds
all by itself in a most
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