CHEMICAL BONDING
Atoms differ from each other in their atomic number and configuration, and
also their reactivity. Some atoms are more reactive and some are less. The
elements in group 0 do not easily react and form compounds. Hence, they are
called inert gases. Earlier, they used to be called noble gases. Inert gas
configuration is stable.
The configurations of fluorine, neon and sodium are F(2,7) , Ne(2,8),
Na(2,8,1). Here Ne is the most unreactive and inert. Fluorine would become
stable and inert if it could get one more electron and complete its valence or
outermost shell. Sodium would become stable if it could lose an electron and
attain the configuration of neon (2,8). Every atom has the tendency to acquire
the configuration of the nearest noble gas and become stable.
What happens when the fluorine atom does so? Having picked up an electron
the atom has become the negatively charged fluorine ion. This negative charge
will electrically attract a positive ion, and repel a negative ion. And what
happens when the sodium atom attains closed shell stability by losing an
electron? It becomes the singly charged positive ion Na +. The sodium ion will
now electrically attract a negative charge like Cl- or F-.
Ions, Atoms and Valency
The properties of Na+ ion and the sodium atom are different from each other.
The positive charge dictates all the properties of Na+ , since otherwise it is
stable and inert with its eight electrons in the valence shell. A sodium atom,
would strive to lose its lone valence shell electron. Single valence electron
makes the Na atom and the group I alkali metal atoms chemically reactive.
The elements of groupI are called alkali metals, they have the tendency to lose
the electron present in the outermost shell and to acquire the electron
configuration of the noble gas.
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Na
 Na+ + e-
Similarly the ionization of chlorine atoms is as
Cl + e-  ClThe properties of Cl- ions and the Cl atom are different from each other.
Ionic Bond
When Na atom and Cl atom are brought closed to each other, they would like to
attain the stable electronic configuration of nearest noble gas. They can achieve
the octet electronic configuration by transferring and electron from Na atom to
the Cl atom.
Na + Cl  Na+ +ClThe ions are oppositely charged, so they will attract each other and hold on to
each other. It will result in the making an ion pair Na +Cl- the common salt (
Sodium Chloride). The two ions are bonded together by their electrical charges.
Such a bond is called ionic bond.
When this ionic bond was formed, one electron from Na has been lost and Cl
gained one electron. The valency of both Na and Cl are one each, or they are
monovalent.
The valency of an atom is the number of electrons it loses or gains in making a
bond with another atom.
For example
Ca
(2,8,8,2)

Ca ++ +2e(2,8,8)
Al
(2,8,3)

Al+++ + 3e(2,8)
O + 2e(2,6)

O
(2,8)
Covalent Bonding
A different types of bonding takes place when the atoms do not lose or gain
electrons easily. For example, In the Chlorine Cl2 molecule, electron transfer
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can not take place because the atoms are not complementary but behave
identically. Still, each atom would like to attain the stable octet configuration
by gaining one electron. In this case, sharing electron method is used.
While sharing electrons only atoms that are in the outermost shell of the
atom can take part in the bond formation. Therefore, only the electrons in the
outermost shell are shown in the figures.
When the Chlorine atoms close enough to each other, the 7th electron can be
shared between them. When this is done, each atom achieves the nearest noble
gas configuration through sharing. This type of bonding between the two atom
takes place through sharing a pair of electrons between them equally. Such a
bond is called the covalent bond.
In a covalent bond, both atoms attract the same pair of electrons. This
shared pair is called the bonding pair of electrons, they act as the cement or the
glue that holds the atoms together in the bond. One pair of bonding electrons
makes one covalent bond. Thus, we have a single bond between the two atoms
in the chlorine molecule.
The valency of an atom is then defined as the number of electrons it loses, or
gains, or shares in bonding with other atoms in a molecule.
More examples:
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