Chemical bonding

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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.
Na  Na+ + e-
Similarly the ionization of chlorine atoms is as
Cl + e-  Cl-
The 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+ +Cl-

The 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  Ca ++ +2e-

(2,8,8,2) (2,8,8)

Al  Al+++ + 3e-

(2,8,3) (2,8)

O + 2e-  O

(2,6) (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 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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