Chemical bond can be defined as :
            " the force of attraction between two atoms or ions that hold them
                               together in a unit is called Chemical Bond".
    Actually chemical bond is the main factor that makes molecules and compounds. By the interaction
    of outer electrons, great forces of attraction are developed between two atoms. This force of
    attraction is called chemical bond.

   There are three types of chemical bonds:
Ionic bond or Electrovalent bond
Covalent bond
Coordinate Covalent bond
   Chemical bond formed between two atoms due to transfer of electron(s) from one atom to the other
   atom is called "Ionic bond" or "electrovalent bond".
In ionic bond formation one atom looses electron(s) and the other picks it up. The atom that looses the    electron acquires positive charge and the other atom which gains the electron becomes a negatively    charged particle. Due to opposite charge an electrostatic force of attraction is setup between them.
      This force hold these atoms together in a unit. This force of attraction is referred to as
      "IONIC BOND".
Electronic arrangement of Na (Z=11) is K = 2 , L = 8, M = 1
Electronic configuration of Na is 1s2, 2s2, 2p6, 3s1
It has one valence electron.
The loss of the valence electron from Na-atom 495 KJ/mole energy is needed.
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Due to low ionization potential value, Na-atom loses its valence electron
Electronic arrangement after losing an electron is K = 2, L = 8 , M = 0
Electronic configuration after losing an electron is 1s2, 2s2, 2p6 .This shows that its octet is complete.
Na è Na+ + e-    : DH = 495 KJ/mole
Due to loss of one electron Na becomes Na+ -ion.
Electronic arrangement of Cl (Z=17) is K = 2, L = 8, M = 7
Electronic configuration of Cl is 1s2, 2s2, 2p6, 3s2 3p5.
Cl-atom needs one electron to complete its octet.
The gain of one electron by chlorine atom releases 348 KJ/mole energy.
Electron lost by Na atom picks up by Cl atom.
Electronic arrangement after acquiring an electron: K = 2, L = 8, M = 8
Electronic configuration of Cl is 1s2, 2s2, 2p6, 3s2, 3p6
. This shows that its octet is also complete.
Cl + e- è Cl-    : DH = -348 KJ/mole
Chlorine atom is now converted into Cl-_ion.
   We know that positive and negative ions attract each other, therefore an electrostatic force of
    attraction is set up between Na+ & Cl- ions. This force unites these ions in a unit. In this way
    ionic bond is formed between Na and Cl atoms which results in the formation of sodium chloride.
In the above two steps we clearly observe that there is a difference of energy i.e.
   This shows formation of one mole of NaCl increases the energy of system by 147 KJ /mole.
   There arises a question "How this difference of energy is compensated?"
   The answer to this question is that when oppositely charged ions i.e. Na+ ions & Cl- ions attract each    other and arrange themselves to form crystal lattice of NaCl which consists of very closely packed
   positive & negative ions. In NaCl crystal each Na+ ion surrounded by six Cl- ions. Similarly each Cl- ion
    is surrounded by six Na+ ions. The force of attraction between these ions in the crystal lattice
    decreases the energy of the system and making it stable.
Na+(g) + Cl-(g) Na+Cl- (s): DH = -788 KJ/mole
   From above description, we conclude that for the formation of ionic bond the sum of energies released in    step-2 and step-3 must be larger than the required in step-1.
   To form a stable ionic compound , there must be a decrease in net energy.
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