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CHEMICAL
BOND
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CHEMICAL
BOND
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| 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. |
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TYPES
OF CHEMICAL BOND
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There are three types of chemical bonds: Ionic
bond or Electrovalent bond Covalent
bond Coordinate
Covalent bond |
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IONIC
BOND OR ELECTROVALENT BOND
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| 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". |
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EXPLANATION
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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". |
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FORMATION
OF IONIC BOND BETWEEN Na & Cl
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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 |
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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. |
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Na è
Na+ + e- : DH
= 495 KJ/mole
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Due
to loss of one electron Na becomes Na+ -ion. |
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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. |
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Electron lost by Na atom picks up by Cl atom. |
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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. |
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Cl + e- è
Cl- : DH
= -348 KJ/mole
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| Chlorine
atom is now converted into Cl-_ion. |
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| 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. |
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In the above two steps we clearly observe that there is a difference of
energy i.e. |
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| 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. |
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Na+(g)
+ Cl-(g)
è Na+Cl- (s):
DH = -788 KJ/mole
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| 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. IMPORTANT: To form a stable ionic compound , there must be a decrease in net energy. |
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