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Ionic Bond Energy

If atoms come together and bond, there should be a net decrease in energy, because the bonded state should be more stable and therefore at the lower energy level. Consider the formation of an ionic bond between a sodium atom and a chlorine atom. You can think of this as occurring in two steps:

  1. An electron is transferred between the two separate atoms to give ions.

  2. The ions than attract to form an ionic bond.  

The transfer of an electron and the formation of an ionic bond occur simultaneously, rather than in discrete steps, as the atoms approach one another. But the net quantity of energy involved in the same weather the steps occur one after the other or at the same time.


Bond Potential Energy:

Bond potential energy is also called as bond strength or bond energy. Where the energy required invariably in breaking a chemical bond. For illustration, in the breaking of one mole of hydrogen gas into atoms, 458 kJ of energy is required. The bond potential energy or bond strength in this case is said to be 458 kJ per mole, i.e., per Avogadro’s number of bonds.


Definition to Bond Potential Energy:


It is defined as the energy required in rupturing one mole of bonds of that type in a substance in gaseous state. The bond energies of some common bonds are given as follows;

For H-H bond the bond energy is 458.0 kJ per mole.

For F-F bond the bond energy is 154.8 kJ per mole.

For Cl-Cl bond the bond energy is 242.7 kJ per mole.

For H-Cl bond the bond energy is 430.0 kJ per mole.

For N≡N bond the bond energy is 945.6 kJ per mole.

For O="O" bond the bond energy is 494.6 kJ per mole. Etc.

The forces of the bond indicate the steadiness of the bond. Thus, N≡N is more stable than O="O" bond. Hence, nitrogen molecule is more stable than oxygen molecule. Consequently, nitrogen is much less reactive than oxygen molecule. The strength of F-F bond is lower than that of Cl-Cl bond. Hence, fluorine is more reactive than chlorine similarly I-Cl is more reactive than chlorin.


Ionic Bond Energy Description


The first step requires removal of the 3s electron from the sodium atom and the addition of this electron to the valence shell of the chlorine atom. Removing the electron from the sodium atom requires energy (the first ionization energy of the atom, which equals 469kJ/mol). Adding the electron to the chlorine atom releases energy (the electron affinity of the chlorine atom, which equals to –349kJ/mol). It requires more energy to remove an electron from the sodium atom that is gained when the electron is added to the chlorine atom. That is, the formation of ions from the atoms is not in itself energetically favorable. It requires additional energy equal to at least (496 - 349) kJ/mol, or 147kJ/mol, to form ions.


Strengths of the Bond Potential Energy by Ionic Bond:


An ionic bond is formed by complete transference of one or more electrons from the outer energy shell (valency shell) of one atom to the outer energy shell of the other atom. In this way, both the atoms acquire the stable electronic configurations of the nearest noble gases according to bond potential energy. Thus, the atom from which the electrons are transferred, i.e., that atom which loses the electrons, acquires a positive charge and becomes, what is called, a positive ion. The atoms which gains the electrons, acquires a negative charge and becomes what is called, a negative ion. The compound formed in this manner is called bond potential energy of ionic compounds or bond potential energy of electrovalent compounds.


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Estimation Of Energy


When positive or negative ions bond, however, more than enough energy is released to supply this additional requirement. What principally determines the energy released when ions bond is the attraction of positively charged ions. You can estimate this energy from Coulomb’s law if you make the simplifying assumption that the ions are spheres, just touching, with the distance between the nuclei equal to that in NaCl crystal. According to coulomb’s law, the energy E obtained in bringing two ions with electric charges Q1 and Q2 from infinite separation of the distance r apart is  

                                E = k Q1 Q2 / r

Where k is the physical constant equals to 8.99 x 109 J.m/C2 (where C is symbol of coulomb).


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