Ionic Bond

Ionic bond is a type of chemical bond that exist between metal and nonmetal forming  an ionic compound which involves transfer of electrons.  Metals having low in ionization energy and electronegativity, tends to lose their valence electrons to nonmetals.  Nonmetals on the other hand accept valence electrons from metal due to their higher ionization energy and electronegativity.

Below is an example of ionic compound LiF:

Complete transfer of electrons occur between metal and nonmetal when the electronegativity difference is 2.0 above.  Just like the example above Li has electronegativiy of 1.0 and F has electronegativity of 4.0.  Their difference is 3.0 therefore there is a complete transfer of valence electrons from  Li to F.  

Let us look at this:

Li has 3 electrons and F has 9 electrons  with the electron configuration of:

Li =  1s² 2s¹      and     F =  1s² 2s² 2p⁵

Li has 1 valence electron while F has 7 valence electrons.  To become stable the atoms should complete 8 valance electrons, but there are some exceptions that we have to consider.

The valence electrons of Li upon giving its 1 valence electron to F,  is 2 from 1s² configuration and the F will have the configuration of 1s² 2s² 2p⁶      , which has the valence electrons of 8.

As you can see the valence electrons now of Li upon giving its valence electron will be 2, same with the valence electrons of H which is exempted to the Octet Rule, therefore stable, and the F will have now 8 valence electrons achieving the octet rule.  Therefore we can say that only one atom of Li is needed to make the F stable.  

The formula now of Lithium flouride is LiF, having 1:1 ratio.

Let us have more examples:

The reaction between Calcium with Chlorine is also an example of ionic bonding, with electronegativity difference of 2.0 (3.0 -1.0) looking at their valence electrons and lewis symbols;

Calcium has 2 valence electrons being in group IIA and chlorine has 7 valence electrons being in group VIIA.  The lewis symbol will be:

Since Ca has 2 valence electrons and Cl only need 1 valence electron, Ca will not be stable yet.  And so we need more another Cl atom to make Ca stable as shown below:

Therefore to become stable, calcium will react with 2 atoms of chlorine, and will have a formula of CaCl₂.

How about the reaction of Sr and N?  Could there be a transfer of electrons?  What is the electronegativity difference?  Electronegativity difference of 2.0 ( 3.0 - 1.0) therefore transfer of electrons will occur.   Sr is in group IIA having 2 valence electrons and N is in group VA having 5 valence electrons.

If we will only allow 1 Sr atom to react with 1 atom N, then both atoms will not be stable yet, so how many Sr is needed to react with N and vice versa.  Lets find out:

Therefore to become stable Sr needs 3 atoms to 2 atoms of N.  Chemical Formula is Sr₃N₂.

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