Q. Explain Ionization Energy?
Ans.
When an electron is removed from an atom, the atom has been ionized. Therefore, the energy required to remove an electron from an atom is called the ionization energy.
When forming a positive ion, an atom undergoes the following reaction:
atom + energy -> positive ion + electron
For example, consider the reaction by which sodium forms a positive ion:
Na + 494 kJ/mole -> Na+ + e-
This means that 494 kilojoules of energy are required to remove one electron from each sodium atom in a mole of atoms.
The first ionization energy is the amount of energy required to remove the first electron from an atom. The second ionization energy is the amount needed to remove the second electron from that atom, and so on.
Moving from left to right within a period in the periodic table, the ionization energy increases. The noble gases have high ionization energy values because they have a very stable, complete eight-electron structure in the outer energy level (recall the octet rule), and therefore do not "willingly" give up any of their electrons.
In general, if loosing an electron will allow an element to achieve a noble gas configuration, then the ionization energy for such a favorable reaction will be very low. However, if loosing an electron will also cause an element to lose its noble has electron configuration, then such a reaction is undesirable, as reflected in a high value for the ionization energy.
For example, elements in Group IA have only one outermost valence electron. As loosing this electron will allow these elements to have a noble gas electron configuration, the values for the first ionization energy for these elements are very low.
However, it would be very difficult to remove a second electron from an element in Group IA, since this would result a loss of the noble gas configuration. Consequently, the second ionization energy values for these elements are very high.