Ionization Potential

Ionization potential 

It is defined as the energy required to remove a electron from a neutral,isolated,gaseous atom to change it into a positively charged ion i.e. cation. This is also called First Ionization energy.

X + energy --> X+ e-

If  we go into depth of the definition it is termed that atom should be neutral , isolated and gaseous . Why is it written so? 
Neutral - atom must be neutral as in that state it has its total number of  electrons and protons so, we are able to know the exact energy required. 

Isolated and gaseous - It is so because in isolated state there is no external force acting which may effect the value of ionization potential . It should be in gaseous because it is the only state where the atoms can exist isolated.

Ionization energy depends on following:

Atomic size         : Larger the atomic size  greater is the distance between nucleus and the valance shell and lesser is the force attracting it . So it requires less force to remove the electrons i.e. ionization potential is less and vise versa.

Nuclear charge : Larger is the nuclear charge larger is the force of attraction between nucleus an valance shell and larger is the energy required to remove an electron from it i.e. ionization potential is more and vice versa.

Configuration  :  As we know that half filled shells and full filled shells are generally more stable therefore it requires more energy to remove an electron from it.

 Second ionization potential 

The Energy required to remove an electron from a single positively charged atom (X+) ion to change it into double positively charged ion(X2+) is called second ionization energy.

X+ + energy--> X2+e-

Comparison of first and second ionization energy

Second  ionization energy is greater than first ionization energy .

The reason behind it is that after removal of an electron from a neutral atom ,it turns positively charged and the net nuclear force  on an electron  increases this decreases size of atom .So, more energy is required to remove an electron from it .
 Therefore second ionization energy is greater than first ionization energy.

Trends in periodic table

Across the period : Ionization potential increases due to decrease in  atomic size (The numbers of shells in a period are same . So, increase in nuclear charge decreases atomic size across period) 

Down the group : Ionization potential decreases due to increase in atomic size(atomic size increase down the group due to increase in number of shells )

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