The Ionization Potential or Ionization Energy of an element is the energy required to remove the most loosely bound electron from a free gaseous atom of the element in its ground state.

        Unit of Ionization Potential  is  KJ/Mole (S.I. Unit)

Strictly speaking,elimination of first electron from the valance shell is known as 1st 

Ionization energy. Elimination of 2nd electron from the monopositive ion is turned as 2nd 

Ionization Potential and so on.1st ionization potential is always less than the 2nd ionization

potential because elimination of electron becomes easier from neutral atom with respect to 

monopositive atom.Less energy is required for the elimination of electron from neutral

atom.So 1st ionization potential is always less than the 2nd ionization potential.    

I1<I2<I3...<In  

       The first ionization potential of Mg 737KJ/Mole but the second ionization potential of 

Mg is 1450KJ/Mole. The enhancement of ionization potential occurs due to presence of 

more pulling capacity by the nucleus on outer most electron . In case of neutral Mg 12 

proton is present with 12 electron.Electron:proton=1:1 but in monopositive ion 12 proton is 

present with respect to 11electron,electron proton ratio 11:12.So pulling capacity by the 

proton increases and more energy required to remove further electron from monopositive 

ion.

Factors Controlling the magnitude of Ionization Potential:-

1. Size of the atom-

With increasing size of atom(down a group) distance between valance outer most electron

 and nucleus increases, under this situation pulling capacity by the electron decreases and 

less required for the removal of valance electron -(IP decreases)

2. Charge on the nucleus-

When other  factor remains constant then with increasing charge in the nucleus pulling 

capacity by the nucleus increasing - more energy is required to remove these electron 

practically outer most electron-Ionization Potential increases.

    We know that charge of nucleus increases when we move from left to right across

a period.But in case of N2-O2,P-S,Arsenic-Selenium abnormal trained is observed 

due to high stability of half filled sub shell of group 15 element (N,P,Ac,S)