The energy required to remove an electron from an atom or ion, or its propensity to give up an electron, is measured by the ionisation energy or ionisation potential in chemistry and physics. The ground state of a chemical species is often where electron loss occurs.

Ionization energy, also called ionisation energy, is the measure of the attractive forces (or strength) that keep an electron in its current location.

Ionization energy:

“The energy required to remove the most loosely bound electron from an isolated neutral atom (gaseous atom) or gaseous ion to make cation is called Ionization Potential or Ionization Energy.”

Ionization energy is an endothermic process in which energy is absorbed so the sign for this energy is +ve. This energy is required to overcome the force of attraction of the nucleus on the electron. After the removal of the first electron hold of the nucleus on the remaining electrons increases so Second I.P is greater than First I.P because more energy is required to remove the second electron and so on.

  e.g (i) Na → Na+ + e- (I.E)1 = +496 kJ/mol

         (2,8,1) (2,8)

 e.g (ii) Mg → Mg+ + e- (I.E)1 = +738 kJ/mol

           (2,8,2) (2,8,1)

   Mg+ → Mg2+ + e- (I.E)2 = +1451 kJ/mol

   (2,8,1) (2,8)

By I.E values we can determine the number of valence electrons and valency of an atom. e.g. 

      1st I.E 2nd I.E 3rd I.E 4th I.E

Li 520            7,298          11,815 –

Be 899 1,757           14,848    21,006

In the case of Li, there is a sudden jump in the value of its 2nd I.E which means this is the electron of its inner shell so its valence shell has only one electron and thus its valency is one.

In the case of Be, there is a sudden jump in the value of its 3rd I.E which means this is the electron of its inner shell so its valence shell has two electrons and thus its valency is two.

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Factors affecting Ionization Energy :

 Nuclear Charge:

With an increase in nuclear charge, the hold of the nucleus increases on electrons so more energy will be required to remove electrons. Thus more is the nuclear charge will be the I.E and vice versa.

 Atomic Size:

Smaller is the size of an atom, more will be the holder of the nucleus on electrons so more energy will be required to remove electrons. Thus smaller the atomic size more will be the I.E and vice versa.

                       I.E α 1/r2 

 Shielding Effect:

Greater the shielding effect of lower shells less will be the holder of the nucleus on electrons of the outer shell so low will be the I.E and vice versa.

 Penetration Effect:

The fraction of time that the electrons of outer orbitals spend near the nucleus is called the penetration effect. The order of this effect and thus I.E is

                  s > p > d > f

Electronic Configuration:

Filled and half-filled orbitals are more stable and thus their electrons are difficult to remove so their I.E is high. Thus elements of Groups 1A, IIA, and VIIA have extraordinary high I.E. 

The trend of 1st ionization energy in Periodic Table:

               Generally, I.E increases in a period from left to right due to decreasing atomic size and increasing nuclear charge. But anomalies are observed in groups IIA and VA they have extraordinary high I.E. This is due to the reason that they have filled or half-filled outer orbitals which are more stable and thus their electrons are difficult to remove so their I.E are high. Group II-A has an outer completely filled s-orbital while group V-A has an outer half-filled p-orbital so they have extraordinary high ionization energies. 

e.g. trend of Ionization Energy in the 2nd and 3rd periods is

        Groups IA IIA IIIA IVA VA VIA VIIA VIIIA

 Elements of period II Li Be B C N O F Ne

 Ionization Energy (kJ) 520 900 801 1086 1402 1314 1681 2081     

Elements of period III Na Mg Al Si P S Cl Ar

Ionization Energy (kJ) 496 738 578 786 1012 1000 1251 1521

 On other hand in a group I.E decrease from top to bottom due to increasing atomic size.

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