CSIR NET Chemical Science: Short Notes on Ionization Energy

By Neetesh Tiwari|Updated : April 10th, 2023

Are you looking for some short and reliable notes during your CSIR-NET preparations? Then, you have come to a perfect place!

Candidates preparing for their CSIR NET exam might need some short study notes and strategies to apply while preparing for the key exam of their life. At this point, We at BYJU'S Exam Prep come up with short notes on Ionization Energy which comes under the Inorganic Chemistry section of the Chemical Science syllabus

This set of short notes on Ionization Energy has been meticulously designed by our experienced subject-matter experts to give you the most standard set of study materials to be focused upon. In this cut-throat competitive world, students need to prepare themselves with the best study materials to help them in their learning and future. Here we offer the best reliable study notes that the students can use during their preparations for the upcoming CSIR-NET 2023 exam.

Short Notes on Ionization Energy

Ionization Energy: It is the amount of energy needed in order to remove the most loosely bound electron of the outermost shell (i.e., the outermost electron) from one mole of an isolated gaseous atom of an element in its ground state to produce a unipositive cation.


The energy required to remove the first, second and third electron from the gaseous atom is called first, second and third ionization energy respectively.


 Enthalpy of ionization is always an endothermic process.

Factors Affecting Ionization Energy

1. Atomic Radius:


2. Effective Nuclear Charge: 

The greater the effective charge on the nucleus of an atom, the more difficult would become to remove an electron from the atom. Ionisation Energy an Effective nuclear charge (Zeff).

3. Penetration Effect of Orbitals: The order of energy required to remove an electron from s, p, d and f orbitals of a shell are:


4. Shielding or Screening Effect:


5. Stability of Half-Filled and Fully-Filled Orbitals: According to Hund's rule, the stability of half-filled or completely filled degenerate orbitals is comparatively high. So, Comparatively more energy is required to separate the electron from such atoms.

Periodicity in Ionization Potential

Generally, ongoing from left to right, ionization potential increases because of the increase of effective nuclear charge. But this trend is always not. Whenever an element has a half-filled or fully filled configuration, its ionization energy will always be high. Example:


According to the general trend, the ionization energy of B should be greater than Be but due to the stable configuration of Be, its ionization energy will be higher than that of B.

The trend in I.E in Groups of Normal Elements



On moving from top to bottom in a group in transition series:

   (a). In a group, the values of I.P. decreases because of an increase in atomic size.

   (b). In moving from the second to third transition series, the value of the I.P of the second and third elements almost becomes the same due to lanthanoid contraction.

Applications of Ionization Potential:

(i) Along the period, ionization energy increases, so electropositive character decreases.      

(ii) For noble gases, ionization energy is very high which makes them almost unreactive.

(iii) As the ionization energy of metals decreases, their reactivity increases.

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