Ionisation Energy and Ionisation Potential for JEE

VSAT 2022

What is Ionisation Energy?

Ionisation energy can be defined as the least amount of energy that an electron in a gaseous atom or ion must absorb to escape the nucleus's influence. Ionisation energy is typically an endothermic process and is also sometimes referred to as the ionisation potential. We can infer that ionisation energy gives us an idea of the chemical compounds' reactivity. Using it, one may also gauge the potency of chemical bonding. Either kJ/mol or electron volts are the units used to measure it. 

The metals generally have low ionisation energy due to their high tendency to lose electrons, whereas non-metals have higher ionisation energies due to their low tendency to lose electrons. Let us understand ionisation energy and ionisation potential in detail in the following sections.

Factors Governing Ionisation Energy

We have discussed the ionisation energy definition in the introduction; now, let's talk about the factors governing Ionisation Energy. Normally, it will be more challenging to remove an electron when the ionisation energy is high. The attraction forces are governed by several additional factors.

  • The electrons are attracted to the nucleus strongly if it has a high positive charge.

  • The attraction will be stronger if an electron is nearby or close to the nucleus than if it is farther away.

  • The attraction forces are weaker if there are more electrons between the outer level and the nucleus.

Ionisation Energy and Bohr’s Atomic Model

Using Bohr's atomic model, it is possible to anticipate further aspects of atomic ionisation energy. His model predicts that there will be a variety of routes for the electron to take around the protons and neutrons in the nucleus. Each orbit or path has a predetermined separation from the nucleus. Additionally, each orbit stands for set energy. Since an electron remains a particle, it will still contain the energy from its orbit. A particle can absorb energy and go to the following higher orbits with more energy. The electron will escape the nucleus's force of attraction and leave the atom if enough energy is available and absorbed.

Ionisation Energy Formula

Ionisation is the process of removing an electron from its orbit and moving it outside of the atom. Ionisation energy is equal to the difference in energy between the energy of the electron in the initial orbit and the energy of the electron outside the atom since each orbit of the electron has distinctive energy (in the infinite orbit from the nucleus).

The Bohr model of an atom calculates the energy of an electron in the nth orbit as –

${{E}_{n}}=-\dfrac{2{{\pi }^{2}}m{{e}^{4}}}{{{(4\pi {{\varepsilon }_{o}})}^{2}}{{h}^{2}}}\times \dfrac{{{Z}^{2}}}{{{n}^{2}}} \\ =R_H\times \dfrac{{{Z}^{2}}}{{{n}^{2}}}J/atom \\ =-13.6\times \dfrac{{{Z}^{2}}}{{{n}^{2}}}eV/atom \\ =-2.18\times {{10}^{-18}}J/atom $

Where, En is the energy of the nth orbit, m is the mass of an electron, e is the unit charge present on an electron, h is Plank’s constant, Z is atomic number, n is the principal quantum number, and RH is the Rydberg constant.

By changing the orbit numbers of the electrons before and after the shift from n1 to n2 in Bohr's energy equation, it is possible to compute the ionization energy required to remove an electron from a neutral atom.

${{E}_{n_1}}=-R_H\times \dfrac{{{Z}^{2}}}{{{n}^{2}}}$ and ${{E}_{n_2}}=-R_H\times \dfrac{{{Z}^{2}}}{{{\infty }^{2}}}$

Since $\dfrac{1}{{{\infty }^{2}}}=0$ and can be neglected.

$\Delta E={{E}_{n_2}}-{{E}_{n_1}} \\ =R_H\times {{z}^{2}}\times \dfrac{1}{{{n}^{2}}} \\ =2.18\times {{10}^{-18}}\times {{z}^{2}}(\dfrac{1}{{{n}^{2}}})J  $

Ionisation Energy Periodic Table

The ionisation energy of an atom lowers with higher energy orbitals and rises with the atomic number. The ionisation energy rises as we progress across the period in the periodic table from left to right because the atomic radius is getting smaller. The outermost electron is closer to the nucleus in this case and experiences a more effective nuclear charge.

The ionisation energy decreases as we proceed from top to bottom, though. As we move down the group, the elements start to have more electron shells, which is the main cause of this. Additionally, the electrons are placed farther away from the nucleus's attractive forces. Thus we can conclude that ionisation energy order is well related to periodic order.

What is Ionisation Potential? 

Let's define ionisation potential. The energy required to eject an electron from the valence shell of a single gaseous atom is known as the ionisation potential. The outermost shell is the valence shell. Mono-positive cation is formed when a neutral atom loses an electron. The first ionisation energy is the quantity of energy required to remove this electron. Second ionisation energy is required when the mono-positive cation loses another electron.

Ionisation Potential Formula

Ionisation energy, work function, or Ionisation Potential are the similar terms. We have the formula for Ionisation potential as follows. $V=\dfrac{E}{e} \\ =13.6\dfrac{{{Z}^{2}}}{{{n}^{2}}}eV  $ 

Ionisation potential of the hydrogen atom is 13.6 eV as n and z for Hydrogen is 1.


Ionisation energy can be defined simply as a measurement of how difficult it is to remove an electron from an atom. Usually, when a chemical species is in its ground state, an electron is lost. Ionisation potential is the potential difference that corresponds to the amount of energy, measured in electron volts, required to completely remove an electron from an atom or ionize a gas molecule. Energy is required to extract one electron from an atom. Energy is needed to add an electron to an atom. Typically speaking, the first and second ionisation energies of alkali metals and alkaline earth metals, respectively, are both quite low.

FAQs on Ionisation Energy and Ionisation Potential for JEE

1. Give any two factors affecting the Ionisation Potential?

Two factors affecting the ionisation potential are as follows.

  • Atomic radii: The nucleus and outermost electron are less attracted to one another as the atomic radius increases. The outermost electron is just weakly held. The removal of the electron hence requires less energy. As a result, the Ionisation Potential decreases as the atomic radius increases.

  • Effective nuclear charge: The nucleus and outermost electron become more attracted to one another as the effective nuclear charge grows. The outermost electron is tightly confined as a result. The removal of the electron therefore requires greater energy. Consequently, the Ionisation Potential rises as the effective nuclear charge does.

2. Explain the effects of Electron Shells on Ionisation Energy.

The division of electron orbitals into different shells has a significant impact on the ionisation energy of the individual electrons. The first element of its period to have electrons in the 3p shell is aluminum. Because only one electron must be lost to create a stable 3s shell, the new valence electron shell, the first Ionisation energy of this element is comparably low to that of other elements in the same period. The energy needed to expel a second electron increases significantly as you pass the first ionisation energy and enter the second ionisation energy, though. This is because you are currently attempting to remove an electron from a 3s electron shell that is full and rather stable.