Ionization Energy

Ionization energy is the energy needed to remove the outermost electron from an atom, creating a positive ion. This energy, measured in electron-volt eV or kilojoules per mole (kJ/mol), is determined using an electric discharge tube. In this process, a fast-moving electron collides with a gaseous atom to eject an electron. The lower the ionization energy, the easier it is for an atom to form cations.


The Bohr model of an atom explains this by describing how electrons revolve around a positively charged nucleus in fixed energy levels due to the coulombic force of attraction. These energy levels are quantized, meaning the electron can only occupy specific energy states.

Where, Z is the atomic number and n is the principal quantum number where n is an integer. For a hydrogen atom, Ionisation energy is 13.6eV.

The Ionisation Energy (eV) is the energy required to take the electron from n = 1 (ground state or most stable state) to infinity. Hence taking 0 (eV) reference at infinity, the Ionisation Energy can be written as :The concept of Ionisation Energy supports the evidence of Bohr model of atom that the electron can revolve around the nucleus in a fixed or discrete energy levels or shells represented by the principal quantum number ‘n’. As the first electron goes away from the vicinity of the positive nucleus, then greater energy is required to remove the next loosely bound electron as the electrostatic force of attraction increases, i.e., the second Ionisation Energy is greater than the first one.

For example, the first ionization energy of Sodium (Na) is given as :
And its second Ionisation Energy is

Hence, IE₂ > IE₁ (eV). This is also true if there are K number of ionisations, then IE₁ < IE₂ < IE₃……….< IE_k

Metals have low Ionisation Energy. Low Ionisation Energy implies better conductivity of the element. For example, the conductivity of Silver (Ag, atomic number Z = 47) is 6.30 × 10⁷ s/m and its Ionisation Energy is 7.575 eV and for Copper (Cu, Z = 29) is 5.76 × 10⁷ s/m and its Ionisation Energy is 7.726 eV. In conductors the low Ionisation Energy causes the electrons to move throughout the positively charged lattice, forming an electron cloud.

Factors Affecting Ionisation Energy

In the periodic table, the general trend is that the Ionisation Energy increases from left to right and decreases from top to bottom. So the factors affecting ionization energy can be summarised below:

• Size of the Atom: The Ionisation Energy decreases with the size of the atom because as the atomic radius increases the columbic force of attraction between the nucleus and outermost electron decreases and vice-versa.
• Shielding Effect: The presence of inner shell electrons shield or weaken the columbic force of attraction between the nucleus and the valence shell electrons. Hence ionization energy decreases. The number of inner electrons means more shielding. However, in the case of gold, the Ionisation Energy is greater than silver even if the size of gold is more than silver. This is due to the weak shielding offered by the inner d and f orbitals in case of gold.
• Nuclear Charge: The more the nuclear charge, the more it will be difficult to ionize the atom due to more attraction force between nucleus and electrons.
• Electronic Configuration: The more stable the electronic configuration of the atom, the more difficult is to withdraw an electron hence more Ionisation Energy.