What is ionisation energy and how IE vary in Periodic table

Ionisation energy is the energy required to remove the most loosely bounded valence electron. Here we will also discuss how IE vary in Periodic table.

What is ionisation energy

The minimum amount of energy required to remove the most loosely bounded electron from the isolated gaseous atom.

M (g) + IE ➡️ M+ + e The isolated gaseous atom converts in to gaseous cation. Units of Ionisation energy is KJ/mol

What is successive ionisation enthalpies

It is the IE required to remove first, second and third electrons from an isolated gaseous atom.

M (g) + IE1 ➡️ M+ + e

M+(g) + IE2 ➡️ M2+(g) + e

M2+(g) + IE3 ➡️ M3+ (g) + e

IE3 > IE2 > IE1 When an electron removes from the neutral from the neutral gaseous atom, Zeff increases on the cation. Therefore, more energy needs to remove the electron from a cation. Hence IE2 > IE. For the same reason, IE3 > IE2

Factors governing ionisation energy

  1. Nuclear charge

IE increases with the increase in nuclear charge. Greater the nuclear charge, more energy needs to remove the valence electron. Therefore, IE increases in the period when we move from left to right.

2. Atomic radius

IE decreases as the atomic radii increases. As the distance from the nucleus increases, attractive force on the outermost electron decreases. Lesser energy needs to remove the outermost valence electron.

3. Penetration effect of the electrons

IE increases as the penetration effect of the electrons increases. In case of multi-electron system, s orbital has the maximum probability of finding near to the nucleus than p, d and f orbitals. Therefore, within the same shell, penetration effect decreases in the order s > p > d > f . For example : First IE of Al is lower than Mg.

4. Shielding effect of inner shell electrons

As the shielding effect of the inner electrons increases, the IE decreases. The actual charge felt by the valence shell electron is Zeff. The repulsive force experienced by the valence electron due to the inner shell electrons is called as screening effect.

Zeff = Z – screening constant. Therefore, as the number of inner shell electrons increases, IE decreases.

5. Electronic configuration

Half filled and full filled electronic configuration are extra stable.

Check out the following post on isoelectronic ions here :

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