What is Ionization Energy?

Many students make a mistake when asked:

“What is ionization energy?”

Let’s learn how to properly and correctly define ionization energy.

The amount of energy required to remove one mole of electrons from one mole of atoms or ions in the gaseous state to produce one mole of positive ions, is called ionization energy / ionization potential of the concerned element.

Symbol: IE

Unit: kJ/mol [Read: Kilojoules per mole]

Expression:

Official IUPAC definition of ionization energy

Successive Ionization Energies

First Ionization Energy

The amount of energy required to remove one mole of electrons from one mole of ground state atoms in the gaseous state to produce one mole of unipositive ions, is called first ionization energy of the concerned element.

It is called first ionization energy because it is the 1st electron removed from the outer / valence shell of the atom in its ground state.

Expression:

Second Ionization Energy

The amount of energy required to remove one mole of electrons from one mole of unipositive ions in the gaseous state to produce one mole of dipositive ions, is called second ionization energy of the concerned element.

It is called second ionization energy because it is the 2nd electron removed from the outer / valence shell of the atom in its unipositive ion state.

Expression:

M⁺(g) → M²⁺(g) + e⁻

Third Ionization Energy

Similarly it goes on like this.

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