(i) The amount of energy required 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 cation is known as ionisation energy of that element.
(ii) Because ionisation energy is generally expressed in electron volts, so it is also known as ionisation potential.
(iii) Energy required for the removal of first, second and third electron from the gaseous atom is called first, second and third ionisation energy respectively.
A(g) + I1 → A(g)+ + e–
A+(g)+I2 → A(g)2+ + e–
A(g)2+ + I3 → A(g)3+ + e–
(iv) The order of first, second and third ionisation energies may be given as
I1 < I2 < I3
This is because second and third electron is removed from monopositive and dipositive cations respectively.
Effective nuclear charge increase with the increase of positive charge. So the attraction between the nucleus and the outermost electron increases and more energy is required for the removal of electron.