When Lithium vapour is taken in a discharge tube and the potential difference between the electrodes is 5.4 ev, there is a sudden increase in the flow of current. The ionisation energy of Lithium is

The minimum potential required to ionize a gas is called Ionization potential. Due to ionization sudden increase in flow of current is observed.  Thus IP of lithium = 5.2ev/atom IE = 5.4(96.45 kJ/mol)=520 kJ/mole