The ionisation energy of `He^(+)` is `19.6xx10^(-18)"J atom"^(-1)` Calculate the energy of first stationary state of `Li^(2+)` ion. 

Explanation

Ionisation energy of hydrogen like species = Ionisation energy of `H xxZ^(2)`
`"I.E of "He^(+)="I.E of H"xx4,"I.E of "H=(1.9xx10^(-18))/(4)"            ______(1)"`
`"I.E of "Li^(2+)="I.E of "Hxx9," I.E of H"=("I.E of "Li^(2+))/(9)"       ____(2)"`
`"Equating (1) and (2) "("I.E of "Li^(2+))/(9)=(19.6xx10^(-18))/(4)`
`"I.E of "Li^(2+)=(19.6xx10^(-18)xx9)/(4)`
The energy of first stationary state of `Li^(2+)`
`=(-19.6xx10^(-18)xx9)/(4)=-4.41xx10^(-17)"J atom"^(-1)`