The excitation energy of a hydrogen like ion in its first excitation state is $40.8 \mathrm{eV}$. The energy needed to remove the electron from the ion in ground state is,

Wavelength of light emitted from second orbit to first orbit in a hydrogen atom is

As the electron in Bohr orbit of hydrogen atom passes from the state $n=2$ to $n=1$, the kinetic energy $K$ and potential energy $U$ changes as

Which of the following atoms has the lowest ionisation potential ?