At what condition is the energy of an electron in an atom considered to be zero?

Calculate the energy associated with the first orbit of $He^{+}$. What is the radius of this orbit?

Supposing the energy of the fourth shell for a hydrogen atom is $-25 \, a.u.$ (arbitrary unit). What would be its potential energy in the ground state?

The space between the proton and the electron in a hydrogen atom is:

For $Li^{2+}$ ion,the ratio of radii $r_2 : r_5$ will be

The minimum energy that must be possessed by photons in order to produce the photoelectric effect with platinum metal is $.... \times 10^{-19} \, J$
[Given: The threshold frequency of platinum is $1.3 \times 10^{15} \, s^{-1}$ and $h = 6.6 \times 10^{-34} \, J \, s$.]