[elliotyang]

The question is like following:
Consider these ground state ionisation energies of one electron species:
H=1.31*10^3kJ/mol
He+=5.2410^3kJ/mol
Li2+=1.1810^4kJ/mol

a)write a general expression for the ionization energy for any one=electron species.
I got answer for this question, that is E=-2.18*10^-18J(Z^2/n^2)

b)what is the minimum wavelength required to remove the electron from the n=3 level of He+?
I wonder how to do this question as the He atom only has 4 electron which 2 in s 1s orbital while the other two in 2s orbital. so the principle quantum number involved is only 1 and 2. there are no electron in n=3. Can anyone explain this question to me and tell me how to solve it? the answer given is 205nm.