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## Homework Statement

In a hydrogenlike ion with atomic number Z, the energies of the allowed states are given by

E(n) = (-13.6eV) (Z^2/n^2)

What is the wavelength asociated with the transition between first excited state and ground state of hydrogen-like helium? (He+)

## Homework Equations

E(n) = (-13.6eV) (Z^2/n^2)

E = hf = hc/Lambda

## The Attempt at a Solution

Hydrogen like helium is He+ which has 2 protons and 1 electron. Atomic number Z is 2.

So I'd sub in 2 for Z (4 for Z^2), but I don't know what to choose n as.

I assume n=1 for ground state and n=2 for first excited state.

An idea I had was to do it twice - sub in n=1 and complete the equation, sub in n=2 and complete the equation, and... take them away?

I'm not sure :(

Thanks very much.