# Finding the energy of an electron from n=4 to n=2?

1. Sep 12, 2013

### robertjordan

1. The problem statement, all variables and given/known data
Find the energy of a He+ electron going form the n=4 state to the n=2 state.

2. Relevant equations
E_n=\frac{m\cdot e^4 \cdot z^2}{2n^2 \cdot \hbar^2}
Where m= mass of electron, z= atomic number, e= charge of an electron, n is the energy level.

^ I think those are the right meanings of the variables...

3. The attempt at a solution

I want to calculate E_4 and E_2 for He+ and compare the difference in their values, but I don't know what to plug in for everything to get an answer that makes sense. When I tried to use the E_n equation in one of my other homework problems, the units came out all wrong...

2. Sep 13, 2013

### ehild

Your equation for the energy is not correct. It should be $$E_n=\frac{m\cdot e^4 \cdot z^2}{2n^2 \cdot \hbar^2\epsilon_0^2}$$

ehild