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

• robertjordan

## Homework Statement

Find the energy of a He+ electron going form the n=4 state to the n=2 state.

## Homework 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...

## 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...

plugging in 1 for n, 3 for z, 9.11*10^-31 kg for m, -1.602*10^-19 Coulombs for e, and 1.054572×10^-34 J*s for h, we get
E_1= ((9.11*10^-31 kg))*((-1.602*10^-19 C)^4)*(3^2)/(2(1.054572×10^-34 J s)^2) = 2.428×10^-37 s^6A^4/(kg m^4) (second to the 6 amperes to the fourth per kilogram meter to the fourth).

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