Question

At what atomic number would transition from $n=2$ to $n=1$ energy level result in emission of photon of $\lambda =3\times {10}^{-8}m$?

Answer 1

The Reidberg wavelength formulae:

$\frac{1}{3.8\times {10}^{-8}}=109678(1-\frac{1}{4})\times Z^2$

$Z^2=405$

$Z=20$