Define $$n_r~:=~n-\ell-1~\geq 0,$$ where $n$ and $\ell$ is the [principal](http://en.wikipedia.org/wiki/Principal_quantum_number) and [azimuthal](http://en.wikipedia.org/wiki/Azimuthal_quantum_number) quantum number, respectively. [Bohr's model](http://en.wikipedia.org/wiki/Bohr_model) works best in the limit 

$$ \ell \gg 1 $$ 

(to get to the [semiclassical limit](http://en.wikipedia.org/wiki/Classical_limit) & the [correspondence principle](http://en.wikipedia.org/wiki/Correspondence_principle)), and

$$  n_r  \text{  small} $$

(to ensure that the orbital has a well-defined radius).