Incoming links: Hydrogen 1-2 spectral line
One reasonable and memorable approximation excluding any fine structure is:see for example example: hydrogen 1-2 spectral line.
Equation 1.
Hydrogen spectral series mnemonic
. 21 cm is very long and very low energy, because he energy split is very small!
Compare it e.g. with the hydrogen 1-2 spectral line which is 121.6 nm!