Here it is...
Stuck in a shell
2n^2 - 1 electrons enjoying universality,
Were looking for a partner to complete orbitality,
They'd soon done a hole-in-one
A spin-down guy with mass 511
But they did not foresee reversal of parity!
"Oh crap," said one, "now our beloved shell,
Once bosonic, is under Fermi's spell.
To get back the fun,
One must transition,
But who it must be, how can we tell?"
"Life in a complete shell is anyway dull,
One of our number will thus, surely, cull."
And there was a bump,
As one tried to jump,
From this to the n+1 orbital.
But he came back, not losing any Joules,
And cried, "All of us, are silly fools!"
We are stuck with this n,
For if we jump, then,
We'll violate nature's selection rules!"
And they had learned to live with fight and bout,
When a photon came leapin' and knocked one out.
- AnonickN.B.: "Selection rules" are rules which forbid transitions between certain levels in atoms.