Starting with ground state 2s, here we assume that the first 2 electrons are in the 1s, one of them has spin up and the other has spin down, so the third electron as I said in 2s.
The first exited state will be 2p, then 3s,3p,3d,4s,4p...
All the above states we call them singly exited states(the first 2 electrons are in 1s)
The first exited state will be 2p, then 3s,3p,3d,4s,4p...
All the above states we call them singly exited states(the first 2 electrons are in 1s)
