Neutral lithium ionization energy for removing an electron from the $1s$ orbital How much energy does it take to singly ionize a neutral lithium atom by removing an electron from the $1s$ orbital? 
 A: You can obtain this directly from the energy-levels part of the NIST Atomic Spectra Database.


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*In Spectrum, put Li I for the energy levels of neutral lithium, or Li II for the energy levels of the first cation.

*Choose whatever Level units you're most comfortable with.


Click Retrieve Data to get the results.
The ionization energy you want is not listed explicitly, and you need to get it as the sum of two factors:


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*The ionization energy from Li I to Li II, which is listed in the spectrum of Li I, at the entry Li II ($1s^2$ $^1S_0$) at the left-hand column.

*The excitation energy from the $1s^2$ of the cation to the relevant $1s\,2s$ of the cation, which are listed in the Li II spectrum.


Note that there are two such states, with a small $\sim 1\:\rm eV$ energy difference between them, depending on how the two remaining spins are aligned. (Though, frankly, given how ridiculously hard it is to excite the cation to anything other than the ground state, this difference may well be negligible.) 
