Ionization energies of the elements (data page) - Wikipedia
https://en.wikipedia.org/wiki/Ionization_energies_of_the_elements_(data_page)
For each atom, the column marked 1 is the first ionization energy to ionize the neutral atom, the column marked 2 is the second ionization energy to remove a second electron from the +1 ion, the column marked 3 is the third ionization energy to remove a third electron from the +2 ion, and so on. "use" and "WEL" give ionization energy in the unit kJ/mol; "CRC" gives atomic ionization energy …
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