Ionization energies of the elements (data page)

Numerical values
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 in the unit eV.

WEL (Webelements)
As quoted at http://www.webelements.com/ from these sources:
 * J.E. Huheey, E.A. Keiter, and R.L. Keiter in Inorganic Chemistry : Principles of Structure and Reactivity, 4th edition, HarperCollins, New York, USA, 1993.
 * A.M. James and M.P. Lord in Macmillan's Chemical and Physical Data, Macmillan, London, UK, 1992.