| MadSci Network: Physics |
Brad, Rigorously, the first ionization energy of an atom in vacuum is the amount of energy necessary to remove the outer valence electron from its orbital to a distance of infinity. I say "first" because an atom can have as many ionization energies as it has electrons. The second ionization energy, for example, is the amount of energy necessary to remove the second outer valence electron to infinity after the first electron has previously been removed. See, you just work your way through the atom, one electron at a time, all the way through the valence and core electrons. You can imagine that the more electrons you remove and the closer the electrons are to the nucleus, the more energy it will take to remove them to infinity. Also note, ionization energy is sometimes confusingly called "ionization potential" since only the charge of one electron is considered. But stick to ionization energy since eV is an energy term, while V is a potential term (e = charge of electron; V = potential. So, e times V = energy of an electron, or eV). In reality, a typical researcher working with a finite apparatus probably takes "infinity" to mean a few meters separation max. At that distance the interaction of the removed electron and the atom are nearly non- existent. The electron is essentially a "free electron". One of the most accurate means to measure ionization potentials (and electron affinities) is via ZEKE spectroscopy (do a Google web search for that term if you're interested). Using ZEKE, ionization potentials of 1meV resolution are possible for even large molecules. Remember however, for single atoms in a vacuum, there are physical constants associated with the process of ionization. These terms are also used to describe ionization of molecules and solids, but the values are not constant because their ionization can be affected by the local chemistry, geometry, and temperature. Here are a couple of URLs that may be useful... => http://www.chem.uidaho.ed u/~honors/ionpot.html => http://spectr- w3.snz.ru/ion.phtml ---* Dr. Ken Beck
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