The Hebrew University, Jerusalem 91904, Israel
Ionization potentials (IP) are fundamental for the under-
standinn of chemistrv. since thev determine the tendencv of
an atom to share itsvalence electrons in a chemical bind.
'I'extbooks of general chemi3trv (1 3) devote some soace to
the discussionof trends on IP';. While the trend of 'creas-
ing IP with atomic number Z in an isoelectronic series is
mentioned, it is difficult to find in textbooks a rationaliza-
tion of the experimental trends by any one simple relation.
The problem of determining the Z dependence of IP's in
isoelectronic serieshas been dealt with in the very early days
of quantum mechanics (4-6).It was then observed that the
total energy of an atom can he written using (4)
where h c R ~= 13.598eV is the IP of the hydrogen atom, S is
a screening constant and n* is an effective principal quan-
tum number. These two parameters vary as afunction of the
number of electrons, which we designate Q.
Slater (5)has interpreted eq 1as giving the energy of an
electron in an atom so that the total electronic energy is the