Ask A Scientist Chemistry Archive

  
name         Lyndsay
status       student
age          16

Question -   Why is there no value is given for the fourth ionisation
energy of lithium?
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The are only 3 electrons.

Dr. Harold Myron
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Lithium only has 3 electrons so the 1st, 2nd, 3rd ionization leaves no more
electrons to ionize.

Vince Calder
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Lindsay,

Ionization by definition means the energy needed to remove an electron from
the atom or ion. If you look at a periodic table, lithium has an atomic 
number
of "3". It has 3 protons - and therefore as a neutral atom has 3 electrons.

Greg (Roberto Gregorius)
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Dear Lyndsay,

Thanks for your question.  Ionization energy for an element is the energy
required for an electron to be removed from orbiting the nucleus of an atom.
The first ionization energy is the energy required to remove the first
electron, the second ionization energy is the energy required to remove the
second electron, and so on.

For lithium, the first ionization energy is 520.2 kiloJoules/mole.  This
means that it requires 520.2 kiloJoules (kJ) of energy to remove one
electron from one mole (6.02 x 10^23) of lithium atoms.  The second
ionization energy for lithium is 7,298.1 kJ/mole.  Can you guess why the
second ionization energy is so much higher for lithium compared to the first
ionization energy?

The third ionization energy for lithium is 11,815 kJ/mole.  If you check on
a periodic table, you can see that lithium, with atomic number 3, only has 3
protons and 3 electrons in its ground state.  After you remove the third
electron, you have an ion (Li+3) that has no more electrons!  So there
cannot be a fourth ionization energy.

Here is the web site that lists ionization energies for different elements:

http://www.webelements.com/webelements/elements/text/periodic-table/ionz.htm
l

Regards,

Todd Clark, Office of Science
U.S. Department of Energy
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Gotcha- because it only has 3 electrons to loose!   Lithium is element 
number 3...
The element number, the atomic number is really the electron number.
Lithium, after 3 ionizations, is a bare nucleus,
a much smaller positive particle with no electron cloud around it, like a 
proton or an alpha particle.

Perhaps you are confusing the "ionization number" with
the "excitation level" of one (outer) electron?
There would of course be an infinite series of excitation levels of the 
first electron,
all having energy less than the first ionization energy.

Likewise, Hydrogen only has 1 ionization energy, and Uranium has 92.
We probably do not know all 92.

cordially-
Jim Swenson
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