|The ionization energy is the energy needed to remove an
from an atom. For an element X, it is written as:
|X + energy ------> X+ + e-|
|The ionization energy is a measure of how tightly the electrons are held by the atom.|
|Successive Ionization Energies in kJ/mole for the First 12
1 2 3 4 5 6 7 8
He 2372 5250
Li 520 | 7297 11,810
Be 899 1757 | 14,845 21,000
B 800 2426 3659 | 25,020 32,820
C 1086 2352 4619 6221 | 37,820 47,260
N 1402 2855 4576 7473 9442 | 53,250 64,340
O 1314 3388 5296 7467 10,987 13,320 | 71,320 84,070
F 1680 3375 6045 8408 11,020 15,160 17,860 | 92,101
Ne 2080 3963 6130 9361 12,180 15,240 ------- -------
Na 496 | 4563 6913 9541 13,350 16,600 20,113 25,666
Mg 737 1450 | 7731 10,545 13,627 17,995 21,700 25,662
K 416 3082
Ca 592 1151 | 4960
Rb 403 | 2662
Cs 378 | 2432
|Atoms with more than one electron have more than one ionization energy. These ionization energies correspond to the stepwise removal of electrons, one after the other. Take for example Be. It is fairly easy to remove the first electron and about twice as much energy to remove a second. To remove the third electron involves a major jump.|
|In general, successive ionization energies always increase because each subsequent electron is being pulled away from an increasingly more positive ion.|
|Periodic Table Trends|
|Ionization energy increases from bottom to top within a
increases from left to right within a period.
The trend within a group can be seen easily by observing
ionization energies vary for either the alkali metals (Li through Cs)
the noble gases (He through Rn). The change in ionization energy from
through Ne or from Na through Ar illustrates the trend across a period.
These periodic trends in ionization energy are the opposite of
trends in atomic size within the periodic table.
|The reason for the left to right trend is the gradual increase in effective nuclear charge felt by the valence electrons. As this charge increases the valence electrons get pulled closer to the nucleus which causes the shrinking of the atom but it means that more energy will have to be expended to pull an electron away.|
|Noble Gas Stability|
|Look at the trends in the chart above. We always get a big increase in the ionization energy needed when we start to remove electrons from a new inner core of electrons. The noble gases have electron configurations that have a full 'p' subshell. In effect the noble gases have very stable cores of electrons. For this reason they are considered to be inert and unreactive.|
|Using the ionization energies chart from above:
1. Get a blank periodic table and fill in the first electron ionization energies in the appropriate element. Verify for yourself that the ionization trends are as stated above.
2. Using your periodic table from above, plot on a piece of graph paper the Ionization Energy vs Atomic number. Connect dot-to-dot. Can you note any trends visually on the graph?