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First ionization energy IE 1 :

A (g) A + (g) + e - (g)

Second ionization energy IE 2 :

A + (g) A 2+ (g) + e - (g)

Third ionization energy IE 3 :

A 2+ A 3+ (g) + e - (g)

The sequential ionization energies for the elements in the second row of the periodic table are shown here .

Successive ionization energies (kj/mol)
Na Mg Al Si P S Cl Ar
IE 1 496 738 578 787 1012 1000 1251 1520
IE 2 4562 1451 1817 1577 1903 2251 2297 2665
IE 3 6912 7733 2745 3231 2912 3361 3822 3931
IE 4 9543 10540 11575 4356 4956 4564 5158 5770
IE 5 13353 13630 14830 16091 6273 7013 6542 7238
IE 6 16610 17995 18376 19784 22233 8495 9458 8781
IE 7 20114 21703 23293 23783 25397 27106 11020 11995

Note that the second ionization energy is always greater than the first, and the third is always greater thanthe second, etc. This makes sense, since an electron should be more strongly attracted to a positively charged atom than to a neutralatom.

However, the data in the table show a surprising feature. In most cases, the ionization energy increases a fairlylarge amount for successive ionizations. But for each atom, there is one much larger increase in ionization in the sequence. In Nafor example, IE 2 is nearly 10 times greater than IE 1 . Similarly, IE 3 is five times greater than IE 2 for Mg, although IE 2 is less than twice IE 1 . The data for Na through S all show a single large step in addition to the smaller increases in IE.

Looking closely and counting electrons, we see that this unusually large increase always occurs for the ionizationwhere we have already removed all of the outer shell electrons and are now removing an electron from the inner shell. This occursuniformly across the second row elements, indicating that our shell model is in fact a very accurate predictor of the higher ionizationenergies. We can now tell how many electrons there are in the outer shell of each atom: it is equal to the number of electrons sincethe last inert gas.

We can conclude that an inner shell is "filled" once we have the number of electrons equal tothe number in an inert gas atom. The subsequent electrons are added to a new outer shell. This is commonly referred to as the valence shell of the atom.

However, we do not know why only a limited number of electrons can reside in each shell. There is no obviousreason at this point why all the electrons in an atom do not reside in the shell closest to the nucleus. Similarly, there is no reasongiven for why the number of electrons in an inert gas atom exactly fills the outer shell, without room for even a single additionalelectron. These questions must be addressed further.

Review and discussion questions

Explain how the scattering of α particles from gold foil reveals that an atom contains a massive,positively charged nucleus whose size is much smaller than that of the atom.

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Explain the significance of the relationship between the frequency of x-ray emission from each atom and theatomic ranking of that atom in the periodic table.

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Provide experimental evidence which reveals that the electrons in an atom are grouped into a valence shell andinner shell electrons.

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State and explain the evidence which reveals that the outershell of each inert gas atom is full.

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Why does the ionization energy for each successive ionizationincrease for every atom? Why is the increase from IE 4 to IE 5 in Si much larger than any of the other increases for Si?

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Source:  OpenStax, Concept development studies in chemistry. OpenStax CNX. Dec 06, 2007 Download for free at http://cnx.org/content/col10264/1.5
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