0.2 The structure of an atom  (Page 6/6)

First ionization energy ${\mathrm{IE}}_1$ :

Second ionization energy ${\mathrm{IE}}_2$ :

Third ionization energy ${\mathrm{IE}}_3$ :

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

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, ${\mathrm{IE}}_2$ is nearly 10 times greater than ${\mathrm{IE}}_1$ . Similarly, ${\mathrm{IE}}_3$ is five times greater than ${\mathrm{IE}}_2$ for Mg, although ${\mathrm{IE}}_2$ is less than twice ${\mathrm{IE}}_1$ . 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