The Foundations of Chemistry

Here is one reason why trends in

ionization energies are important.

Noble gas electron configurations are

stable only for ions in compounds.In

fact, Li(g) is less stable than Li(g) by

520 kJ/mol.

6-4 Electron Affinity 245

charged ions). Elements with intermediate ionization energies generally form molecular
compounds by sharing electrons with other elements. Elements with very high ionization
energies, such as Groups VIA and VIIA, often gain electrons to form anions(negatively
charged ions).
One factor that favors an atom of a representativeelement forming a monatomic ion in
a compound is the formation of a stable noble gas electron configuration. Energy consid-
erations are consistent with this observation. For example, as one mole of Li from Group
IA forms one mole of Liions, it absorbs 520 kJ per mole of Li atoms. The IE 2 value is
14 times greater, 7298 kJ/mol, and is prohibitively large for the formation of Li^2 ions
under ordinary conditions. For Li^2 ions to form, an electron would have to be removed
from the filled first shell. We recognize that this is unlikely. The other alkali metals behave
in the same way, for similar reasons.
The first two ionization energies of Be (Group IIA) are 899 and 1757 kJ/mol, but IE 3
is more than eight times larger, 14,849 kJ/mol. So Be forms Be^2 ions, but not Be^3 ions.
The other alkaline earth metals—Mg, Ca, Sr, Ba, and Ra—behave in a similar way. Only
the lower members of Group IIIA, beginning with Al, form 3ions. Bi and some d- and
f-transition metals do so, too. We see that the magnitudes of successive ionization ener-
gies support the ideas of electron configurations discussed in Chapter 5.

Due to the high energy required, simple monatomic cations with charges greater than

3 do not form under ordinary circumstances.

EXAMPLE 6-2 Trends in First IEs

Arrange the following elements in order of increasing first ionization energy. Justify your order.

Na, Mg, Al, Si

Plan

Table 6-1 shows that first ionization energies generally increase from left to right in the same
period, but there are exceptions at Groups IIIA and VIA. Al is a IIIA element with only one
electron in its outer porbitals, 1s^22 s^22 p^63 s^23 p^1.

Solution

There is a slight dip at Group IIIA in the plot of first IE versus atomic number (see Figure
6-2). The order of increasing first ionization energy is NaAlMgSi.

You should now work Exercise 26.

ELECTRON AFFINITY

The electron affinity (EA)of an element may be defined as

the amount of energy absorbedwhen an electron is added to an isolated gaseous atom

to form an ion with a 1
charge.

The convention is to assign a positive value when energy is absorbed and a negative value
when energy is released. Most elements have no affinity for an additional electron and

6-4

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 8.13, Electron Affinity.

This is consistent with thermodynamic

convention.