Chemistry Reference
In-Depth Information
energy
is the energy required to remove the third electron from the dipositive ion;
and so on. For every element, the second ionization energy is higher than the first
because it is harder to remove an electron (a negative particle) from a positively
charged body than from the corresponding neutral one. The ionization energy
increases dramatically as the octet of electrons is disrupted (Table 13.1). The term
ionization energy
always means
first
ionization energy unless otherwise specified.
Table 13.1
Successive
Ionization Energies for Some
Main Group Elements
(kilojoules per mole)
First
Second
Third
Na
495.8
4562
6912
Mg
737.7
1451
7733
EXAMPLE 13.5
Al
577.6
1817
2745
Predict which of the first three ionizations of potassium has the greatest
increase
in ionization energy from the preceding ionization.
Solution
The second ionization energy of potassium is much greater than the first because
the octet of electrons in the third shell is broken in ionizing that electron. (The
actual values for the first three ionization energies are 418 kJ/mol, 3050 kJ/mol,
and 4400 kJ/mol, respectively.)
Practice Problem 13.5
Predict which of the first four ionizations
of calcium has the greatest
increase
in ionization energy from the preceding
ionization.
Electron Affinity
Electron affinity
is defined as the energy
liberated
when an electron is added
to a
gaseous atom
to form a
gaseous anion.
For example,
Positive electron affinity values
signify that energy is liberated,
in contrast to ionization energy
values and values of other
variables to be introduced in
Chapter 14.
e
----------------------
ยก
Electron affinity
Cl
(g)
Cl(g)
Note that this energy is
not
the energy for the more familiar reaction of gaseous
chlorine
molecules
to form chloride ions in a solid lattice or in solution.
Table 13.2 presents the electron affinities of a number of elements. The
electron affinities of some elements are negative, meaning that energy is
required
to add an electron to the gaseous atoms. Periodic variation of electron
affinity is more difficult to rationalize than is the variation of ionization energy,
but note that the halogens have the greatest electron affinities of all the elements.
EXAMPLE 13.6
Determine from Table 13.2 which periodic groups of elements generally require
energy when an electron is added to their atoms.
Solution
The alkaline earth metals and noble gases are the only groups of elements with
negative (first) electron affinities.
