Chemistry Reference
In-Depth Information
2
Atomic radius
Ionic radius, An4+
Ionic radius, An3+
1.8
1.6
1.4
1.2
1
Å
0.8
0.6
0.4
0.2
0
aUNp
Pu Am Cm Bk Cf
Es
Fm Md No
Lr
Actinides
FIGURE 2.12
The variation of atomic and ionic radii for the two oxidation states 3+ and
4+ of the actinides.
that the removal of another electron requires more energy. The secondary ionization
energy is bigger than the initial energy, the third is bigger than the second, and so on.
The ionization energy is one of the few properties of the atom that can be measured
directly by studying electrical discharges of gases. Among the indirect methods, the
most used is based on the study of the emission of optical spectra.
Ionization energy values are determined by several factors, such as:
•
the nuclear charge increases for the atoms in each period and determines an
increase in the ionization energy due to the increase in the attraction force
of the electrons by the nucleus,
•
the atomic radius increases with period and produces a decrease of the ion-
ization energy,
•
the shielding effect of the inner electrons produces a decrease of the attrac-
tion exerted by the nucleus upon the outer electrons, and
•
the degree of penetration of the outer orbitals varies for the electrons
that have the same principal quantum number according to the order
ns > np > nd > nf, which means that the electrons in the ns layers are the most
strongly retained, followed by the np electrons, nd electrons, and so on.
The ionization energy is dependent on all these factors and they must be considered
in order to obtain the correct evaluation.
Figure 2.13
shows the modification of the first ionization energy according to
the atomic number. It can be seen that for each period, the first ionization energy
increases to maximum values in rare gases, which have very stable ns
2
np
6
structures.
Minimum values are found in the elements in group 1 because the distinctive
electron that is lost is situated on a new energetic layer. The atomic radius increases
compared to the radius of the atoms in the previous period, which determines a
stronger shielding compared to the electron of interest. There are remarkably high
values for ionization energies of the elements situated at the end of the periods in
