Chemistry Reference
In-Depth Information
2500
He
Ne
2000
Ar
1500
Kr
Xe
Hg
Rn
1000
No
Tl
500
In
Lu
Li
Ga
Na
K
Rb
Cs
Fr
0
15913 17 21 25 29 33 37 41 45 49
Atomic Number, Z
53 57 61 65 69 73 77 81 85 89 93 97 101
FIGURE 2.13
The variation of the first ionization energy according to the atomic number Z.
the d-block, i.e., zinc, cadmium and mercury. This situation is a consequence of the
more stable electron configurations of the type: (n−1)d
10
ns
2
. The ionization energy
is decreasing per group in general from period 2 to 7. This is a result of increasing
atomic radius and a concurrent increase in the shielding effect.
With some exceptions, the first ionization energy increases generally from left
to right in periods in the periodic table and decreases downward in groups. The
nuclear effective charge (Z
eff
) increases in periods, and as a consequence, it is natural
that the valence electrons are more strongly linked to the nucleus (Housecroft and
Constable 1997). The number of layers in groups increases with each layer that is
further from the nucleus. The electrons become more amenable to removal.
The ionization energy increase is not uniform in periods. In each period there are
some values that are bigger or smaller than expected. In group 13 (B, Al, Ga, In, Tl)
for example, as we expect, the first ionization energy decreases sharply from boron
to aluminum, but it is slightly higher for gallium than for aluminum. The explanation
is that there is the first row of d-block elements before gallium, such that gallium
has 10 more positive charge units and 10 electrons on the 3d orbitals. Although the
d electrons shield less than the s or p electrons, the outer electrons are very strongly
attracted by the nucleus so that the ionization energy is slightly higher than that of Al
despite the larger size of the atom. The same phenomenon is found for indium, which
has slightly lower ionization energy than gallium. For thallium there is a significant
increase of the ionization energy and Tl has the highest value in its group after
boron. This can be explained by the presence of the row of elements that fill the 4f
subshell before thallium, resulting in an increase of the effective nuclear charge and
simultaneous decrease of the energy of the 6p orbitals.
The elements whose atoms have low ionization energy values have metallic char-
acter and are situated on the bottom left side of the periodic table.
