Example 3.4 a) Use the relative orbital energies given in Figure 3.4 to order the electronegativities
of silicon, sulfur, and chlorine. Silicon has the highest energy valence orbi
tals of the three, so it is the least
electronegative, while the valence orbitals of
chlorine are the lowest in energy, so Cl is the
most electronegative:
χSi
, <
χS
<
χCl
. Orbital energies increase going down a group, so we
also predict the following:
χSi
, <
χC
; χ
, < S
χO
; χ
, < Cl
χF
.
b) Use Figure 3.4 to predict which is mo
re electronegative, oxygen or chlorine.
The valence orbitals of oxygen are lower t
han those of chlorine, so oxygen is more
electronegative.
4 3 2 1 0
c
atomic number
H BeLi
B
ON C
F Na
Al Mg
SP Si
Cl K
Ga Ca
Se Ge
Br
As
Rb
In Sr
SbSn
I Te Cs
PbTl Ba
Bi
Po
At
Figure 3.8 Electronegativities (
χ) of main group elements
Elements within the same group are identified by circles with the same color. Table 3.2 Electronegativities of some late metals*
Metal Electronegativity
Ag
1.9
Sn
2.0
The periodic behavior of electronegativity
is shown in Figure 3.8. Electronegativity
increases left to right in a period due to an
increase in effective nuclear charge and
decreases going down a group due to an increase in the n quantum number. Consequently, fluorine (high Z
and low n) is the most electronegative atom, while cesium (low Zeff
and eff
high n) is the least electronegative element shown in Figure 3.8. In summary,
Nonmetals have high electronegativities, so th
ey tend to gain electrons to form anions, but
metals have low electronegativities, so they do not gain electrons to become anions. An exception to the above generality about
the electronegativities of metals arises
from the fact that d and f electrons do not sh
ield very well because they contain two and
three nodal planes, respectively. Therefore, the
effective nuclear charge experienced by the
valence orbitals in late metals (metals that lie on the right side of the Periodic Table) can be quite large. For example, Pb has 27 more protons and electrons than does Cs, but 24 of those electrons are d and f electrons, which
do not shield the 27 additional protons very
well. Thus, the 6p electrons in Tl and Pb experience relatively high Z
(12.25 and 12.39, eff
respectively), which makes both of these meta
ls fairly electronegative. Indeed, the
electronegativity of Pb is much greater than
that of Si even though they are in the same
Group, and the valence orbitals in Pb ha
ve a much higher n quantum number. We
conclude that
due to their
high effective nuclear charges,
late metals have unusually high
electronegativities (Table 3.2), which impacts
significantly on their chemical properties
.
Hg
2.0
Tl
2.0
Pb
2.3
* Late metals, such as those list
ed in Table 3.2, are those that
lie on the right side of the periodic table. Early metals, such as the Group 1A and 2A metals, lie on the left side of the table.
Chapter 3 Atomic Structure and Properties
© by
North
Carolina
State
University