Chemistry - A Molecular Science

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

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