Burdge/Overby, Chemistry: Atoms First, 2e FM | Page 9

42

CHAPTer 2

Atoms and the Periodic Table

Number of Stable Isotopes with Even and
Odd Numbers of Protons and Neutrons

TA b l e 2.2

Protons

New and Updated Chapter Content

Neutrons

Odd
Odd
Even
added to
Even

PREFACE ix

Number of stable isotopes

Odd
Even
Odd
the concept
Even

Chapter 2—A new section (2.4) has been
introduce
of nuclear
provide students insight into why some nuclei are stable, and others are not.

4
50
53
stability
164

and

Figure 2.9 Plot of neutrons versus
protons for various stable isotopes,
represented by dots. The straight line
represents the points at which the
neutron-to-proton ratio is 1. The shaded
area represents the belt of stability.

(n/p ratio 1.5:1)

206
82 Pb

120

100

Number of neutrons

Belt of stability

s e C T I O N 7.1

80

Molecular Geometry

223

which represents the central atom (A). When they are as far apart as possible, they adopt the five
geometries shown in the figure. When there are only two balloons, they orient themselves to point
60
in opposite directions [Figure 7.1(a)]. With three balloons, 1.3:1)arrangement is a trigonal plane
(n/p ratio the 83Kr
36
[Figure 7.1(b)]. With four balloons, the arrangement adopted is a tetrahedron [Figure 7.1(c)]. With
five balloons, three of them adopt positions in a trigonal plane whereas the other two point opposite
40
to each other, forming an axis that is perpendicular to the trigonal plane [Figure 7.1(d)]. This
geometry is called a trigonal bipyramid. Finally, with six balloons, the arrangement is an octahe1:1 neutron-to-proton ratio
dron, which is essentially a square bipyramid [Figure 7.1(e)]. Each of the ABx molecules we consider will have one of these five electron-domain20
geometries: linear, trigonal planar, tetrahedral,
trigonal bipyramidal, or octahedral.
232

CHAPTer 7

Molecular Geometry and Bonding Theories

(n/p ratio 1.1:1)23Na
11

0

0
20
40
electron-Domain Geometry and Molecular Geometry

60

80

protons
Dipole moments can be used to the electron-domain geometry,Number of the same chemical
It is important to distinguish betweendistinguish between molecules that have is the arrangement
which
formula but different arrangements of atoms. Such compounds are and the molecular geometry,
of electron domains (bonds and lone pairs) around the central atom,called structural isomers. For
example, there are two structural isomers Figure 7.2 illustrates the molecular geometries of AB
which is the arrangement of bonded atoms.of dichloroethylene (C2H2Cl2). Because the individual
x
bond dipoles sum to zero electron domains are bonds—that is, there are no lone pairs
molecules in which all the in trans-dichloroethylene, the trans isomer is nonpolar: on any of the

Chapter 6—A small section Figure 2.9 shows a plotand bases has been versus thein conjunctionin various isotopes.
on Lewis acids of the number of neutrons added number of protons with
central atoms. In these cases, the molecular geometry is the an area of electron-domain geometry. stability. Most
The the nuclei are located of acids and bases—and the of
Lewis structures. The importance ofstable Lewis conceptinsame as thethe graphBknown as the beltimportance radioacIn an ABx molecule, a bondnuclei lie outside this belt. Above the belt of A
bonds. In an AB2
tive angle is the angle between two adjacent stability, the nuclei have higher neutron-to-proton
of molecular structure are determining acid-base properties(for angle, and, to a student’s understanding
are critical provided that there
molecule, there in only two bonds and therefore only one bond the same number of protons). are
ratios than those within the belt
no reactivity; and we atom, the bond angle is 180°. introduce ?x early have three and
of chemical lone pairs on the centralbelieve it is beneficial toAB3 and AB4 moleculesin this context. (More
it = 0
x
?y possible between any
four bonds, respectively. However, in each case there is only one bond angle= 0 16.)
comprehensive coverage of Lewis acids and bases also remains in Chapter
overall: ? = 0
two
Chapter AisB bonds. In anadded Section 2.4tono loneis 120°, and in an AB4 molecule, theand equatorial
7—We have AB3 molecule, the bond angle pairs on the central atoms. Similarly, bond
a graphic Review
illustrate more clearly the axial in an
angle 109.5°—again, provided that there are
positions ABtrigonal bipyramidal structures.
in 6 molecule, the bond angles between adjacent bonds are all 90°. (The angle between any two
y
A B bonds that point in opposite directions is 180°.)
Nuclear Stability
The bond dipoles in the cis isomerdifferent bond one another, so cis-dichloroethylene isreason for
AB5 molecules contain two do not cancel angles between adjacent bonds. The polar:
this is that, unlike those in the2.4.1 ABx molecules, the positions occupied by bonds in a trigonal nuclear radius of
other What is the density of the nucleus of an oxygen-16 atom with a
6.05 bonds that are mass of in a trigonal plane are referred
bipyramid are not all equivalent. The three × 10–3 pm and aarranged16 amu?
to as equatorial. The bond angle between any two following isotopes are ?x = 0 is 120°. The two
2.4.2
Which of the of the three equatorial bonds to be unstable?
predicted
bonds that form an axis perpendicular to thextrigonal plane are referred to as axial.
?y ? 0
(a) 46Ti (b) 20Ne (c) 72Rb
overall: ? ? 0
Axial

y
Student Annotation: Polarity is an important property that determines, in part, the
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