Chapter 24
Simple molecular substances with
non-octet structures and shapes of
simple molecules
24.1 Simple molecular substances with nonoctet structures
24.2 Shapes of simple molecules
Key terms
Progress check
Summary
Concept map
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24.1 Simple molecular substances with
non-octet structures
Noble gases are very stable. Their atoms have
either a duplet or an octet structure.
The strong tendency of all atoms to attain the
stable electronic arrangement of the nearest
noble gas is known as the octet rule.
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Atoms of non-metallic elements other than noble
gases can share their outermost shell electrons to
achieve an octet structure, forming molecules.
(a)
(b)
(c)
Figure 24.1 Electron diagrams of molecules of (a) chlorine, (b) carbon
dioxide and (c) methane.
24.1 Simple molecular substances with non-octet structures
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Are there any exceptions
to the octet rule?
24.1 Simple molecular substances with non-octet structures
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Non-octet structure with less than eight outermost
shell electrons
Boron trifluoride (BF3)
In a BF3 molecule, the boron atom forms a single
covalent bond with each of the three fluorine atoms.
The boron atom contains only six outermost
shell electrons .
BF3 has a non-octet structure.
Figure 24.2 Electron diagram of a boron
trifluoride, BF3 molecule. (Outermost shell
electrons of boron atom in blue colour.)
Example 24.1
24.1 Simple molecular substances with non-octet structures
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Non-octet structure with more than eight
outermost shell electrons
Phosphorus pentachloride (PCl5)
In a PCl5 molecule, the phosphorus atom forms a
single covalent bond with each of the five chlorine
atoms.
The phosphorus atom contains a total of 10
outermost shell electrons.
PCl5 has a non-octet structure.
Figure 24.3 Electron diagram of a phosphorus
pentafluoride, PCl5 molecule.(Outermost shell
electrons of phosphorus atom in blue colour.)
24.1 Simple molecular substances with non-octet structures
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Sulphur hexafluoride (SF6)
In a SF6 molecule, the sulphur atom forms a
single covalent bond with each of the six fluroine
atoms.
The sulphur atom contains a total of 12
outermost shell electrons.
SF6 has a non-octet structure.
Figure 24.4 Electron diagram of a sulphur
hexafluoride, SF6 molecule. (Outermost shell
electrons of sulphur atom in blue colour.)
24.1 Simple molecular substances with non-octet structures
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Key point
Some simple molecular substances have nonoctet structures. Certain atoms in their molecules
have less than or more than eight electrons in
their outermost shells.
Class practice 24.1
24.1 Simple molecular substances with non-octet structures
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24.2 Shapes of simple molecules
The shape of a simple molecule is mainly
determined by the arrangement of the electron
pairs around the central atom.
The electrons pairs around the central atom
tend to minimize the repulsion between them;
are arranged as far apart from each other as
possible.
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How can we predict the
shape of a molecule?
24.2 Shapes of simple molecules
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To predict the shape of a molecule, we can follow
the steps in the flow chart below:
Draw the electron diagram of the molecule
Count the total number of electron pairs around
the central atom of the molecule
Find out the spatial arrangement of electron pairs
that can minimize the repulsions between them
Predict the shape of the molecule based on the
positions of the bonded atoms
24.2 Shapes of simple molecules
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Molecules with central atom obeying octet rule
Methane (CH4)
In a CH4 molecule, there are four bond pairs of
electrons around the central carbon atom. They
repel each other.
The four hydrogen atoms lie at the corners of a
tetrahedron with the carbon atom at the centre.
24.2 Shapes of simple molecules
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The CH4 molecule is tetrahedral in shape.
All four H–C–H bond angles are 109.5°°.
(a)
(b)
(c)
Figure 24.5 (a) Electron diagram of a methane, CH4 molecule. (b) The CH4
molecule is tetrahedral in shape. (c) A ball-and-stick model of a CH4
molecule.
Concept check
24.2 Shapes of simple molecules
Skill corner 24.1
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Do all molecules with four
electron pairs around the
central
atom
have
a
tetrahedral shape?
24.2 Shapes of simple molecules
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Ammonia (NH3)
In an NH3 molecule, there are one lone pair and
three bond pairs of electrons around the central
nitrogen atom.
The four electron pairs repel each other and take
up the tetrahedral arrangement.
24.2 Shapes of simple molecules
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The shape of a molecule depends only on the
positions of the bonded atoms.
∴ An NH3 molecule is trigonal bipyramidal in
shape.
(a)
(b)
(c)
Figure 24.6 (a) Electron diagram of an ammonia, NH3 molecule. (b) The
NH3 molecule is trigonal pyramidal in shape. (c) A ball-and-stick model of an
NH3 molecule.
24.2 Shapes of simple molecules
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The lone pair of electrons
is not shared between two atoms;
is held closer to the nitrogen atom than the
bond pair of electrons;
exerts a greater repelling effect than the
bond pair of electrons.
The H–N–H bond angles are squeezed to a
value of about 107°°.
24.2 Shapes of simple molecules
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Water (H2O)
In a H2O molecule, there are two lone pairs of
electrons and two bond pairs of electrons
around the central oxygen atom.
The four electron pairs repel each other and take
up the tetrahedral arrangement.
24.2 Shapes of simple molecules
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Based on the positions of the bonded atoms, a
water molecule is V-shaped.
(a)
(b)
(c)
Figure 24.7 (a) Electron diagram of a water, H2O molecule. (b) The H2O
molecule is V-shaped. (c) A ball-and-stick model of a H2O molecule.
24.2 Shapes of simple molecules
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The two lone pairs of electrons exert a greater
repelling effect than the two bond pairs of
electrons.
The H–O–H bond angles are squeezed to a
value of about 104.5°°.
Example 24.2
24.2 Shapes of simple molecules
Example 24.3
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Class practice 24.2
Molecules with central atom not obeying octet rule
and with no lone pair of electrons
Beryllium fluoride (BeF2)
In a BeF2 molecule, there are two bond pairs
of electrons around the central beryllium
atom.
The two electron pairs repel each other and lie
on the opposite sides of the beryllium atom.
24.2 Shapes of simple molecules
Think about
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The BeF2 molecule is linear in shape.
The F–Be–F bond angle is 180°°.
(a)
(b)
(c)
Figure 24.8 (a) Electron diagram of a beryllium fluoride, BeF2 molecule.
(b) The BeF2 molecule is linear in shape. (c) A ball-and-stick model of a
BeF2 molecule.
24.2 Shapes of simple molecules
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Boron trifluoride (BF3)
In a BF3 molecule, there are three bond pairs
of electrons around the central boron atom.
The fluorine atoms lie at the corners of an
equilateral triangle with the boron atom at the
centre.
The molecule is trigonal planar in shape.
24.2 Shapes of simple molecules
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All three F–B–F bond angles are 120°° and all
atoms are lying in the same plane.
(a)
(b)
(c)
Figure 24.9 (a) Electron diagram of a boron trifluoride, BF3 molecule. (b)
The BF3 molecule is trigonal planar in shape. (c) A ball-and-stick model
of a BF3 molecule.
24.2 Shapes of simple molecules
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Phosphorus pentachloride (PCl5)
In a PCl5 molecule, there are five electron pairs
around the central phosphorus atom.
The five electron pairs repel each other.
Three chlorine atoms are in the same plane and
lie at the corners of an equilateral triangle.
The other two chlorine atoms lie above and
below the plane of the triangle.
24.2 Shapes of simple molecules
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The PCl5 molecule is trigonal bipyramidal in
shape.
(a)
(b)
(c)
(d)
Figure 24.10 (a) Electron diagram of a phosphorus pentachloride, PCl5
molecule. (b) The PCl5 molecule is trigonal bipyramidal in shape. (c) A trigonal
bipyramid showing the spatial arrangement of electron pairs around the central
P atom in a PCl5 molecule. (d) A ball-and-stick model of a PCl5 molecule.
24.2 Shapes of simple molecules
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The Cl–P–Cl bond angles within the plane of the
equilateral triangle are 120°°.
The chlorine atoms above and below the plane of
the triangle
are 180°° apart; and
are perpendicular (at 90°) to the plane of the
triangle.
24.2 Shapes of simple molecules
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Sulphur hexafluoride (SF6)
In a SF6 molecule, there are six electron pairs
around the central sulphur atom.
The six electron pairs repel each other.
Four fluorine atoms are in the same plane and
lie at the corners of a square.
The other two fluorine atoms lie above and
below the plane of the square.
24.2 Shapes of simple molecules
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The SF6 molecule is octahedral in shape.
All F–S–F bond angles are 90°°.
(a)
(b)
(c)
(d)
Figure 24.11 (a) Electron diagram of a sulphur hexafluoride, SF6
molecule.(b) The SF6 molecule is octahedral in shape. (c) An octahedron
showing the spatial arrangement of electron pairs around the central S
atom in a SF6 molecule. (d) A ball-and-stick model of an SF6 molecule.
24.2 Shapes of simple molecules
Example 24.4
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Number of
electron pairs
around central
atom (a double
bond or a
triple bond is
treated as ‘one’
electron pair)
Number
of bond
pairs
around
central
atom
2
2
0
Linear
3
3
0
Trigonal
planar
4
4
0
Tetrahedral
Number
Shape of
of lone
molecule
Example Example
pairs (determined (obeying
(not
around
by the
octet
obeying
central position of
rule)
octet rule)
atom
atoms)
F–Be–F
Table 24.1 Shapes of molecules with different numbers of electron pairs
around the central atoms.
Simulation (Shapes of molecules)
24.2 Shapes of simple molecules
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Number of
electron pairs
around central
atom (a double
bond or a
triple bond is
treated as ‘one’
electron pair)
Number
of bond
pairs
around
central
atom
4
3
1
Trigonal
pyramidal
4
2
2
V-shaped
Number
Shape of
of lone
molecule
Example Example
pairs (determined (obeying
(not
around
by the
octet
obeying
central position of
rule)
octet rule)
atom
atoms)
Table 24.1 Shapes of molecules with different numbers of electron pairs
around the central atoms.
24.2 Shapes of simple molecules
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Number of
electron pairs
around central
atom (a double
bond or a
triple bond is
treated as ‘one’
electron pair)
Number
of bond
pairs
around
central
atom
5
5
0
Trigonal
bipyramidal
6
6
0
Octahedral
Number
Shape of
of lone
molecule
Example Example
pairs (determined (obeying
(not
around
by the
octet
obeying
central position of
rule)
octet rule)
atom
atoms)
Table 24.1 Shapes of molecules with different numbers of electron pairs
around the central atoms.
24.2 Shapes of simple molecules
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Key point
The total number of electron pairs around the
central atom of a molecule determines their spatial
arrangement, but the shape of the molecule
depends only on the positions of the bonded
atoms.
24.2 Shapes of simple molecules
Class practice 24.3
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Key terms
1.
2.
3.
4.
5.
6.
7.
8.
9.
linear 線形
non-octet structure 非八隅體結構
octahedral 八面體形
octet rule 八隅體規則
tetrahedral 四面體形
trigonal bipyramidal 三角雙錐體形
trigonal planar 平面三角形
trigonal pyramidal 三角錐體形
V-shaped V形
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Progress check
1. Which simple molecule has a central atom with
less than eight electrons in the outermost shell?
2. Which simple molecule has a central atom with
more than eight electrons in the outermost shell?
3. How do we draw electron diagrams of molecules
with non-octet structures such as BF3, PCl5 and
SF6?
4. What is the main factor that determines the shape
of a molecule?
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5. What are the shapes of simple molecules such as
BeF2, BF3, PCl5, SF6, CH4, NH3, H2O, CO2 and
HCOH?
6. How do we draw three-dimensional diagrams to
represent the shapes of simple molecules such as
BeF2, BF3, PCl5, SF6, CH4, NH3, H2O, CO2 and
HCOH?
Progress check
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Summary
24.1 Simple molecular substances with non-octet
structures
1.
Some simple molecular substances have nonoctet structures. Certain atoms in their
molecules have less than or more than eight
electrons in their outermost shells.
2.
The boron atom in BF3 has only six outermost
shell electrons.
3.
The phosphorus atom in PCl5 and the sulphur
atom in SF6 have 10 and 12 outermost shell
electrons respectively.
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24.2 Shapes of simple molecules
4.
The electron pairs surrounding the central atom
tend to minimize the repulsion between them.
They are arranged as far apart from each other
as possible.
5.
The shape of a molecule is mainly determined
by the arrangement of the electron pairs
around the central atom. It is distorted by the
presence of lone pair of electrons. The actual
shape of a molecule refers to the positions of
the bonded atoms.
Summary
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6.
The shapes and three-dimensional diagrams of
simple molecules such as BeF2, BF3, CH4, NH3,
H2O, PCl5, SF6, CO2 and HCOH are
summarized in Table 24.1 on p.14.
Summary
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Concept map
BeF2
NH3, H2O, CH4
examples
4 electrons
in the
outermost
shell
octet
___________
structure
all atoms have 8
electrons in the
outermost shell
BF3
PCl5
6 electrons
in the
outermost
shell
SF6
10 electrons
in the
outermost
shell
12 electrons
in the
outermost
shell
examples
non-octet
___________
structure
certain atoms have more or
less than 8 electrons in the
outermost shell
SIMPLE MOLECULAR SUBSTANCES
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SIMPLE MOLECULAR SUBSTANCES
Molecular shape
mainly determined by
the arrangement of
electron pairs
Number of ________________
around the central atom
2 bond
pairs
Linear
e.g. BeF2,
CO2
3 bond
pairs
trigonal
_______
planar
_______
e.g. BF3,
HCOH
Concept map
4 electron pairs
1 lone pair
2 lone pairs
4 bond
3 bond pairs
2 bond pairs
pairs
trigonal V-shaped
________
tetrahedral
________ __________
pyramidal
________ e.g. H2O
e.g. CH4
e.g. NH3
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SIMPLE MOLECULAR SUBSTANCES
Molecular shape
mainly determined by
the arrangement of
electron pairs
Number of ________________
around the central atom
5 bond
pairs
trigonal
___________
bipyramidal
___________
e.g. PCl5
6 bond
pairs
___________
octahedral
e.g. SF6
Concept map
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