Group IV Elements

1
Group IV Elements
42.1 Introduction 42.2 Characteristic Properties
of the Group IV Elements 42.3 Composition of
Chlorides and Oxides of the Group IV
Elements 42.4 Silicon and Silicates
2
Introduction
The Group IV elements
? carbon ? silicon ? germanium ? tin ? lead
? exhibit a marked change (dissimilarity) among
the elements in the same group
3
Introduction
Carbon ? dull black in the form of graphite
4
Introduction
Carbon ? hard and transparent in the form of
diamond
5
Introduction
Silicon and germanium ? dull grey or black
Ge
Si
6
Introduction
Tin and lead ? shiny grey
Sn
Pb
7
Introduction
The Group IV elements ? outermost shell
electronic configuration of ns2np2
Element Electronic configuration
Carbon He 2s22p2 2,4
Silicon Ne 3s23p2 2,8,4
Germanium Ar 3d104s24p2 2,8,18,4
Tin Kr 4d 105s25p2 2,8,18,18,4
Lead Xe 4f 145d 106s26p2 2,8,18,32,18,4
8
Structure and Bonding
Moving down the group
? non-metal
carbon silicon germanium tin lead
9
Structure and Bonding
Most common structure giant covalent structure
Examples ? carbon ? silicon ? germanium
? grey tin (an allotrope of tin)
Allotropes are different structures of the same
element
10
1. Carbon
? two important allotropic forms ? diamond and
graphite
11
Structures of diamond
12
each carbon atom is bonded to four other C atoms
13
extremely hard and chemically inert
14
All electrons are localized ? non-conductor
15
Structure of graphite
16
Graphite ? layered structure
17
The layers slide over each other easily ? brittle
and soft
18
Electrons between layers are delocalized ?
conducts electricity along the layers
19
2. Silicon and Germanium
? network lattice ? the atoms are covalently
bonded to one another
20
3. Tin and Lead
Tin ? two allotropes ? white tin and grey tin
21
3. Tin and Lead
White tin ? stable form ? metallic lattice
structure ? atoms are held together by metallic
bonding
? conducts electricity ? shows the properties of
a typical metal
22
3. Tin and Lead
Grey tin ? network lattice structure ? similar to
that of diamond
White tin expands and crumbles on
cooling Napoleons retreat from Russia
23
3. Tin and Lead
Lead ? typical metallic lattice ? atoms are held
together by metallic bonding
24
Some physical properties of the Group IV elements
C Si Ge
Electronegativity value 2.5 1.74 2.0
Electronic configuration 1s22s22p2 Ne 3s23p2 Ar 3d104s24p2
Atomic radius (nm) 0.077 0.117 0.122
Bond enthalpy (kJ mol1) 347 226 188
Melting point (?C) 3527 1414 1211
Boiling point (?C) 4027 3265 2833
Enthalpy change of atomization (kJ mol1) 716 456 376
25
Some physical properties of the Group IV elements
Tin (Sn) Lead (Pb)
Electronegativity value 1.7 1.55
Electronic configuration Kr4d10 5s25p2 Xe 4f145d10 6s26p2
Atomic radius (nm) 0.140 0.154
Bond enthalpy (kJ mol1) 150
Melting point (?C) 232 327
Boiling point (?C) 2602 1749
Enthalpy change of atomization (kJ mol1) 302 195
26
Variation in Melting Point
? on going down the group
C 3527?C
Si 1414?C
Ge 1211?C
Sn 232?C
Pb 327?C
27
Variation in Melting Point
The very high m.p. of diamond is due to the
strong C C bonds the giant structure
C 3527?C
Si 1414?C
Ge 1211?C
Sn 232?C
Pb 327?C
Going from C to Ge ? bond length ? ? bond
strength ? ? melting point ?
28
Variation in Melting Point
C 3527?C
Si 1414?C
Ge 1211?C
Sn 232?C
Pb 327?C
Sn and Pb have exceptionally low m.p. because
1. metallic structures ? extent of bond breaking
on melting is small
29
Variation in Melting Point
C 3527?C
Si 1414?C
Ge 1211?C
Sn 232?C
Pb 327?C
Sn and Pb have exceptionally low m.p. because
2. only two (ns2) of the four valence electrons
are involved in the sea of electrons
Tin (Sn) Lead (Pb)
Kr4d10 5s25p2 Xe 4f145d10 6s26p2
30
Variation in Boiling Point
The general trend and explanation ? similar to
those for m.p.
C 4027?C
Si 3265?C
Ge 2833?C
Sn 2602?C
Pb 1749?C
31
Chlorides
Two series of chlorides formed by the Group IV
elements ? the dichlorides (MCl2) ? the
tetrachlorides (MCl4)
32
Chlorides
All Group IV elements ? form tetrachlorides
CCl4
SiCl4
GeCl4
SnCl4
PbCl4
? liquids at room temperature and
pressure ? all are simple covalent molecules
with a tetrahedral shape
33
M Cl bonds are polar with ionic character
Molecules as a whole are non-polar
34
Reactions with water
CCl4 H2O ? no reaction SiCl4 H2O ?
Si(OH)Cl3 HCl
35
?
Si in SiCl4 is more positively charged than C in
CCl4 ? More susceptible to nucleophilic attack
Si, unlike C, can expand its octet to accept an
additional electron pair
36
?
H4SiO4, silicic acid
37
Reactions with water
CCl4 H2O ? no reaction SiCl4 H2O ?
Si(OH)Cl3 HCl
Si(OH)Cl3 H2O ? Si(OH)2Cl2
HCl Si(OH)2Cl2 H2O ? Si(OH)3Cl
HCl Si(OH)3Cl H2O ? Si(OH)4 HCl
H4SiO4, silicic acid
38
Chlorides
? tendency to form dichlorides, MCl2 down the
group
-
-
GeCl2
SnCl2
PbCl2
? all possess covalent character though they
exist as crystalline solids at room temperature
and pressure
39
Chlorides
On moving down the group, Metallic character of
elements ? Ionic character of MCl2 ?
-
-
GeCl2
SnCl2
PbCl2
40
Chlorides
On moving down the group, ? the relative
stability of 4 oxidation state ? ? the relative
stability of 2 oxidation state ?
41
Tin (Sn) Lead (Pb)
Kr4d10 5s25p2 Xe 4f145d10 6s26p2
The outermost ns2 electrons are less shielded by
the more diffused inner d and/or f electrons.
? They are attracted more by the positive
nucleus ? Less available for forming
bonds ? Form only two bonds using np2
42
Oxides
Two series of oxides formed by the Group IV
elements ? the monoxides (MO) ? the dioxides (MO2)
43
Oxides
All Group IV elements ? form the dioxides
Carbon dioxide ? the only Group IV dioxide which
consists of simple molecules ? exists as a gas
at room temperature and pressure
44
Oxides
The dioxides of other Group IV elements ? crystall
ine solids of high melting points ? either giant
covalent or giant ionic structures
45
Oxides
All Group IV elements (except silicon) ? form the
monoxides at normal conditions
CO
-
GeO
SnO
PbO
? Stability of MO ? down the group
46
Oxides
CO2
SiO2
GeO2
SnO2
PbO2
47
Tin (Sn) Lead (Pb)
Kr4d10 5s25p2 Xe 4f145d10 6s26p2
The outermost ns2 electrons are less shielded by
the more diffused inner d and/or f electrons.
? They are attracted more by the positive
nucleus ? Less available for forming
bonds ? Form only two bonds using np2
48
The bond type and the relative stabilitiy of the
monoxides and dioxides formed by the Group IV
elements
Group IV element Oxides formed Bond type of the oxide Relative stability
Carbon CO Covalent Unstable (reducing)
Carbon CO2 Covalent Stable
Silicon (SiO) Very unstable
Silicon SiO2 Covalent Stable
Germanium GeO Predominantly ionic Unstable in the presence of O2
Germanium GeO2 Partly ionic, partly covalent Stable
49
The bond type and the relative stabilitiy of the
monoxides and dioxides formed by the Group IV
elements
Group IV element Oxides formed Bond type of the oxide Relative stability
Tin SnO Predominantly ionic Unstable (reducing)
Tin SnO2 Partly ionic, partly covalent Unstable (oxidizing)
Lead PbO Ionic Stable
Lead PbO2 Predominantly ionic Unstable (oxidizing)
50
Silicon and Silicates
Silicon ? the second most abundant element in
the Earths crust ? about 28 by mass
? commonly found as silicon oxide (also known
as silica)
51
(No Transcript)
52
Silicon
Example ? in a variety of forms such as sand,
quartz and flint ? also found as silicates in
rocks and clay
53
Preparation of Silicon
1. by reduction of silica with carbon in an
electric furnace SiO2(s) 2C(s) ? Si(s) 2CO(g)
2. Extremely pure silicon can be obtained by
the reaction of silicon(IV) chloride with
hydrogen SiCl4(s) 2H2(g) ? Si(s) 4HCl(l)
followed by zone refining of the resultant
silicon
54
Applications of Silicon
Silicon is the basic material ? for making
semi-conductors used in the construction of
transistors and rectifiers ? for making steel and
aluminium alloys
55
Chemistry of Silicon
? dominated by its strong tendency to form SiO
single bond ? reflected by its formation of
silica and a variety of silicates
Silicates consist of Si, O and one or more
metals
56
Structures and Bonding of Silicates
1. SiO44 as the Basic Chemical Unit of Silicates
one electron (symbol O) of each oxygen atom is
gained from another atom (usually metal atom)
57
SiO44?
58

• For simplicity, a SiO44 tetrahedron can be
  represented by a pyramid as follows

Si gt O not drawn to scale
59
Types of silicates

• Nesosilicates (lone tetrahedron) - SiO44-, eg
  olivine.
• Sorosilicates (double tetrahedra) - Si2O76-, eg
  epidote.
• Cyclosilicates (rings) - SinO3n2n-, eg
  tourmaline group.
• Inosilicates (single chain) - SinO3n2n-, eg
  pyroxene group.
• Inosilicates(double chain) - Si4nO11n6n-, eg
  amphibole group. (not required in A-Level)
• Phyllosilicates (sheets) - Si2nO5n2n-, eg micas
  and clays.
• Tectosilicates (3D framework) -
  AlxSiyO2(xy)x-,
• eg quartz, feldspars.

http//www.windows.ucar.edu/tour/link/earth/geolo
gy/silicates2.html
60
Different structures of silicates
1. Isolated silicates

• Contain isolated SiO44 tetrahedra.

61
1. Isolated silicates

• are not polymerized.

• are bonded to the metal ions (e.g. Mg2 or Fe2)
  by ionic bonds.

• tend to have high densities and are not easy to
  cleave.

62

• Olivine ((Mg,Fe)2SiO4) is an example of isolated
  silicates.

• It is the most abundant mineral in the Earths
  mantle.

63
Deducing the chemical formula of an isolated
silicate, (Fe,Mg)2SiO4
The diagram below shows an incomplete part of an
isolated silicate. This part, when complete, can
repeat infinitely to give a three-dimensional
structure of the silicate.
64
(a) Add an appropriate number of
(representing iron(II) ion) in the figure above
to make up a complete part of the silicate.
Charge on each Mg2 ion is 2 Charge on each SiO4
4 ion is 4, the net charge of the incomplete
part (2)(8) (4)(8) 16
To balance the charge, the number of Fe2
ions needed 8
65
For regular packing of particles, one of the
feasible way of putting Fe2 ions is shown below
(b) Hence, deduce the chemical formula of the
silicate.
From the picture, the formula of the part is
Fe8Mg8(SiO4)8 ? FeMgSiO4.
66
M1 Mg2 M2 Fe2
Fe4Mg4(SiO4)4 ? FeMgSiO4.
No. of Fe2 4
No. of Mg2 4?¼ 4?½ 1 4
67
1. SiO44 as the Basic Chemical Unit of Silicates

• Zircon (ZrSiO4)
• ? an example of isolated silicate mineral
• ? the principal ore of zirconium metal

68
1. SiO44 as the Basic Chemical Unit of Silicates

• Zircon (ZrSiO4)
• ? brilliant appearance
• ? high refractive index
• ? used as a diamond- like gem

69
Double tetrahedra Sorosilicates
The Si2O76 anion is formed by joining two SiO44
tetrahedra together through a common oxygen atom
The negative charges are present only on the
oxygen atoms that are not shared by the two
silicon atoms
70
Double tetrahedra Sorosilicates
71
Double tetrahedra Sorosilicates
72
2. Structures of Silicates

• The SiO44 tetrahedra can be joined up
• ? by sharing oxygen atoms
• ? form ring, chain, sheet or network silicates

73
Cyclosilicates (rings) - SiO3n2n-
SiO362?6?
Si6O1812?
74
Cyclosilicates (rings) - SiO3n2n-
SiO362?6?
Si6O1812?
75
Single chain silicates

• Each SiO44 tetrahedron possesses 1 Si atom and 3
  (i.e. 1 1 0.5 0.5) O atoms.

• The ratio of Si to O is 1 3.

• Stoichiometry (SiO3)n2n

76
Single chain silicates
77

• Draw a diagram to show that when two oxygen atoms
  of each SiO44 tetrahedron are used for sharing,
  an infinite polymer chain can be formed.
• Your diagram should have four SiO4 4 tetrahedra.

78
(b) Hence, show that the general formula of the
silicate is (SiO3)n2n.
Number of Si atoms 4 Number of O atoms 11
(inside the chain) ? 0.5 2 (ends of chain)
12 Charge on oxide ?8 (bridging O has no
charge) ? the formula is Si4O128? ? the general
formula is (SiO3)48? or (SiO3)n2n?
79

• Single chain silicates - pyroxenes

Generally dark-coloured Commonly found in
igneous rocks
80
Pyroxene minerals

• Schefferite, Ca(Mg,Fe,Mn)Si2O6
• Zinc schefferite, Ca(Mg,Mn,Zn)Si2O6
• Jeffersonite, Ca(Mg,Fe,Mn,Zn)Si2O6
• Leucaugite, Ca(Mg,Fe,Al)(Al,Si)2O6
• Calcium-Tschermak's molecule, CaAlAlSiO6

3
81
Double chain silicates

• Every other tetrahedron in a single chain shares
  a third oxygen atom with a neighbouring chain.

82
Double chain silicates
In every two SiO44 tetrahedra,
Si O 2 5.5 4 11
No. of Si 2
Stoichiometry Si4O116
No. of O
5.5
83
-6?2
-1?2
2?7

• Amphibole (Ca2Mg5(Si4O11)2(OH)2) is an example of
  double chain silicates.

84
It is more difficult to cleave across the chains
For both single chain silicates and double chain
silicates, the SiO covalent bonds within the
chain silicate crystals are strong.
85
It is easier to cleave along the chain direction
The ionic bonds between the metal ions and
polymeric silicate chains are comparatively
weaker than the SiO covalent bonds.
86
Also, cleave between the front and the back
chains
Two planes of cleavage are perpendicular to each
other
87
Chain silicate anions are found in fibrous
asbestos
88
Single chain silicates

• Glass CaSiO3
• Limestone CaCO3

• long chain anions
• ? can be drawn into long wire or glass wool.

? Individual anions ? hard and brittle
89
Carbonate ions do not exist as long chains. Why?
Carbon can form strong ? bond with
oxygen. Moreover, carbonate ions can be
stabilized by delocalization of ? electrons.
90
Also, polymerization of CO32? ions gives
polymeric (CO2)2 which is electrically neutral
91
Sheet silicates
Each SiO44 tetrahedron shares three oxygen atoms
with neighbouring SiO44 tetrahedra
92
Each tetrahedron
has 1 Si atom and 2.5 O atoms (10.50.50.5)
carries one negative charge
93
(SiO2.5)nn?
or (Si2O5)n2n?
or (Si4O10)n4n?
94
Structure of sheet silicates
95

• Talc (Mg3(Si4O10)(OH)2) is an example of sheet
  silicates.

• It is an important filler material for paints,
  rubber and insecticides.

Talc(??)
96
Only weak van der Waals forces exist between the
sheets of the SiO44 anions ? only one plane of
cleavage
97

• Since talc is a relatively soft silicate, it can
  be pulverized to make talcum powder. It is a soft
  and fine powder used after bath to make our body
  feel smooth and dry.

• It is slippery because the layers can slide over
  one another

98
Sheet silicates
Mica and clay readily cleave into thin slices
(only one plane of cleavage)
99
Sheet silicates
Mica is used as window glass and waveguide cover
in microwave oven
100
4. Network silicates
Consist of SiO44 tetrahedra in which ALL oxygen
atoms are shared with adjacent SiO44 tetrahedra
? Electrically neutral
Each tetrahedron has one Si atom and 2 O atoms
(4?0.5)
? Si O 1 2
? SiO2 (Quartz)
101
4. Network silicates

• No plane of cleavage
• much harder than other types of silicates
• very high melting points

102
4. Network silicates
Examples-
103
4. Network silicates
Feldspar
- the most abundant group of minerals in the
Earths crust
- every oxygen atom is shared between SiO44
tetrahedra - some Si atoms are replaced by Al
atoms
Formula of anions (AlO2)x(SiO2)yx-
or AlxSiyO2(xy)x?
104
4. Network silicates
Feldspar
Formula of anions (AlO2)x(SiO2)yx- or
AlxSiyO2(xy)x?
when x0 ? SiO2
105
Types of Feldspar

• K-feldspar endmember
• KAlSi3O8
• Albite endmember
• NaAlSi3O8
• Anorthite endmember
• CaAl2Si2O8

? K(AlO2)(SiO2)3?
? Na(AlO2)(SiO2)3?
? Ca2(AlO2)2(SiO2)22?
106
Comparison of different types of silicates
107
(No Transcript)
108
(No Transcript)
109
(No Transcript)
110
Predict the type of each silicate from its
chemical formula.
(a) CaMg(SiO3)2
single chain
(b) Fe3Al2Si3O12
isolated
(c) Si12Mg8O30(OH)4
sheet
(d) Na2Mg3Al2Si8O22(OH)2
double chain
(e) Mg(Al2Si2O8)
network (feldspar)
single chain
(f) MgAl(AlSiO6)