Download STRUCTURES OF CRYSTALS

L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
METALLIC
I.
Chpt.10:p.1
BONDING
Metallic Bonding
<1> The outershell electrons of a metal (the valency electrons) are relatively easily removed,
with the formation of cations.
<2> Metallic bonding can be illustrated by a model of cationic lattice and mobile valency
electrons. In this model,
• Metals are regarded as existing as ions occupying fixed positions in a crystal lattice.
• The valency electrons are moving in orbitals enveloping the whole lattice.
• The valency electrons are delocalized through the lattice and attract the positive ions
electrostatically, thereby, holding the lattice together.
<3> According to the model of the metal 1attice:
• In a metallic crystal. positive ions of the metal are packed in a regular array within a
“sea of electrons” denoted by the metal atoms.
• The electrons are delocalized throughout the lattice. producing a mobile electrons
cloud.
• Since neigiibouring cations are mutually attracted to this electron cloud, they will also
be attracted to each other. These attractive forces exactly balance the repulsion
between the like-charged cations. This constitutes the metallic bond.
<4> CONCLUION
Metallic bonding comprises an array of cations held together in a sea of electrons
liberated from atoms.
It is the “sea of electrons” that binds the positive atoms together.
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
Chpt.10:p.2
Note : Metallic conduction or electricity
<1> Metals can conduct an electric current because their valency electrons are delocalized in
orbitals extending over the whole lattice.
<2> The electrons are relatively mobile so that they can move throughout the crystal (to the
positive terminal) under applied potential difference.
II.
Metallic Radius
Metallic radius is defined as half the internuclear distance between tha atoms in a metallic
crystal.
Note : Metallic radii
<1> increase down a qroup : Reason______________________________________________
<2 > decrease across a period : Reason ____________________________________________
<3> Values of metallic radius are slightly higher than the corresponding covalent radii. This
suggests that bonding in a metallic crystal is weaker than the covalent bonding.
III.
Strength of Metallic Bonds
The strength of metallic bond is a measure of the electrostatic attraction between the
delocalized electron cloud and the position metallic cations.
The strongest metallic bonds will be formed between the cations and electrons cloud which
have the highest charge density, which in turn leads to greater mutual attraction between them.
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
Chpt.10:p.3
The strength of metallic bonds increases as
• the number of valency electrons per atom increases (such that the charge density of
electrons cloud increases)
• the nuclear charge increases and the metallic radii decreases (such that the charge
density of the metallic cation increases)
Consider the following representations of metallic bonds:
Interpretation
<1> From A to B to C, there is an increase in
(a) the density of the mobile electron cloud because
______________________________________________________________
(b) the charge density (i.e. charge/size) of the cations.
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<2> Metallic bond strength increase in the order : ____________
Note
In general. if the metallic bonds are strong, the metal will be hard and has a high
melting point.
Example : Metallic elements in period three
Na (m.p. 98°C)
soft
Mg (m.p. 650°C)
Al (m.p. 659°C)
hard
Aluminium has the greatest metallic bond strength among the three elements,
and hence the highest melting point.
Exercise 1
Arrange the following elements in order of increasing melting points:
potassium, calcium, scandium
Explain your order in terms of the metallic bond strength involved
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
Chpt.10:p.4
Exercise 2
Arrange the following group I elements in order of decreasing melting points:
lithium, sodium, potassium, rubidium. caesium
Explain your order in terms of the metallic bond strength involved.
___________________________________________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
IV.
Metallic and non—metallic character
(A)
Metallic character
Metallic character is defined as the tendency of an atom to form positive ions:
M  Mn+ + ne
<1> Large atoms have their valency electrons most far away from the attraction of the nucleus.
Thus, they tend to undergo the above change to form positive ions.
<2> Metallic character follows a similar pattern in the Periodic Table as that for ionization
enthalpies and atomic size:
Metallic Charcter increases down a group and towards the left hand side of the
PeriodicTable.
<3> The alkali metal (Group I element) exhibits the most metallic character (and reactivity) in
each Period.
(B)
Non-metallic character
Non-metals are electronegative in nature. They tend to acquire electrons to form negative ions.
The general pattern for increasing non-metallic character and reactivity follows that of
increasing electronegativity values.
The halogens (Group VII elements) are therefore the most reactive non-metals in each Period.
Fluorine is the most reactive nonmetal.
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
III.
Chpt.10:p.5
Metallic Crystals
Most metals crystallize with one of the three structures:
Close packed structure
Hexagonal close
packed
Packing efficiency
High (74%)
Arrangement of atoms
ababab... pattern
Coordination number
Note:
Face centred cubic
or
Cubic close packed
Open structure
Body centred cubic
Lower (68%)
abcabcabc ... pattern
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ababab ... pattern
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The type of packing in a metal crystal adopts is important because it affects such
physical properties as melting point, boiling point, density, hardness and malleability,
etc.
(A) Close packed structures : Cubic close-packed and Hexagonal close-packed
<1> In a close packed structure,
• the spheres occupy the minimum volume
• maximum lattice stability is attained.
• each sphere is in contact with 12 other spheres, i.e. has a coordination number of 12.
<2> In close-packed structures, the units are packed together in the tightest possible way to fill
the maximum amount (74%) of available space. Each sphere is surrounded hexagonally
by six others - Consider this layer as layer A.
A second layer B of spheres is then filled into the holes an the first layer such that each
sphere in B touches 3 sphere in A. This gives the tightest packing for two layers
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
Chpt.10:p.6
There are now two alternative ways of fitting in the third layer of spheres on B, each giving
equally close-packed structure. This gives rise to hexagonal close-packed and cubic
close-packed structures.
Hexagonal close-packed structure (h.c.p.)
The spheres of the third layer are packed directly above the first layer in hollows marked “x”. This
gives a pattern that repeat itself every third layer, i.e. abababa. . .. structure.
Any particular sphere is in contact with 6 spheres in the same layer, 3 spheres in the layer above
and another 3 in the layer below. Thus, the coordination number of close-packed structure is 12.
Cubic close-packed structure (c.c.p.)/face-centred cubic structure (f.c. p.)
The third layer of spheres are packed in hollows marked “” .This gives a pattern that repeats itself
every forth layer, i.e. abcabcabc. . . structure.
This structure is also known as the face-centred cubic structure for it has atoms at each corner and each
face centre of a cube.
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
Note:
Chpt.10:p.7
tetrahedral holes and octahedral holes
<1> Tetrahedral hole (t) ( 4-spheres )
It is the space between 3 spheres of one layer and a fourth sphere on the upper layer.
The centres of the four spheres form a regular tetrahedron.
<2> Octahedral hole(o)
( 6-sphere )
It is the space between 3 spheres of on~ layer and three other spheres on the upper
layer. The centres of the six spheres form a regular octahedron.
Exercise: The diagram below shows a unit cell representation of a crystal structure.
(a) What type of crystal structure does this unit cell represent?
(b) What is the coordination number in this crystal structure? Calculate the number of atoms present in
a unit cell.
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
(B)
Chpt.10:p.8
Body-centred cubic structure
This is the less efficient mode of packing commonly found in alkali metals (Group I elements).
It utilizes only 68% of the available space.
Body-centred cubic structure has 8 atoms at the corners of a cube and 1 atom at the centre.
Each sphere has 8 nearest neighbours, thus the coordination number is 8.
The first layer of atoms are organized in this manner:
A second layer of spheres fit on the first layer such that each sphere on the first layer is in
contact with 4 other spheres in the second layer. This gives an ababab... arrangement.
Note:
Almost all metals crystallize into one or more of the above structures. Some examples are given below:
Hexagonal close packing (h.c.p.) Mg, Zn, Ni, Ti, Be (most are transition metal)
Cubic close packing (c.c..p.) or
face-centred cubic (f.c.p.)
Cu, Ag, Au, Al, Ca, Fe
(Group II ,III and transition metal )
Body centred cubic packing
(b.c.c.)
Li, Na, K, Rb, Cs (alkali metals)
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
Chpt.10:p.9
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
Chpt.10:p.10
STRUCTURES OF CRYSTALS
I. Ionic Crystals
<1> The arrangement of ions within an ionic lattice is governed by the number of, and the
charges on, the ions and their relative size.
<2>
The stoichiometric ratio of cations and anions also affect the type of crystal structure
formed.
<3> The ions are packed in an orderly manner for stability such that
• each ion tends to have the maximum number of neighbours (ions of unlike charge),
• the lattice is electrically neutral and
• maximum lattice enthalpy is attained.
<4> Coordination number
The coordination number of an ion in a crystal of ionic compound is the number of
oppositely charged ions immediately surrounding that ion.
<5> Unit cell
A unit cell is the smallest part of the crysta that
contains all the characteristics of the structure.
The whole structure can he generated by repeating the unit cell
in 3 directions.
II.
Structure of Ionic crystal
<1> Ionic structure can be regarded as interpenetrating lattices of cations and anions.
<2>
The formula of the compound can be c3lcuiated from the coordination numbers of the
ions’ in the crystal
<3>
For MmXX (where M represents the cation and X represents the anion ).
C.N. (X) : C.N. (M) = m : x
Sodium chloride
-
The sodium chloride lattice is close packed. It consists of two penetrating face centred
cubic lattices, one of Na+ ions and the other Cl- .
-
Each Na+ has six Cl- ions as nearest neighbours; the coordination number of sodium is 6.
Each Cl- is surrounded immediately by six Na+ ions; the coordination number of
chlorine is 6. Therefore, sodium chloride exhibits 6:6 coordination.
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
Chpt.10:p.11
Sodium chloride exhibit 6:6 coordination
Caesium chloride
• Since caesium ch1oride is larger than sodium ion, caesium chloride has to adopt an open structure.
caesium chloride is not close packed.
• The caesium chloride lattice consists of two interpenetrating cubic lattices of Cs+ and Cl- ions
• Each ion has 8 oppositely charged ions as its nearest neigbhours. Therefore, caesium chloride has
8:8 co-ordinataon.
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
Chpt.10:p.12
Exercise 1:
Sodium and caesium are both Group I elements. Both of them form ionic chlorides with the
formula MCl (where M represents the metal) with different crystal structures.
(a) Describe the interaction within the crystals of these chlorides.
(b) Why do these two chlorides have different structures?
Exercise 2 :
The diagram below is a unit cell of an oxide of titanium:
(a)
(h)
(c)
Give the co-ordination number of titanium.
State the number of titanium ions and oxygen ions per unit cell. Explain your answer.
Write down the formula of the compound.
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
II.
Chpt.10:p.13
Covalent Crystals : Diamond, graphite and Quartz
In giant covalent crystals. atoms are bonded by strong covalent bonds only. The properties of
the crystals are governed by the geometry and the strength of the bonds.
Diamond and graphite are allotropes of carbon. They have different structures.
(A)
Diamond
Each carbon atom is bonded tetrahedrally to four neighhouring carbon atoms by identical and
directional strong C—C covalent bonds (0.154 nm). This gives a three dimensional covalent
network structure.
Diamond: Tetrahedral arrangement of carbon atoms around a central carbon atom
(B) Graphite
<1> Bonding within a layer
Each carbon atom is bonded trigonally to three carbon atoms in the same layer by
identical and directional C—C covalent bonds to give a planar structure of hexagonal
rings.
The C—C bond in graphite has some double bond character due to the delocalized
electrons in between the bond. Hence, the C—C bond length (0.142 nm)is smaller than
that in diamond (0.154 nm). Therefore, the C—C bond in graphite is stronger than in
diamond.
<2> Interaction between graphite layers
Weak van der Waal’s forces attract planes of carbon atoms together. Hence, the planes
are widely spaced (0.34 nm) . This account for the lubricating properties of graphite.
-
Electrons are delocalized in between the layers and account for the electrical
conductivity of graphite.
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
Chpt.10:p.14
Delocalized electrons above or
below planes of carbon atoms.
Graphite : Layered structure
(C)
Quartz (Silicon (IV) oxide)
• Each silicon atom is tetrahedrally bonded to four oxygen atoms by identical and directional
strong Si—O covalent~ bonds. Thus, the co-ordination number of Si is 4.
• Each oxygen atom is bonded to 2 silicon atoms. Thus, the coordination number of 0 is 2. The
formula of silicon (IV) oxide is __________
• The gives rise to tetrahedral diamond-like structure as shown below
Structure of silicon (IV) oxide
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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 10: Metallic Bonding
Chpt.10:p.15
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