Section 7.3
BONDING IN
METALS
Understanding Metal Atoms
• The behavior of a metal can be better
explained if we understand that it is actually a
collection of cations, rather than atoms.
• The valence electrons that
the atoms hold onto so
loosely are free to travel
within the metal sample,
moving from cation to
cation.
Metallic Properties
• Understandably, the
bonding in metals is
different from ionic bonding, since electrons
are not transferred from one atom to another.
• Electrons are free to move around and are
frequently pictured as a “sea of electrons”
moving around the metal cations.
• As can be seen in the animation, the free-
floating sea of electrons explains why metals
are such good conductors of electricity.
Metallic Bonding
• The delocalized electrons
in the sea of electrons
enable the metal atoms to
roll over each other when a stress is applied.
• This explains the malleability and ductility
exhibited by metals.
• Metallic bonds are a result of the attraction of
the free-floating electrons for the metal cations
they move around. Realplayer\7.3 Metallic Bonding sea of electrons.flv
Comparing to Ionic Compounds
• Why do metals deform while ionic compounds
break when struck? Consider the image:
Metallic Crystal Structure
• When viewed at the atomic level, metal atoms
will pack together in an arrangement that fills
as much space as possible (called closepacking).
• This regular arrangement of the atoms result
in metals having a crystalline structure (a very
compact and orderly pattern).
• In a pure metal sample, the atoms are all the
same size. Several close-packing
arrangements are possible.
Some Close-packed Arrangements
Using Unit Cells
• When identifying
crystalline patterns, a unit
cell is often used.
• A unit cell is the smallest
building block of a crystal,
consisting of atoms, ions,
or molecules, whose
geometric arrangement
defines a crystal's
characteristic symmetry
and whose repetition in
space produces a crystal
lattice.
Body-centered Cubic (bcc)
• In the bcc arrangement, each
metal atom has 8 neighbors.
• Metallic examples include
chromium, iron, potassium,
sodium and tungsten.
Face-centered Cubic (fcc)
• In the fcc arrangement, each
atom in the structure has 12
neighbors.
• Metallic examples include
aluminum, copper, gold, lead
and silver.
Hexagonal Close-packing
• The hexagonal arrangement
is different from the cubic;
each atom in this structure
also has 12 neighbors.
• Metallic examples
include cadmium,
magnesium and
zinc.
Mixing Metals
• When metals are mixed, they may
be mixed in any proportions. The
result is an alloy. Actually, an alloy
must contain at least one metal,
but nonmetals or metalloids may
be mixed in as well to change the
properties of the alloy.
• The proportion used will result in
different properties being
imparted to the alloy.
Realplayer\7.3 Alloys and
metal properties.mp4
Sterling Silver
• Sterling silver is superior in
performance to pure silver
because it is harder to
deform and is more durable,
yet is still soft enough to be
shaped into jewelry and
flatware.
• The ideal proportion is
92.5% silver, 7.5% copper.
Copper alloys
• Brass is an alloy of copper and zinc; the
proportions of zinc and copper can be
varied to create a range of brasses with
varying properties.
• Historically, bronze has principally
been an alloy of copper and tin.
However, bronze does not necessarily
have to contain tin, and a variety of
alloys of copper, including alloys with
arsenic, phosphorus, aluminum, manganese
and silicon are commonly termed "bronze."
Steels
• Note that there are many varieties
of steel (a lot more than those
shown here).
• Each type of steel has different
properties, due to its composition.
• As can be seen on matweb.com (a
material property data website)
there are over 4000 currently
recognized grades of steel.
Realplayer\7.3 Making Stainless Steel Alloys.flv
Steels
• Most steels are corrosion resistant (which is
why they are preferred over pure iron), ductile,
hard and tough, making them good building
materials for machines and structures.
• Cast iron is not a type of steel, although it
does contain iron and carbon (two things
commonly found in steel). There are 4 basic
types of cast iron, having different properties:
Atoms in Alloys
• When alloys are created, the atoms being
added can create different crystal lattices,
depending on the relative sizes of the atoms.
• If the atoms are about the same size, they just
replace each other in crystal (called a
substitutional alloy).
• If atoms are of significantly different sizes, the
smaller atoms can fit into the spaces
(interstices) between the larger atoms.
Types of alloys
• Zinc is very close in size to copper; in brass,
the zinc atoms take the place of some of the
copper atoms in the crystal. Brass is a
substitutional alloy.
Types of Alloys
• Carbon is much smaller than iron; in steels, the
carbon atoms fit into the spaces between the
iron atoms. Steels are interstitial alloys.
References
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http://www.maryrose.org/learning/science/images/thumb2.jpg
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http://1.bp.blogspot.com/_coc4dIAtydY/SqpzHg36ejI/AAAAAAAAAAk/-djkG68lQfc/s320/metallic_bond.jpg
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http://www.ausetute.com.au/metallic.html
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http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch13/unitcell.php#simple
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http://www.sutree.com/upload/uygpkmcdzlg/captured.jpg
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http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch13/unitcell.php#simple
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http://image.wistatutor.com/content/solid-state/co-ordination-number.jpeg
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http://static-p1.artedona.com/z1/Christofle-Marly-cutlery-sterling-silver,c00920.jpg
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http://www.baublesbabe.com/mc_images/product/detail/qsk129.jpg
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http://en.wikipedia.org/wiki/Brass
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http://www.imglego.co.cc/data/Others/brass-lamp.jpg
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http://user9530.websitewizard.com/images/Birds/AN0253_bronze_eagle.jpg
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http://info.lu.farmingdale.edu/depts/met/met205/castiron.html
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http://knol.google.com/k/-/-/a8npan5yj7ut/hp1erd/figure-2.png
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http://knol.google.com/k/-/-/a8npan5yj7ut/hp1erd/figure-3.png