Section 7.3

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 close- packing). 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.

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