Applied Chemistry
Mr. Gensits
Class Notes 11/10/2014
Bonding in Metals
Metals are made up of closely packed cations.
The valence electrons of metal atoms can be modeled as a sea
of electrons (i.e., the valence electrons are mobile and can
drift freely from one part of the metal to another.)
Metallic bonds consist of the attraction of the free-floating
valence electrons to the positively charged metal ions.
The sea-of-electrons model explains many physical properties
of metals.
(1) They are good conductors of electricity because
electrons can move freely in them.
(2) Metals are ductile and malleable because the sea of
electrons shields (insulates) the positive metal ions
and allows them to slide when a force is applied.
Crystalline Structures of Metals
Metal atoms are arranged in very compact and orderly
patterns.
For atoms of approximately the same size, there are three
common closely packed arrangements.
(1) Body-centered cubic
Every atom has eight neighbors (except the atoms
on the surface)
Examples: Na, K, Fe, Cr, and W
(2) Face-centered cubic
Every atom has twelve neighbors.
Examples: Cu, Ag, Au, Al, and Pb
(3) Hexagonal
Every atom also has twelve neighbors.
Examples: Mg, Zn,and Cd
Alloys – mixtures of two of more elements, at least one of
which is a metal
Alloys are important because their properties are often
superior to those of their component elements.
(1) Sterling Silver – 92.5% Ag, 7.5% Cu
Harder and more durable than pure silver but still soft
enough to be made into jewelry and tableware.
(2) Bronze – usually composed of 7 parts Cu/1 part Sn
Harder than copper and more easily cast.
(3)
Steels – iron and carbon (B, Cr, Mn, Mo, Ni, W,
and V are commonly added)
Wide range of useful properties, such as corrosion
resistance, ductility, hardness, and toughness.
Substitutional Alloys – the replacement atoms are
approximately the same size as the original atoms.
Interstitual Alloys – if the atomic sizes are quite different,
the smaller atom can fit into the interstices (spaces) between
the larger atoms.
In steels the carbon atoms occupy spaces between iron
atoms.