Chemistry Reference
In-Depth Information
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36
METALS AND ALLOYS
The 92 chemical
elements
that occur naturally in the earth can be divided
into two main groups:
metals
and
nonmetals
. Although the distinction
between the two is not always sharp and clear, it can be said that over 70
of the 92 elements are metals; among the fewer than 22 remaining non-
metals, six are known as
metalloids
, which have properties that fall
between those of metals and nonmetals (see Appendix I).
In common parlance, the term
metal
is used to refer to two different
types of
metallic materials
:
metals
and
alloys
. The
metals
are chemical ele-
ments; each metal (e.g., copper, iron, and gold) is composed of only one
type of atom. The
alloys
are mixtures that have metallic properties. All
alloys include two or more elements in their composition; some are made
up of two or more metals, others of one or more metals mixed with one
or more nonmetals. Bronze, for example, is made up of two metals: copper
(60-85%) and tin (40-15%);
steel
includes iron, a metal (98-99.97%) and
carbon, a nonmetal (2-0.03%). Metals and alloys share many common
properties:
1.
They are
malleable
, that is, they can be shaped by applying an
external pressure, as when hammering a lump of metal or alloy into
different shapes or into thin sheets; they are also
ductile
(can be
drawn into wire), although cast iron is brittle.
2.
They have a distinctive metallic
luster
, although some nonmetals
have a similar luster.
3.
They are good conductors of electricity and heat.
4.
All the metals and therefore the components of the alloys combine
with nonmetals to form
compounds
. In the compounds thus
formed, the metals always form
cations
, which have a positive elec-
tric charge, and the nonmetals form negatively charged anions. The
compounds formed by the metals with oxygen, for example, known
as
metal oxides
, include a positively charged metal (the cation) and
oxygen, a nonmetal that has a negative charge (the anion). Since
the metal oxides generally react with water to form
basic sub-
stances
, the metal oxides are said to be
basic
(or
alkaline
).
Relatively soon after ancient humans recognized the metals and their
special properties, they also discovered ways to make alloys. Some alloys
were produced in antiquity directly, by the smelting of ores that include
two metals in their composition or mixtures of ores of different metals.
Arsenical copper
,
bronze
, and
brass
, for example, three alloys of copper
