Almost three-quarters of all elements are metals, such as
gold and silver. There are also some elements we may not think of as metals,
such as the calcium in our bones, and the sodium in table salt (sodium
chloride). Metals are defined by their METALLIC PROPERTIES, such
as high melting points. Mixtures of metals are called ALLOYS.
Solder is an alloy that is used to join metals in plumbing and electrical
wiring. It is mainly tin with lead or silver.
Metals are classified according to where they are found in the
periodic table. Each group has a set of properties that make the metals useful
for different purposes.
These include potassium and sodium, and form Group I of the
periodic table. They are extremely reactive metals: they react strongly with
water to form strong alkalis.
These elements make up Group II of the periodic table. They
combine with many elements in the Earth’s crust. Their oxides react with
water to form alkalis.
This group includes copper, silver, and gold. They are hard and
shiny, have high melting points, and are good conductors of heat and
electricity.
Also called poor metals, these metals are fairly soft and melt
easily. They include tin and aluminium and are often used in alloys. Bronze is
an alloy of tin and copper.
Some metals, such as gold, are found naturally as pure metals in
rocks. Gold is unreactive, so it does not combine with other elements. Most
metals are more reactive and are found combined with other elements in rocks.
Iron, for example, is usually combined with oxygen. The rocks in which metals
are found are called ores.
To extract gold from its ore, huge grinders crush the ore to a fine
powder. The powder is mixed with a solution of cyanide. Only the gold from the
ore dissolves in the solution. Powdered zinc is added to bring the gold out of
the solution. The gold is melted down and poured into moulds.
Metals are usually shiny solids with high melting points
and are very good conductors of heat and electricity. They are malleable, so
they can be beaten into sheets, and ductile, which means they can be drawn into
wires. Most are strong and cannot be broken easily. Of course, there are
exceptions: mercury, for example, is a metal that has a low boiling point and
is liquid at room temperature.
The transmission lines (electric cables) that bring electricity to
our homes, schools, and offices all rely on copper. Copper is a red-orange
metal that is one of the best electrical conductors. Metals conduct electricity
well because when metal atoms bond (join together), the electrons in their
outer shells move freely. If electricity passes through one part of the metal,
the electrons carry the electricity quickly to other parts.
Alloys are mixtures of metals with properties that make
them more useful than pure metals. A mixture of chromium and iron resists
rusting much better than iron on its own. Most alloys are made of two or more
metals, but some contain a non-metal. Steel is an alloy of iron and carbon.
Alloys are made by melting the different materials together. Changing the
proportions of the materials can change the properties of the
alloy.
