Hard, shiny, and tough—metals are the macho poster boys of the material world. Learning how to extract these substances from the Earth and
turn them into all kinds of useful materials was one of the most
important developments in human civilization, spawning tools,
jewelry, engines, machines, and giant static constructions like
bridges and skyscrapers. Having said that, "metal" is an almost
impossibly broad term that takes in everything from lead (a
super-heavy metal) and aluminum (a super-light one) to mercury (a
metal that's normally a liquid) and sodium (a metal soft enough to
cut like cheese that, fused with chlorine, you can sprinkle on your
food—as salt!). What exactly are metals and what makes them so
useful? Let's take a closer look!
Photo: Metals like iron help bridges hold their proud heads high.
The Golden Gate bridge, shown here, is made from
75 million kg of steel (an alloy of iron)... so why isn't it a cold, dull gray? It's
painted to protect it from rusting in the salt-water air (in a warm, distinctive color known as International Orange). Detail of the Golden Gate Bridge by Carol M. Highsmith. Photo from the Jon B. Lovelace Collection of California Photographs in Carol M. Highsmith's America Project,
Library of Congress, Prints and Photographs Division.
You might think Earth is a big lump of rock, hard on the outside
and soft in the middle—but quite a lot of it is actually metal. What
exactly is metal? Over three quarters of the chemical elements
that occur naturally on our planet are metals, so it's almost easier
to say what metal isn't.
Chart: Roughly half of Earth's crust is made from metallic or semi-metallic atoms. Apart from ~46 percent oxygen, most of the remaining ~54 percent is made from elements that are either metallic (elements like aluminum and iron) or semi-metallic (silicon).
When we talk about metals,
we're usually referring to chemical elements that are solid (with
relatively high melting points), hard, strong, durable, shiny,
silvery gray in color, good conductors of electricity and
heat, and easy to work into various different shapes and forms (such as thin
sheets and wires). The word metal is quite a broad and vague term,
and not something you can define precisely.
When we talk about nonmetals, it ought to mean everything else—although things
are a bit more complex than that. Sometimes you'll hear people refer
to semi-metals or metalloids, which are elements whose
physical properties (whether they're hard and soft, how they carry
electricity and heat) and chemical properties (how they behave when
they meet other elements in chemical reactions) are somewhere in
between those of metals and nonmetals. Semi-metals include such
elements as silicon and germanium—semiconductors (materials that
conduct electricity only under special conditions) used to make
integrated circuits in
and solar cells. Other semi-metals include
arsenic, boron, and antimony (all of which have been used in the
preparation—"doping"— of semiconductors).
Artwork: The periodic table is dominated by metals. Over three quarters of the natural elements are metals of one kind or another.
Take a look at the periodic table (the way we draw the
elements in a chart so ones with similar physical and chemical
properties line up together) and you'll see two types of metal get
special treatment. The middle of the table is dominated by a large
group of elements called the transition metals (or transition
elements); most of the familiar metals (including iron,
copper, silver, and gold) live here,
along with less well known metals such as zirconium, osmium, and tantalum. The other group, called the
rare-earth metals, is often printed separately from the main
periodic table and includes the fifteen elements from lanthanum to
lutetium plus scandium and yttrium (which are chemically very similar). They're called "rare-earth"
metals for the rather obvious reason that they were originally
believed to exist in extremely scarce deposits in Earth, though some
are now known to be relatively common. Like many conventional metals,
the rare-earths don't occur naturally in their pure form, but only as
When is a metal not a metal? Look at a car, plane, or motorcycle
and you see lots of metal—or do you? Quite a lot of the metal-like
materials around us are actually alloys: metals that have been
mixed with other materials (metals or nonmetals) to make them
stronger, harder, lighter, or superior in some other way. Steel is an
alloy of iron, for example, that contains a small amount of carbon
(different types of steel contain more or less carbon). Bronze is an
alloy of copper and tin, while brass is the alloy you get when you
mix copper and zinc.
It's also very common to find plastics that have been
electroplated (coated with a thin layer of a metal element, using electricity) to make
them look like shiny metals. Plastics treated this way look shiny and attractive, like metals,
but are usually cheaper, lighter, and rustproof—but also weaker and less durable.
How are metals made?
Whether you're assembling airplanes or building
need metals—and lots of them. The snag is that metals generally
don't occur in the ground in exactly the way that we'd like to find
them, in their pure form and in deposits large enough to make it
worth our while to extract them. Often they're buried in rock with
deposits of other metals and they exist not as pure elements but
oxides (metals fused with oxygen atoms) and other compounds.
Producing large quantities of a metal like iron, aluminum, or copper
therefore involves two distinct operations: extracting an ore
(a deposit consisting usually of a huge amount of useless rock and
smaller amounts of useful metals) from a mine or quarry and then
refining the ore to get the metals away from their oxides (or
other compounds) into the pure form that we need.
Photo: Metals are buried inside Earth, usually mixed in with useless rock. Extracting them is often difficult, dangerous, expensive, and polluting. Photo by Carol M. Highsmith courtesy of Gates Frontiers Fund Wyoming Collection within the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.
Exactly how this is done varies from metal to metal and from
place to place, but usually involves a mixture of mechanical
processing (such as grinding, filtering, or using water to wash away
unwanted materials), chemical treatment (using acids, perhaps),
heating (smelting iron ore, for example, involves roasting it in air
to remove the impurities), and electrical treatment (such as
electrolysis—separating a chemical solution into its constituent
elements by passing an electric current through it).
What are metals like?
With so many chemical elements classified as metals, you might
think it would be difficult to generalize about them. But that
problem is true of pretty much any generalization: every house is
different, but we can still say that houses tend to have doors,
walls, windows, and a roof and provide shelter from the weather—and
we can all sketch one on paper.
The generalizations we make about metals are:
They are mostly solid (at everyday temperatures), crystalline (their atoms are stacked up in orderly patterns like cans in a supermarket), hard, strong, and dense (most metals will sink if
you drop them in water, for example).
Metals are malleable (relatively easy to work into new shapes and forms) and
ductile (with the right equipment, you can tease them out into long, thin wires).
Even so, they don't wear out quickly or break easily, though they can
and do fracture (crack or snap) eventually through repeated stresses and strains because
of metal fatigue (a gradually developing weakness).
Photo: Metal objects that are repeatedly stressed and strained (pulled, pushed, bent, twisted,
or otherwise subject to forces) will undergo fatigue and then suddenly fracture. Here's a teaspoon that I bent
one time too many showing (left) a closeup of the fracture and (right) the broken spoon for context.
Most metals are opaque (unless extremely thin) and shiny and silvery gray in color
(because they tend to reflect all wavelengths of light to the same
extent). Some metals are colored (because they reflect certain light
wavelengths better than others); the best-known examples are probably
gold (a yellowish color) and copper (normally reddish, though it
turns blue after exposure to air converts it into copper oxide).
Most metals conduct electricity well (they have a low electrical
resistance, in other words) and feel instantly cold to the touch
(because they conduct heat well too, carrying heat energy
quickly away from your body).
Metallic elements such as iron, nickel, cobalt, and neodymium
(and alloys based on them) power the best magnets;
most other metals make such poor magnets that they're usually thought of as non-magnetic.
How do metals conduct heat and electricity?
Why are metals such good conductors of heat and electricity? The
simplest explanation is to think of the atoms in a metal as ions
(positively charged nuclei) surrounded by a sort of sea of free electrons that wash readily
through the entire structure, carrying heat or electrical energy as
they go. Remember that opposites attract, in electricity as well as
magnetism, so the static-electric pull between the positively charged
ions and the negatively charged electrons makes for a tightly bonded
structure that is strong and hard. The atoms can still move past one
another (with difficulty) and that's why metals are relatively easy
to work and shape.
Animation: Metals conduct electricity because they have "free" electrons (blue)
that are not fixed to any particular atom (black). The electrons
can move through the whole metal carrying electrical energy from one end to the other.
What is the band theory?
Metallurgists (scientists who study metals) prefer to explain the
properties of metals using a more complex idea called the band
theory. You've probably learned in school that the electrons in a
single, isolated atom are arranged in energy levels (sometimes
referred to as shells, sometimes as orbitals—different ideas but
they're broadly talking about the same thing). In a solid, there are lots of atoms
sitting next to one another, but they don't behave as isolated units.
Instead, their electron orbitals overlap, forming what are called
bands that extend between atoms (they're molecular orbitals, in other
words), and spread across the entire solid.
According to this theory, solids have two bands called the valence band (containing electrons
that are involved in bonding) and the conduction band (which allows
electrons to move freely through a metal, carrying heat or electrical
energy). What distinguishes metals, nonmetals, and semi-metals is the
way electrons move between the bands:
In metals (conductors), the two bands overlap, so when
energy (in the form of heat or electricity) is added to the material,
electrons are readily promoted from the valence band to the conduction band and carried
through the material, giving rise to an electric current or heat
In nonmetals (insulators), there is a large "band gap"
between the valence and the conduction band; in other words,
it takes a great deal of energy to get an electron from one band to the other.
Under normal circumstances, electrons don't get promoted from the valence band
to the conduction band and the material doesn't conduct electricity or heat.
On this theory, semi-metals (such as semiconductors) are midway between metals and nonmetals: they're
effectively insulators with a much lower band gap than normal nonmetals.
Metals react easily with other elements, their atoms giving up
electrons to form positive ions and compounds known as salts. The
"salt" you might sprinkle on your dinner is a typical example
(it's the compound that forms when sodium metal reacts with chlorine
gas), but it's only one of hundreds of different salts. The general
willingness of metals to react with other elements is the main reason
why they're often so difficult to extract from ores: they react so
readily with oxygen in the air (or sulfur in the ground) that they're
more likely to exist as oxides (or sulfides) than in their pure form.
Artwork: We can understand how different metals and their alloys behave by looking at the arrangement of atoms inside them. Left: Iron is easy to hammer into new shapes because its atoms are arranged in neat rows that can slip past one another. Right: Stainless steel (an alloy of iron) is much harder because small carbon atoms (red) in the "interstitial" spaces stop the iron atoms from moving. It also contains chromium (yellow) and other atoms (green). The chromium atoms react with oxygen in the air to form a protective outer "skin" of chromium oxide (dotted border line). This helps to stops stainless steel from rusting.
Find out more
On this site
Our website has more detailed articles about some of the more common and useful metals, including their
properties, how they're extracted, and where in the world they occur in abundance:
A Short History of Metals by Alan W. Cramb, Department of Materials Science and Engineering, Carnegie Mellon University. A good overview of how and when people discovered the most common useful metals from 6000BCE onward. [Archived via the Wayback Machine.]
ASM International: The world's largest society for metallurgists and engineers who work with metals.
Stuff Matters by Mark Miodownik. Penguin, 2013. A great, non-technical introduction to materials science. Chapter 1, Indomitable, covers steel, while Chapter 10, Immortal, looks at titanium and alloys.
Physical Metallurgy by Robert Cahn and Peter Haasen. Elsevier, 1996. Classic undergraduate textbook in print since the 1960s.
For younger readers
Science: A Children's
Encyclopedia by Chris Woodford and Steve Parker. DK, 2018/2014. Written by Steve Parker, and me, this is a more general introduction, with matter and materials covered in detail in the first two sections. Ages 8–10.
Metal by Abby Colich. Raintree, 2014. A very simple 24-page introduction for ages 4–6. Part of a series titled "Exploring Materials."
Metals by Chris Oxlade. Heinemann, 2007. A basic 48-page overview for readers aged 9–12.
Metals by Carol Baldwin. Heinemann/Raintree, 2005. A similar 48-page book for the same audience (ages 9–12).
A short sheet metal history: Metalworking World Magazine. June 9, 2014. How and why sheet metal has become so important over the last few hundred years.
African Metalworking by Bryony Reid, The Pitt Rivers Museum, October 2005. Explores traditional metalworking techniques used in different African nations. [Archived via the Wayback Machine.]
Metalworking throughout history by Professor R. Carlisle "Carl" Smith. The Fabricator, May 5, 2014. A whistle-stop tour of metal welding, from ancient times to modern.
↑ "Three quarters" is a widely quoted figure from, for example, Mixed Crystals by A. I. Kitaigorodsky, Springer, 2012, p.142. If you count them yourself on the Periodic Table, you'll get about 80–94 (depending on how you choose to define metal).
↑The CRC Handbook of Chemistry and Physics (table 14-18), gives 46.1% oxygen, followed by 53.5% metals or semimetals (28.2% silicon, 8.2% aluminum, 5.6% iron, 4.2% calcium, 2.4% sodium, 2.3% magnesium, 2% potassium, and 0.6% titanium), and then everything else.
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