The science of metals: a simple introduction (2024)

by Chris Woodford. Last updated: May 8, 2023.

Hard, shiny, and tough—metals are the macho poster boys of the material world. Learning how to extract these substances from the Earth andturn them into all kinds of useful materials was one of the mostimportant developments in human civilization, spawning tools,jewelry, engines, machines, and giant static constructions likebridges and skyscrapers. Having said that, "metal" is an almostimpossibly broad term that takes in everything from lead (asuper-heavy metal) and aluminum (a super-light one) to mercury (ametal that's normally a liquid) and sodium (a metal soft enough tocut like cheese that, fused with chlorine, you can sprinkle on yourfood—as salt!). What exactly are metals and what makes them souseful? 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 from75 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.

What are metals?

You might think Earth is a big lump of rock, hard on the outsideand soft in the middle—but quite a lot of it is actually metal. Whatexactly is metal? Over three quarters of the chemical elementsthat occur naturally on our planet are metals, so it's almost easierto say what metal isn't.[1]

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).[2]

When we talk about metals,we're usually referring to chemical elements that are solid (withrelatively high melting points), hard, strong, durable, shiny,silvery gray in color, good conductors of electricity andheat, and easy to work into various different shapes and forms (such as thinsheets 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 thingsare a bit more complex than that. Sometimes you'll hear people referto semi-metals or metalloids, which are elements whosephysical properties (whether they're hard and soft, how they carryelectricity and heat) and chemical properties (how they behave whenthey meet other elements in chemical reactions) are somewhere inbetween those of metals and nonmetals. Semi-metals include suchelements as silicon and germanium—semiconductors (materials thatconduct electricity only under special conditions) used to makeintegrated circuits incomputer chipsand solar cells. Other semi-metals includearsenic, boron, and antimony (all of which have been used in thepreparation—"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 theelements in a chart so ones with similar physical and chemicalproperties line up together) and you'll see two types of metal getspecial treatment. The middle of the table is dominated by a largegroup of elements called the transition metals (or transitionelements); 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 therare-earth metals, is often printed separately from the mainperiodic table and includes the fifteen elements from lanthanum tolutetium plus scandium and yttrium (which are chemically very similar). They're called "rare-earth"metals for the rather obvious reason that they were originallybelieved to exist in extremely scarce deposits in Earth, though someare now known to be relatively common. Like many conventional metals,the rare-earths don't occur naturally in their pure form, but only asmetal oxides.

Not-quite metals

When is a metal not a metal? Look at a car, plane, or motorcycleand you see lots of metal—or do you? Quite a lot of the metal-likematerials around us are actually alloys: metals that have beenmixed with other materials (metals or nonmetals) to make themstronger, harder, lighter, or superior in some other way. Steel is analloy of iron, for example, that contains a small amount of carbon(different types of steel contain more or less carbon). Bronze is analloy of copper and tin, while brass is the alloy you get when youmix copper and zinc.

Photo: An intermetallic compound (similar to an alloy) made from silver (a transition metal) and yttrium (a rare-Earth metal). Photo taken at Ames Laboratory courtesy of US Department of Energy.

It's also very common to find plastics that have been electroplated (coated with a thin layer of a metal element, using electricity) to makethem 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 buildingbatteries, youneed metals—and lots of them. The snag is that metals generallydon't occur in the ground in exactly the way that we'd like to findthem, in their pure form and in deposits large enough to make itworth our while to extract them. Often they're buried in rock withdeposits of other metals and they exist not as pure elements butoxides (metals fused with oxygen atoms) and other compounds.Producing large quantities of a metal like iron, aluminum, or coppertherefore involves two distinct operations: extracting an ore(a deposit consisting usually of a huge amount of useless rock andsmaller amounts of useful metals) from a mine or quarry and thenrefining the ore to get the metals away from their oxides (orother 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 fromplace to place, but usually involves a mixture of mechanicalprocessing (such as grinding, filtering, or using water to wash awayunwanted materials), chemical treatment (using acids, perhaps),heating (smelting iron ore, for example, involves roasting it in airto remove the impurities), and electrical treatment (such aselectrolysis—separating a chemical solution into its constituentelements by passing an electric current through it).

What are metals like?

Physical properties

With so many chemical elements classified as metals, you mightthink it would be difficult to generalize about them. But thatproblem is true of pretty much any generalization: every house isdifferent, but we can still say that houses tend to have doors,walls, windows, and a roof and provide shelter from the weather—andwe 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 ifyou 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 canand do fracture (crack or snap) eventually through repeated stresses and strains becauseof 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 bentone 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 sameextent). Some metals are colored (because they reflect certain lightwavelengths better than others); the best-known examples are probablygold (a yellowish color) and copper (normally reddish, though itturns blue after exposure to air converts it into copper oxide).
• Most metals conduct electricity well (they have a low electricalresistance, in other words) and feel instantly cold to the touch(because they conduct heat well too, carrying heat energyquickly 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.

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How do metals conduct heat and electricity?

Why are metals such good conductors of heat and electricity? Thesimplest 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 readilythrough the entire structure, carrying heat or electrical energy asthey go. Remember that opposites attract, in electricity as well asmagnetism, so the static-electric pull between the positively chargedions and the negatively charged electrons makes for a tightly bondedstructure that is strong and hard. The atoms can still move past oneanother (with difficulty) and that's why metals are relatively easyto work and shape.

Animation: Metals conduct electricity because they have "free" electrons (blue)that are not fixed to any particular atom (black). The electronscan 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 theproperties of metals using a more complex idea called the bandtheory. You've probably learned in school that the electrons in asingle, isolated atom are arranged in energy levels (sometimesreferred to as shells, sometimes as orbitals—different ideas butthey're broadly talking about the same thing). In a solid, there are lots of atomssitting next to one another, but they don't behave as isolated units.Instead, their electron orbitals overlap, forming what are calledbands that extend between atoms (they're molecular orbitals, in otherwords), and spread across the entire solid.

According to this theory, solids have two bands called the valence band (containing electronsthat are involved in bonding) and the conduction band (which allowselectrons to move freely through a metal, carrying heat or electricalenergy). What distinguishes metals, nonmetals, and semi-metals is theway electrons move between the bands:

• In metals (conductors), the two bands overlap, so whenenergy (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 carriedthrough the material, giving rise to an electric current or heatconduction.
• 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 circ*mstances, electrons don't get promoted from the valence bandto 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'reeffectively insulators with a much lower band gap than normal nonmetals.

Chemical properties

Metals react easily with other elements, their atoms giving upelectrons 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 chlorinegas), but it's only one of hundreds of different salts. The generalwillingness of metals to react with other elements is the main reasonwhy they're often so difficult to extract from ores: they react soreadily with oxygen in the air (or sulfur in the ground) that they'remore 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 theirproperties, how they're extracted, and where in the world they occur in abundance:

• Aluminum
• Copper
• Iron
• Platinum
• Tin
• Titanium

We also have a more general introduction to materials science.

On other websites

• 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.
• The Minerals, Metals, and Materials Society: A US-based materials organization with international membership and events all across the world.

Books

For older readers

• The New Science of Strong Materials (or Why You Don't Fall Through the Floor) by J.E. Gordon. Penguin (various editions). This classic, very readable book explains how and why materials such as wood and metals behave as they do. Part III, The Metallic Tradition (Chapters 9–11), covers 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.
• Metalworking—Doing it Better: Machining, Welding, Fabricating by Tom Lipton. Industrial Press, 2013; and and Metalworking Sink Or Swim: Tips and Tricks for Machinists, Welders, and Fabricators by Tom Lipton. Industrial Press, 2009. Practical guides to metalworking techniques aimed at the hobbyist.
• 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).

Articles

• 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.

1. ↑"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).