When you ponder the origin of iron, your mind likely wanders into visions of steel mills, medieval-age forges or some other manufacturing process characterized by hard, hands-on work and very high temperatures. But apart from being a type of metal used in various ways in human industry, iron as also an element, not a compound or alloy, meaning that it is possible to isolate a single atom of iron. This is not true of most familiar materials; for example, the smallest amount of water than can still be called water includes three atoms, one of them oxygen and the other two hydrogen.
Interestingly, although people associate iron with unusually high temperatures in manufacturing settings here on Earth, iron as an element owes its existence to events so hot and so far away that the numbers involved scarcely make sense. Thus, undertaking a study of how iron is made requires two parallel processes: Exploring how iron came to be and how it reached Earth, and how people on Earth make and use iron for everyday as well as specialized activities. These topics in turn invite discussion on the use of iron in and by living systems and a general look at how the various elements both originate and spread throughout the cosmos.
A Brief History of Iron
Iron has been known to humankind since about 3500 B.C., or over 5,500 years ago. Its name is derived from the Anglo-Saxon version, which was "iren." The periodic table iron symbol Fe comes from the Latin word for iron, which is ferrum. If you're perusing a pharmacy and happen to see iron supplements, you will notice that most of their names are "ferrous" something-or-other (such as sulfate or gluconate). Anytime you see the word "ferrous" or "ferric" in a chemistry context, you should immediately recognize that iron is being discussed; "ironic," though a splendid and useful word, has no role in the world of physical science.
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Chemistry Facts About Iron
Iron (abbreviated Fe) is classified as a metal not only for everyday purposes but also on the periodic table of the elements (see Resources for an interactive example). This probably comes as little surprise, but in fact, metals outnumber nonmetals in nature by a wide margin; of the 113 elements humans have discovered or created in laboratory settings, 88 are classified as metals.
Atoms, as you may already know, consist of a nucleus containing a mixture of protons and neutrons of roughly equal mass surrounded by a "cloud" of nearly massless electrons. Protons and electrons carry a charge of equal magnitude, but protons' charge is positive while that of electrons is negative. Iron's atomic number is 26, meaning that iron has 26 protons and 26 electrons in its electrically neutral state. Its atomic mass, which when rounded off is simply the sum or protons and neutrons, is just shy of 56 grams per mole, meaning that its most chemically stable form contains (56 - 26) = 30 neutrons.
Iron possesses some formidable physical properties. It has a density of 7.87 g/cm3, making it nearly eight times as dense as water. (Density is mass per unit volume; water's is defined as 1.0 g/cm3 by convention.) Iron is a solid at 20 degrees Celsius (68 F), generally considered "room temperature" for chemistry purposes. Its melting point is an extremely high 1538 C (2800 F), while its boiling point – that is, the temperature at which liquid iron begins to evaporate and become gas – is a scorching 2861 C (5182 F). It is no wonder, then, that in metalworking, the kinds of furnaces used must be extraordinarily powerful indeed.
Iron, by mass, is the fourth-most-abundant element in the Earth's crust. Iron's total share of Earth may be considerably greater, however, given that the planet's molten core is believed to consist chiefly of liquefied iron, nickel and sulfur. When iron is extracted from the ground in mining operations, it is in the form of ore, which is elemental iron mixed with one or more types of rock. The most common type of iron ore is hematite, but magnetite and taconite are also significant sources of this metal.
Iron rusts, or corrodes, very easily compared to other metals. This creates problems for engineers because at present, nine-tenths of the metal that is refined includes iron.
Uses of Iron
Most of the iron mined for human use winds up in the form of steel. "Steel" is an alloy, meaning a mixture of metals. A popular form of this product today is called carbon steel, which is somewhat misleading because carbon contributes only a tiny fraction of the mass of this steel in all its forms. In the highest-carbon form of carbon steel, carbon accounts for about 2 percent of the mass of the metal; this figure can range down to 1/10th of 1 percent without the metal losing the title of "carbon steel."
Carbon steel can in turn be strategically adulterated with other metals to yield alloys with certain desirable properties. Stainless steel, for example, is a form of carbon steel that has a significant amount of chromium – over 10 percent by mass. This material is renowned for its durability and its tendency to maintain its shiny, lustrous appearance for long periods owing to its high resistance to corrosion. Stainless steel features prominently in architecture, ball bearings, surgical instruments and tableware. Chances are good that if you can see your reflection clearly in a purely metal surface, you are looking at a kind of stainless steel.
When judicious amounts of metals such as nickel, vanadium, tungsten and manganese are integrated into steel, it makes an already hard substance even harder; these alloy steels are therefore well-suited for inclusion in bridges, cutting instruments and electrical-grid components.
A non-steel type of iron called cast iron includes a great deal of carbon (by the standards of iron metalworking, at least): 3 to 5 percent. Cast iron is not as tough as steel, but it is considerably cheaper, so in moving from steel to cast iron, you make the same general trade-off you do when going from prime rib to 70 percent lean hamburger.
How Is Iron Made?
Iron on Earth is made, or more properly extracted, from iron ore. The "rock" portion of iron ore contains oxygen, sands and clays in varying amounts depending on the type of ore. The job of an iron works, as the earliest such factories were called, is to remove as much of the rock and other grit as possible while leaving iron behind – little different in principle from shelling a peanut or peeling an orange to get to the good part, except that in the case of iron ore, the iron is not merely surrounded by disposable material; it's mixed right in with it.
Despite the daunting temperatures and overall physical challenges of iron works, humans were already using them in pre-Christian times. Iron working first reached the British Isles by way of mainland Europe and western Asia in the 5th century B.C. Back then, iron was physically separated from the unwanted material to the fullest extent possible using only charcoal, clay and the ore itself, heated to temperatures that were modest compared to what would follow. At any rate, smelting was underway by 1500 B.C., but nearly 30 centuries later, in the 1400s, the blast furnace was invented, changing the "industry" (such as it was) radically and forever.
Today, iron is made by heating hematite or magnetite in a blast furnace along with with a form of carbon called "coke" as well as calcium carbonate (CaCO3), better known as limestone. This yields a compound that contains about 3 percent carbon and other adulterants – not ideal in quality, but good enough to make steel. Every year, about 1.3 billion metric tons (roughly 1.43 billion U.S. tons, or nearly 3 trillion pounds) of crude steel are produced around the globe.
Where Did Iron Come From?
Where the iron in your stainless-steel dishwasher or your wood stove "comes from" is perhaps a far less interesting question than how iron came to exist anywhere in the universe in the first place. Iron is considered a heavy element, and elements of this type can only be created in catastrophic "star death" events called supernovae. Whereas most stars kind of fizzle out as they burn through their fuel supply of hydrogen, some stars literally go out with a bang.
These are statistically rare events, occurring only a few times every hundred years throughout the extent of the entire Milky Way Galaxy, the massive slowly rotating pile of stars and other matter humans call home. But they are also vitally important. Without them, the forces necessary to cause sizable smaller elements to fuse together on impact and create even larger elements such as iron, copper, mercury, gold, iodine and lead wouldn't exist. And all the time, a certain fraction of these elements travel long distances through space and settle on Earth, sometimes in the form of meteorite strikes.
How Are Elements Formed in Nature?
Iron is believed to represent the approximate cut-off point in terms of elements that can be generated by ordinary star-combustion processes (as if these processes themselves are truly "ordinary" in any way) and those that can only be created by supernovae.
Most elements – oxygen, atomic number 8, through but probably not including iron, atomic number 26 – are made once a star begins to exhaust its hydrogen supply. The reason a star "burns" is that it is constantly undergoing countless fusion reactions, with hydrogen, the lightest element (atomic number 1) colliding with other hydrogen atoms to form helium (atomic number 2). Eventually, in the innermost part of the star, helium atoms collide in groups to form carbon (atomic number 6).
Iron in the Human Body
You probably recognize iron as being essential in the human diet based solely on advertising claims by food manufacturers ("This cereal contains 100 percent of the U.S. recommended daily allowance of iron!"). You may not know why this is, however.
As it turns out, the typical human body contains about 4 grams of elemental iron. That may not sound like a great deal, but why would your body need any metal in it whatsoever? In fact, iron is an essential part of hemoglobin, the oxygen-binding protein found in red blood cells (RBC). RBCs transport oxygen from the lungs to the tissues, where it us used in cellular respiration.
When people become deficient in iron thanks to insufficient dietary intake (iron is found in meats, particularly organ meats, as well as certain grains) or systemic disease states, their RBCs cannot do their job properly. In this condition, called anemia, people become short of breath after a modest amount of exertion, and often suffer from fatigue, headaches and general weakness. In severe cases, a blood transfusion may be required to correct the anemia, although typically correction is done using supplementation with iron-containing pills and liquids.