IRON

Name: Iron

Symbol: Fe

Group: 8

Period: 4

Block: d

Category: Transition Metals

Atomic Number: 26

Atomic Mass: 55.845 u

Electrons per Shell: 2, 8, 14, 2

Electronegativity: 1.83

Density: 7.874 g/cc

Melting Point: 1538ºC

Boiling Point: 2861ºC

Thermal Conductivity: 79 W (m·K)

Electrical Conductivity: 1 × 10^7 S/m

Magnetic Order: Ferromagnetic

Ordinary State: Solid

Oxidation States: -4, -2, -1, 1, 2, 3, 4, 5, 6, 7

Mohs Hardness: 4

Vickers Hardness: 608 MPa

Brinell Hardness: 490 MPa

Most stable isotopes: Fe-54 (5.85%), Fe-56 (91.75%), Fe-57 (2.12%), Fe-58 (0.28%)

Discoverer: Known since ancient times

Iron, Fe, is not only the most relevant metal from a material standpoint but also one of the most decisive elements for life as we know it. To say "metal" is, in most contexts, to say iron: the image of a generic "metallic" gray color in manuals and reviews is because, when no other is specified, this element is referenced. Such is its relevance that, on occasion, both terms are used interchangeably.

It is the most abundant metal on Earth and the fourth most frequent element in the Earth's crust. If we could slice the planet like a hard-boiled egg, we would find that the core and a large part of the inner mantle are composed almost entirely of iron. This dominance is not exclusive to our world: it is also the most abundant element, by mass proportion, on the other three rocky planets of the solar system — Mercury, Venus, and Mars — as well as on satellites with a composition similar to Earth's, such as the Moon, Phobos, Deimos, Europa, or Titan. Even in the Sun, although its concentration does not reach 1%, the colossal stellar mass means that this fraction is equivalent to billions of tons.

In the cosmic context, iron is the most abundant metal in the observable universe. It represents the final product of nuclear fusion in massive stars, where, after the exhaustion of lighter fuels, the "silicon burning" process leads to the formation of the radioactive isotope Ni⁵⁶, which decays into Co⁵⁶ and finally into Fe⁵⁶ within a mere three months. The nuclei of Fe⁵⁶ and Fe⁵⁸, along with Ni⁶², possess the highest binding energies among all known stable isotopes, making them practically perfect nuclear configurations. Once formed, iron marks the energetic limit of stellar fusion: beyond it, only collapse and supernova remain.

From a mechanical and physical point of view, iron is an irreplaceable material due to its balance of strength, malleability, abundance, and cost. It is cheap, recyclable, versatile, and essential in more fields than any other metal. Even before being worked in furnaces and forges, it was already valued in its natural state: meteoric iron, alloyed with nickel in proportions that could reach 60%, offered superior oxidation resistance to terrestrial iron. Approximately one in ten meteorites that impact Earth is metallic, composed of natural iron and nickel alloys called kamacite or taenite, depending on the percentage of nickel present. These meteorites also contain minute traces of precious metals such as rhodium, ruthenium, and iridium, suggesting that the Earth's core could be rich in them, although they are extremely scarce in the crust.

The history of iron is inseparable from the history of humanity. Its use has facilitated conquests, wars, and defenses, but it has also driven agricultural, architectural, and industrial advances that have transformed human life. Without iron, steel would not exist, and without steel, modern civilization would lack much of its basic infrastructure and tools.

The first iron objects that have reached us date back to at least the 4th millennium BC and, for the most part, come from meteoric iron, whose nickel content delayed corrosion. Terrestrial iron, more reactive and difficult to preserve in its pure state, was used somewhat later. Of the "seven metals" known in Antiquity, it was the most difficult to work due to its high reactivity and relatively high melting point.

The so-called "Iron Age" began in ancient Sumeria, expanding eastward (India, China, Japan) and westward (Persia, Greece, Rome, Iberia, Britannia, and Scandinavia). Some cultures went directly from stone to iron; others barely worked it. Its strategic value lay in the manufacture of stronger and more effective weapons than those made of bronze, which led to its intensive development in regions such as Syria and, later, in metallurgical centers such as Damascus or Toledo, famous for the quality of their steels. In Central Europe, especially in Germanic territories, metallurgy was consolidated even before the Middle Ages.

The mass production of steel, however, did not arrive until the Industrial Revolution in England and Germany, at which point it became the basis of the economic and military power of both nations. Spain, France, and Portugal, previous powers, could not compete in productive volume despite having quality techniques. In the Basque Country, the commitment to blast furnaces laid the foundations for modern industry in the region, while in Japan, methods of Chinese origin were perfected to achieve levels of excellence.

Today, steelmaking remains one of the world's industrial pillars. Throughout the 20th century, it was the backbone of economies such as the American, which, since the automotive revolution driven by Henry Ford, turned steel into a symbol of development. Although the electronics industry is gaining prominence, we still live in an era where steel — and, therefore, iron — is irreplaceable. As in the past, it conditions our present and, probably, will continue to define our future.

There is a curious phenomenon when people unfamiliar with metallurgy are asked about the color of metallic iron: most answer "black". This confusion is due to the fact that, in everyday life, the "iron" we see is rarely pure iron, but rather pieces made of cast iron or wrought iron — railings, rusted nails, manhole covers, sculptures, bridges, railway tracks, old locomotives, kitchen utensils such as pans and shovels — all of which are altered by natural oxidation or surface treatments. Consequently, we associate it with dark or reddish tones.

In reality, pure iron has a bright metallic gray color, comparable to platinum, with a barely bluish tint depending on the lighting. However, it quickly loses that luster due to its high reactivity with oxygen, which explains why we almost never observe it in its pristine state outside of controlled environments.

The essential difference between iron and steel does not lie in color, but in chemical composition. Iron (Fe) is an element of the periodic table and can be obtained with purities exceeding 99.999% through methods such as electrolysis. This "electrolytic iron" is rare and expensive: its price per gram approaches that of sterling silver. In nature and industry, pure iron is exceptional, as even minimal traces of carbon, silicon, or manganese modify its properties.

Steel, on the other hand, is an alloy of iron and carbon, with a carbon content ranging between 0.002% and 2.14% by mass. Below 0.002% carbon (>99.998% Fe) we still speak of pure iron; above 2.14%, the material is considered cast iron. Cast iron is neither pure iron nor steel, but iron with a saturation of partially dissolved carbon, which gives it greater brittleness but also ease of molding. Despite this, many old cast iron pieces are colloquially called "irons," when their composition brings them closer to steel.

Stainless steel constitutes a particular case: it is any iron-based alloy containing a minimum of 10.5% chromium (Cr) by mass. Chromium forms a thin passive layer of chromium oxide (Cr₂O₃) on the surface that protects the material from corrosion. Less than 10.5% is not enough to classify it as stainless, although the oxidation resistance may be high. There is no maximum chromium limit except in extreme compositions where its percentage exceeds that of iron, as in special Cr⁹⁵Fe⁵ alloys. Alloys rich in cobalt or nickel where the sum of these elements equals or exceeds the iron content, as occurs in certain Co-Ni superalloys with 20-25% Fe, are also not considered stainless steels.

In everyday use, it is common to call carbon steel "iron" and stainless steel "steel." This loose usage is not incorrect from a colloquial point of view, but in technical and scientific contexts it is important to distinguish precisely between pure iron, steel, cast iron, and stainless steel. Understanding this difference allows for a better understanding of the properties and applications of each material and, consequently, choosing the most suitable for each function.

Iron, with a purity of 99.999% or higher, presents as a transition metal of metallic gray color, devoid of the intense luster or characteristic nuances of metals like cobalt, nickel, iridium, titanium, or tantalum, which can exhibit pink, golden, or blue tones. Its appearance is sober, without distinctive reflections, but its value lies in its physical and chemical properties. Malleable and ductile, pure iron has a moderate hardness of 4 on the Mohs scale. In dry air, it can remain uncorroded for some time, though not indefinitely without maintenance. Contrary to the belief that ultrapure iron (>99.9999%) does not oxidize, this metal is intrinsically reactive, forming oxides such as hematite (Fe₂O₃) and magnetite (Fe₃O₄) relatively quickly, especially in the presence of humidity or liquid water, where it produces hydroxides. These compounds are distinguished by their colors: anhydrous oxides show reddish, brown, or black tones, while hydroxides exhibit more vivid colors, from yellow and orange to carmine red and brown.

Physically, iron is an exceptionally complete metal. It combines malleability and ductility with remarkable toughness, resisting impacts without fracturing. It is an acceptable conductor of heat and electricity, moderately rigid, easily weldable, and highly recyclable. Its sensitivity to alloying elements is unique: small variations in composition, such as 2% or 4% manganese in steel, generate drastic changes in its properties, unlike alloys such as bronze, where the differences are less marked. This versatility allows pure iron, initially soft, to achieve hardnesses of up to 7 on the Mohs scale or rigidities greater than 2000 MPa on the Vickers and Brinell scales, depending on the alloy. Some alloys are more malleable than pure iron, while others are so brittle that they fracture on impact without deforming.

Iron shares its ferromagnetic nature with only three out of the 94 elements in the periodic table, which makes it sensitive to magnetic fields and capable of being temporarily magnetized with proper treatment. Furthermore, it can dissolve small quantities of carbon without forming binary carbides, as occurs with chromium, tungsten, or titanium, making it an ideal matrix to contain carbon or even pure graphite without chemically combining. Another notable characteristic is its ability to generate sparks by friction, due to its high coefficient of friction, which requires lubricants in ferrous alloy parts to minimize wear, unlike copper alloys, which exhibit lower friction. From a biological point of view, iron is an essential trace element for life. Its balanced affinity with oxygen allows it to be part of complex organic molecules, such as hemoglobin, where its presence is irreplaceable, facilitating vital processes by binding and releasing oxygen.

Pure iron (Fe) possesses corrosion resistance slightly superior to that of common steels. It is a non-flammable and non-volatile material, but it corrodes easily in the presence of humidity, even in the form of vapor. Although it is stable in dry air, environmental conditions rarely guarantee this stability. Therefore, the metal is much better preserved in closed, dry spaces than outdoors.

Iron parts require regular maintenance that includes periodic cleaning, followed by the application of oils or other coatings to prevent contact with water and moisture. The reaction with water is not spontaneous but slow, yet it still produces rust in a few days. Unlike the oxides of other metals such as titanium, vanadium, or chromium, which are stable and form a protective surface layer, iron oxide does not stop. It advances until it completely compromises the integrity of the piece. This explains why iron objects submerged in the sea, like ancient shipwrecks, disintegrate much more rapidly than those made of other, more noble metals.

Iron is vulnerable to most chemical solutions. Reactions with highly oxidizing elements, such as halogen-containing acids, are rapid and very detrimental. In fact, attack with pure halogens is even more aggressive. However, there are exceptions: concentrated nitric acid (HNO₃) and sulfuric acid (H₂SO₄) can be beneficial to iron to some extent. These acids create passive layers on its surface, protecting it from further corrosion. Iron's tolerance to concentrated sulfuric acid is similar to that of lead (Pb), which also develops a passivation layer on its surface. Additionally, iron is resistant to alkalis, even when hot.

Much of this chemical behavior applies to most common carbon steels. This is because, although the mechanical properties differ significantly between pure iron and high-carbon steel, the percentage of iron remains very high (typically >97%). Iron is special because, unlike aluminum, whose corrosion resistance decreases when alloyed, anything added to it (except sulfur and carbon itself in certain contexts) tends to increase its corrosion resistance. This characteristic is observed in high-speed steels, which contain high levels of chromium (Cr), vanadium (V), molybdenum (Mo), and tungsten (W). Similarly, the addition of nickel (Ni) and silicon (Si) improves corrosion resistance, although this often results in a loss of toughness. The percentage of carbon, however, plays a more complex role and alters this resistance, especially in stainless steels. Most metals alloyed with iron, with some exceptions such as manganese, contribute to greater corrosion resistance.

Iron (Fe) is one of the most important metals in folklore and culture, on par with gold, silver, and copper. It has become synonymous with strength, resistance, and tenacity, attributes applied to people, animals, and inanimate objects in countless literary works and everyday expressions. We often use phrases like "strong as iron" or "iron-willed" to describe something or someone unbreakable, even if not literally. In mythology and religions, iron has held a central place. It is strongly associated with the god of war, Mars (Ares), and with the zodiac signs Aries and Scorpio. This connection links it to the masculine principle, belligerence, aggression, and other combative traits.

The planet Mars, named after the Roman god, owes its characteristic red color to the abundance of iron oxides on its surface. The association of iron with war is mainly due to its use in the manufacture of weapons and armor, such as swords, axes, and spearheads. Fire, an element of great folkloric relevance, is used to mold this metal, allowing humanity to overcome animals that physically outperform them. The gods of the forge, like Hephaestus or Vulcan in Greco-Roman mythology, were responsible for working the metal to forge the weapons of the greater gods. An iconic example is Thor's hammer, the prince of the Norse gods, which, according to the Prose Edda, was forged from a mixture of iron and gold. Iron is mentioned in sacred texts such as the Bible, the Quran, and the Talmud, sometimes with positive and other times with negative connotations.

An example is Jesus' famous phrase to Peter: "he who lives by the sword dies by the sword" (often translated as "whoever kills by iron, by iron dies"). In animistic folklore, iron was traditionally used to repel demons and evil spirits. Although today we more readily associate silver with fighting dark forces, iron was the original metal used in making amulets, probably due to its scarcity and value in antiquity. Invulnerability and indestructibility are often symbolized by the term "iron" or "steel." Examples include the "Iron Curtain" or Superman's nickname, the "Man of Steel." In some sub-Saharan African tribes, blacksmith clans are highly respected, as their craft is considered a form of magic, an art that transcends the material. The forging of metal is not seen simply as a technical skill, but as a mystical act of creation.

In summary, iron and its alloys, such as steel, possess a cultural value far superior to almost any other metal, comparable only to that of gold. Although each symbolizes distinct values, both are omnipresent and fundamental in the narratives, beliefs, and expressions of cultures around the world.