Iron is where the fusion inside giant stars finally stops
Iron is the most common element on Earth by mass, and the reason lies in dying stars. Fusion in the most massive stars runs up the periodic table until it reaches iron and then stalls, so supernovae scatter vast amounts of it into space. Some physicists think everything may eventually become iron.
Inside extremely massive stars, lighter nuclei fuse by adding helium nuclei until they reach nickel-56. Past that point the balance tips toward nuclei breaking apart instead, so the chain effectively ends. Nickel-56 decays within days, through cobalt, into stable iron-56, the most common final product of stellar fusion. Anything heavier needs a supernova, where iron nuclei rapidly soak up neutrons. Type Ia explosions in particular scatter iron widely, which is why rocky planets like Earth are so rich in it; iron ranks sixth among elements across the universe.
On Earth it sits mostly out of sight. The inner and outer core, together about 35 percent of the planet's mass, are thought to be largely iron alloy, and currents in the liquid outer core probably generate the magnetic field. Mercury, Venus, Mars and the Moon likely have iron cores too. At the surface, native metal is rare, since iron rusts readily. Unlike some metals, whose oxides form a protective skin, rust takes up more room than the metal, flakes off and exposes fresh iron.
Its structure changes with temperature. Molten iron freezes at 1538 °C into one crystal form, shifts to another at 1394 °C and a third at 912 °C. Below 770 °C it becomes ferromagnetic, its atoms aligning in tiny domains about 10 micrometres across; an outside field lines the domains up, and defects can lock them in place to make a permanent magnet. Natural lodestones of magnetite gave sailors their first compasses.
Smelting it needs furnaces near 1,500 °C, about 500 degrees hotter than copper requires, so iron tools spread only in the 2nd millennium BC, in places around 1200 BC, ending the Bronze Age. An adult body holds about 4 grams, mostly in the oxygen-carrying proteins hemoglobin and myoglobin.
Source: Iron
