Iron (Fe) is element 26. Its symbol comes from the Latin word for iron, ferrum.
There are 4 stable isotopes of iron: Fe 54, Fe 56, Fe 57, and Fe 58. The element's atomic mass is 55.845 amu (Da), which is close to 56. About 92% of all iron is Fe 56. There is a reason for this isotope's dominance. At high temperature (2-5 billion K) and density (10^9 g/cm^3) a mixture of silicon group elements will exchange alpha particles until the concentration of elements in the mix is controlled by their relative stability. Since iron, and elements adjacent to it, are at a peak of stability, the silicon-group elements gradually evolve into a mixture which is mainly iron-group elements. (This is called "silicon burning".) Ni 56 is the most stable and abundant of these elements produced by alpha-particle exchange, but it is not completely stable. Ni 56 decays through Co 56 to Fe 56, which is stable. That is why Fe 56 is so abundant in the universe.
Steady-state silicon burning occurs in massive stars, with the resulting iron forming an inert core which grows until it colllapses into a neutron star (or black hole), This occurs slowly enough that formation of Ni 56 and its decay to Fe 56 both occur in the reaction zone. The same is not true when silicon burning takes place explosively following gravitational collapse of a core made of silicon group metals. In that case, there is no time for Ni 56 to decay, so it predominates in material thrown off by the explosion. Decay of Ni 56 to Fe 56 then takes place inside the cloud of debris thrown off. This is called a Type 1A supernova, and a great deal of its luminosity comes from nickel decaying to iron.
In earth as a whole, iron is one of the four elements which dominate the planet's makeup: O, Si, Mg, and Fe. Atomic concentrations are [O] = 0.482, [Mg] = 0.164, [Si] = 0.150, and [Fe] = 0.148. Total metal (chemical sense) concentration is [M] = 0.462. Since Si and Fe can take oxidation states higher than 2, earth does not have enough oxygen to completely oxidize these metals. Since Si and Mg are more active than iron, they will reduce it. As a result, earth has a large quantity of reduced (metallic) iron. Due to its density, this iron has collected to form a core to the planet. (This happens on all planets.)
Molar mass of iron is much higher than Si, Mg, or O. For this reason, iron ranks number one in mass, accounting for some 32% of the planet's mass.
Iron oxidizes readily, particularly in moist air. The resulting reddish-to-brown oxide (Fe2O3) accounts for the color in many terrestrial soils and rocks, as well as the color of that large rock known as Mars.
There are a number of uses for iron, principally as alloys.