Atomic Mass of Iron

Iron's 55.845 g/mol is dominated by ⁵⁶Fe at 91.75 %, the nuclide with very nearly the highest binding energy per nucleon of any isotope. That is the reason a massive star's core fuses its way up the periodic table and then stops: past iron, fusion consumes energy instead of releasing it, the core collapses, and the supernova that follows makes everything heavier. Every iron atom in your blood was assembled in a star that ran out of profitable fuel.

Chemically, the two accessible oxidation states are what matter. Ferrous Fe²⁺ is soluble and colourless in oxygen-free groundwater; the moment it reaches air it oxidises to ferric Fe³⁺, which hydrolyses and drops out as rust-coloured Fe(OH)₃: the reason well water runs clear at the tap and stains the sink an hour later. Corrosion arithmetic converts a measured metal loss to moles through this number, and each mole of iron oxidised to Fe²⁺ releases two moles of electrons.