Iron filings (Fe). Atomic weight 55.845. Atomic number 26. Iron is the 4th most common element in the earth's crust but, by mass, the most common on the entire planet. Alloys of iron, like steel, are the most common industrial metals. Iron reacts readily with oxygen and water to form brown to black hydrated iron oxides, rust. It could not be smelted until a furnace could be designed that produced a temperature 1500 degrees Celsius or higher to smelt the ore. Iron in the blood's haemoglobin interacts with oxygen and gives it its red colour.