Iron

Iron was, according to the Tenth Doctor, "a malleable ductile ferromagnetic metallic element, found mainly in haematite and magnetite." (PROSE: Forever Autumn) Its chemical symbol on the periodic table was Fe. (AUDIO: Escape Room)

The First Doctor noted that iron had a melting point of 1,539 degrees centigrade. (TV: "The Unwilling Warriors") Its atomic weight was 55.84, and its specific gravity was 7.84. (TV: The Evil of the Daleks (episode 7) ^[+]Loading...{"ep":"7","1":"The Evil of the Daleks (TV story)"})

Iron could rust. (AUDIO: The Macros)

Iron ore was not pure iron, but it was something that the Fourth Doctor would not have been surprised to find underground. (TV: The Creature from the Pit (episode 3) ^[+]Loading...{"ep":"3","1":"The Creature from the Pit (TV story)"})

Humans needed a source of iron. (COMIC: The Metal Eaters) Red blood cells contained iron. (TV: Bad Wolf ^[+]Loading...{"ep":"","1":"Bad Wolf (TV story)"})

History

In human mythology, iron was an "old magical defence". Because the animated gargoyle Bok believed in magic, the Third Doctor kept Bok at bay with an iron trowel and an old Venusian lullaby. (TV: The Dæmons) The Hervoken were also similarly vulnerable to iron. (PROSE: Forever Autumn) Mithras used a sword of pure iron to kill a Nimon scout. (AUDIO: Seasons of Fear)

The Tenth Doctor tasted iron after he licked a portable television. (TV: The Idiot's Lantern)

In the 1970s, a meteorite containing sentient iron filings crashed on Earth. The filings were released and began corroding metal wherever they went until they were stopped by the Third Doctor. (COMIC: The Metal Eaters)

The Eighth Doctor used iron fillings and gunpowder to create flash-charges. With these he fought off a group of Windigos. (COMIC: Bad Blood)

The planet Megerra was rich in valuable minerals such as iron. (PROSE: Shakedown)

Prior to the Thousand Year War, the Kaleds had discovered how to turn iron into gold. (AUDIO: The Lights of Skaro) The Daleks inherited this knowledge and used it to buy the loyalty of Theodore Maxtible. (TV: The Evil of the Daleks (episode 7) ^[+]Loading...{"ep":"7","1":"The Evil of the Daleks (TV story)"})

Brittany Mordley needed iron-rich nutrients after she administered Cyberon. Thus, she ate pre-made meals. (PROSE: Silver-Tongued Liars)

Other references

"Strike while the iron's hot" was a saying which Vivien Rook used as she pressed Lucy Saxon for an interview. (TV: The Sound of Drums)