Iodine (I)

Iodine sublimes readily, going straight from a lustrous grey-black solid to a violet vapour, and the name comes from the Greek for violet. It is the heaviest element the human body genuinely requires: the thyroid concentrates it to make hormones, and deficiency remains the leading preventable cause of intellectual disability worldwide, which is why salt iodisation is one of the cheapest public health wins ever found. That same thyroid appetite is why potassium iodide is handed out after a reactor accident — saturating the gland with stable iodine stops it taking up radioactive iodine-131.

Properties

Atomic number

53

Symbol

I

Standard atomic weight

126.9 u

Category

Halogen

Group

17

Period

5

Block

p-block

Electron configuration

[Kr] 5s² 4d¹⁰ 5p⁵

Electrons per shell

2, 8, 18, 18, 7

Melting point

386.85 K — 113.7 °C — 236.7 °F

Boiling point

457.55 K — 184.4 °C — 363.9 °F

Density

4.93 g/cm³

Electronegativity

2.66 (Pauling)

Oxidation states

-1, +1, +5, +7

Discovered

1811 by Bernard Courtois

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁵

Noble gas shorthand

[Kr] 5s² 4d¹⁰ 5p⁵

Shells

K: 2 · L: 8 · M: 18 · N: 18 · O: 7

What it is used for

• Iodised salt, which effectively ended endemic goitre where it is used
• Povidone-iodine antiseptic for surgical skin preparation
• X-ray and CT contrast agents
• Potassium iodide tablets to block radioactive iodine uptake
• Tungsten-halogen lamps, where iodine recycles evaporated tungsten

Hazards

• Solid iodine and its vapour irritate skin, eyes and airways
• Tincture of iodine stains and can burn skin on prolonged contact

Neighbours in the table

Group 17

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Iodine sits in group 17 with fluorine, chlorine, bromine, astatine, tennessine.