Atomica
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52. Tellurium54. Xenon
53I126.9

Iodine

Halogens

Very reactive non-metals with seven outer electrons. They form salts with metals.

[Kr] 4d¹⁰ 5s² 5p⁵

Atomic number
53
Atomic mass
126.9
Protons
53
Neutrons
74 (most common)
Electrons
53
Shells
2, 8, 18, 18, 7
State at 25 °C
Solid
Position
Group 17, period 5
Proton (+)Neutron (0)Electron (−)
Discovered byBernard Courtois, 1811Bernard Courtois: 1 element

Courtois was making gunpowder ingredients from seaweed ash when violet fumes appeared. The name comes from the Greek iodes, meaning violet.

Did you know?

  • Solid iodine turns straight into violet vapour when warmed.
  • Your thyroid gland needs iodine to make hormones.

Where it's found

  • Seaweed
  • Seawater
  • Iodised salt

What it's used for

  • Iodised salt, which prevents goitre
  • Antiseptic tincture
  • X-ray contrast dyes

Try it

The starch detectorAdult helps
  1. 1An adult puts one drop of iodine tincture on a slice of raw potato.
  2. 2Try the same on a slice of cucumber and a piece of bread.

What you'll see: It turns blue-black where there's starch, like on the potato and bread.