Iodine

Symbol: I
Atomic number: 53
Category: halogens
Group: 17
Period: 5
Block: p
Atomic weight: 126.90447 g/mol
Electrons per shell: 2, 8, 18, 18, 7
CAS number: 7553-56-2
Phase: solid
Density: 4.933 kg/dm³ (near room temperature)
Melting point: 386.85 K (113.7 °C, 236.66 °F)
Boiling point: 457.4 K (184.3 °C, 363.7 °F)
Crystal structure: orthorhombic
Oxidation states: 1, 5, 7
Electronegativity: 2.66 (Pauling scale)
Name's origin: iodes
Name's meaning: a Greek word which means 'violet' due to its color
Facts:
- is the second least reactive of the halogens, and the second most electropositive halogen
- does not occur in the free state in nature
- iodine and its compounds are primarily used in medicine, photography, and dyes
- its compounds, the iodides, are very soluble in water, and the element is concentrated in seawater
- the element is required in trace amounts by all animals and some plants, being the heaviest element commonly used by living organisms (only tungsten, used in enzymes by a few bacteria, is heavier)
- iodine is a famous tester for starch - it dyes the starch blue
- its main role in animal biology is as constituents of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3)
- excess iodine has symptoms similar to those of iodine deficiency - commonly encountered symptoms are abnormal growth of the thyroid gland and disorders in functioning and growth of the organism as a whole