All practice pages/Interatomic theory/Elements
Element 53, a halogen
A shiny violet-black solid that turns straight into purple vapor, and the one halogen your body cannot do without.
Period 5, group 17: the bottom of the 7. The other halogens are tinted, and every cell opens its own article.
Iodine has 53 electrons, one for every proton, so the atom carries no charge. Below are the same 53 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁵
Short form: [Kr] 4d¹⁰ 5s² 5p⁵, where [Kr] stands for the 36 electrons krypton already has.
That list runs by shell. In the order the subshells fill it reads [Kr] 5s² 4d¹⁰ 5p⁵, because a subshell fills by its energy, not by its shell number.
The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 3 here is 3s² 3p⁶ 3d¹⁰, and 2 + 6 + 10 is 18.
Iodine sits in period 5 and has 5 shells. The period number counts the shells.
The outer shell holds 7 electrons. Those are the ones iodine can lose, gain or share, so they are the ones that decide what it bonds with. For a main-group element the group number gives that count: group 17 means 7.
Seven valence electrons, one short of a full shell, so it gains one to form the I⁻ ion or shares one in a covalent bond.
Seven valence electrons, one short of eight. Two iodine atoms share one pair, a single bond, the longest and weakest of the seven.
The rarest of the stable halogens, about 0.00005% of the crust. Seawater has only 0.05 mg per liter, but seaweed concentrates it thousands of times over. Today most comes from natural brines in Japan and from caliche ore in Chile, where it occurs as iodate.
Bernard Courtois found it in 1811 in the ash of seaweed he was processing into saltpeter for Napoleon's gunpowder: too much sulfuric acid gave off a violet vapor that condensed into dark crystals. Joseph Gay-Lussac named it from the Greek iodes, violet-colored.
Iodized salt prevents iodine deficiency, iodine solutions and povidone-iodine disinfect skin before surgery, and iodine compounds make X-ray contrast agents, industrial catalysts and cloud-seeding silver iodide. Radioactive iodine-131 treats thyroid disease.