All practice pages/Interatomic theory/Elements
Element 77, a transition metal
The most corrosion-resistant metal, and the element whose thin layer in rock revealed the asteroid that ended the dinosaurs.
Period 6, group 9. The other transition metals are tinted, and every cell opens its own article.
Iridium has 77 electrons, one for every proton, so the atom carries no charge. Below are the same 77 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d⁷ 6s²
Short form: [Xe] 4f¹⁴ 5d⁷ 6s², where [Xe] stands for the 54 electrons xenon already has.
That list runs by shell. In the order the subshells fill it reads [Xe] 6s² 4f¹⁴ 5d⁷, 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 4 here is 4s² 4p⁶ 4d¹⁰ 4f¹⁴, and 2 + 6 + 10 + 14 is 32.
Iridium sits in period 6 and has 6 shells. The period number counts the shells.
The outer shell holds 2 electrons. Those are the ones iridium can lose, gain or share, so they are the ones that decide what it bonds with.
Like other transition metals, iridium loses its outer s electrons, and often some d electrons as well, to form positive ions, and it can form more than one kind.
One of the rarest elements in the crust, found with platinum in South Africa and Russia. Meteorites carry much more, which is why a thin iridium-rich clay layer, found by Luis and Walter Alvarez in 1980, marks a giant asteroid impact 66 million years ago.
Smithson Tennant found it in 1803, together with osmium, in the residue of platinum ore. He named it after Iris, the Greek goddess of the rainbow, for the many colors of its salts.
Spark plug tips, crucibles for growing crystals at very high temperatures, and electrodes for making chlorine and hydrogen. Iridium compounds give off the light in many OLED screens, iridium-192 is used in cancer radiotherapy and to X-ray welds, and the old international prototype kilogram was an alloy of platinum and iridium.