All practice pages/Interatomic theory/Elements
Element 61, a lanthanide
The only lanthanide with no stable isotopes, so scarce in nature that it was finally found among the products of nuclear fission.
Period 6, in the lanthanide row set below the main table. The other lanthanides are tinted, and every cell opens its own article.
Promethium has 61 electrons, one for every proton, so the atom carries no charge. Below are the same 61 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f⁵ 5s² 5p⁶ 6s²
Short form: [Xe] 4f⁵ 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 4f⁵, 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 4f⁵, and 2 + 6 + 10 + 5 is 23.
Promethium sits in period 6 and has 6 shells. The period number counts the shells.
The outer shell holds 2 electrons. Those are the ones promethium can lose, gain or share, so they are the ones that decide what it bonds with.
Like the other lanthanides, it gives up three electrons to form the Pm³⁺ ion in most of its compounds.
Almost none exists in nature: only about half a kilogram is thought to exist in the Earth's crust at any moment, made by the rare spontaneous splitting of uranium atoms. Nearly all of it is made in nuclear reactors.
Its place in the table sat empty for decades while several claimed discoveries fell apart. Jacob Marinsky, Lawrence Glendenin and Charles Coryell identified it in 1945 at Oak Ridge, among the products of uranium fission. Coryell's wife, Grace Mary, suggested naming it after Prometheus, who stole fire from the gods.
Promethium-147 once powered nuclear batteries for pacemakers and made luminous paint for watch dials. Today it is used mainly in research and in gauges that measure the thickness of materials.
Radioactive. It is handled behind shielding and must be kept out of the body.