Periodic table of the chemical elements
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61 Pm,Promethium
Lanthanides · Solid
Atomic mass [145] mass number of the most stable isotope
61
Average mass of an atom in unified atomic mass units (1 u is 1/12 of the mass of a carbon-12 atom). Accounts for the natural mix of the element’s isotopes.
Number of the vertical column, 1 to 18 in IUPAC numbering. Elements in a group share the same outer-shell layout, so their properties are alike.
Number of the horizontal row. Usually equal to the number of electron shells in the atom; palladium is the exception, with four shells in period 5.
The subshell (s, p, d or f) that receives the last electrons in the usual filling order; it divides the table into four blocks. Real atoms sometimes differ: lanthanum starts the f-block but has no f electrons.
How electrons are arranged in subshells. The bracket stands for the previous noble gas; the superscript is the number of electrons in a subshell.
Number of electrons in each shell, from the nucleus outwards.
Temperature at which the solid turns liquid at standard pressure. Helium, carbon and arsenic do not melt at normal pressure: helium needs about 25 atm, carbon and arsenic turn straight into vapour.
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
How strongly an atom pulls electrons towards itself in a chemical bond. On the Pauling scale it runs from 0.79 for caesium to 3.98 for fluorine.
Energy needed to remove the most loosely bound electron from an atom. The lower it is, the more easily the element gives up an electron, which is typical of metals.
Energy released when a neutral atom gains an electron. Highest for the halogens. Noble gases, nitrogen and some metals form no stable negative ion.
Half the length of a single covalent bond between two atoms of the element. It shows the size of an atom inside a molecule or crystal: 76 pm for carbon, 132 pm for iron.
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Promethium (Pm) is a chemical element with atomic number 61. It sits in period 6 of the periodic table, in the f-block. It is a lanthanide. At room temperature it is a solid. It has no standard atomic weight; the mass number of its most stable isotope is 145.
Promethium is named after Prometheus, the Titan of Greek myth who stole fire from the gods and gave it to people. In 1902 Bohuslav Brauner predicted that an element was missing between neodymium and samarium, and several groups later claimed to have found it but could not prove it. It was finally identified at the end of the Second World War by Jacob Marinsky, Lawrence Glendenin and Charles Coryell in the products of uranium fission from a nuclear reactor at Oak Ridge.
All forms of promethium are radioactive and short-lived, so there is practically none on Earth. Its light has been detected in the spectra of stars in the constellation Andromeda. Promethium is made in nuclear reactors.
Promethium-147 gives off beta rays and is used in gauges that measure the thickness of materials. Mixed with a phosphor, it makes a paint that glows by itself, once used on watch dials, and it can power small nuclear batteries.