River Sand - River Sand

In his report onThe Periodic Law of the Chemical Elements, in 1869,Dmitri Mendeleevhad implicitlypredicted the existenceof a heavier analog of titanium and zirconium. At the time of his formulation in 1871, Mendeleev believed that the elements were ordered by theiratomic massesand placedlanthanum(element 57) in the spot below zirconium. The exact placement of the elements and the location of missing elements was done by determining the specific weight of the elements and comparing the chemical and physical properties.^[21]

TheX-ray spectroscopydone byHenry Moseleyin 1914 showed a direct dependency betweenspectral lineandeffective nuclear charge. This led to the nuclear charge, oratomic numberof an element, being used to ascertain its place within the periodic table. With this method, Moseley determined the number oflanthanidesand showed the gaps in the atomic number sequence at numbers 43, 61, 72, and 75.^[22]

The discovery of the gaps led to an extensive search for the missing elements. In 1914, several people claimed the discovery after Henry Moseley predicted the gap in the periodic table for the then-undiscovered element 72.^[23]Georges Urbainasserted that he found element 72 in therare earth elementsin 1907 and published his results onceltiumin 1911.^[24]Neither the spectra nor the chemical behavior matched with the element found later, and therefore his claim was turned down after a long-standing controversy.^[25]The controversy was partly because the chemists favored the chemical techniques which led to the discovery ofceltium, while the physicists relied on the use of the new X-ray spectroscopy method that proved that the substances discovered by Urbain did not contain element 72.^[25]By early 1923, several physicists and chemists such asNiels Bohr^[26]and Charles R. Bury^[27]suggested that element 72 should resemble zirconium and therefore was not part of the rare earth elements group. These suggestions were based on Bohr's theories of the atom, the X-ray spectroscopy of Mosley, and the chemical arguments ofFriedrich Paneth.^[28][29]

Encouraged by these suggestions and by the reappearance in 1922 of Urbain's claims that element 72 was a rare earth element discovered in 1911,Dirk CosterandGeorg von Hevesywere motivated to search for the new element in zirconium ores.^[30]Hafnium was discovered by the two in 1923 in Copenhagen, Denmark, validating the original 1869 prediction of Mendeleev.^[31][32]It was ultimately found inzirconin Norway through X-ray spectroscopy analysis.^[33]The place where the discovery took place led to the element being named for the Latin name for "Copenhagen",Hafnia, the home town ofNiels Bohr.^[34]Today, theFaculty of Scienceof theUniversity of Copenhagenuses in itsseala stylized image of the hafnium atom.^[35]

Hafnium was separated from zirconium through repeated recrystallization of the doubleammoniumorpotassiumfluorides byValdemar Thal Jantzenand von Hevesey.^[13]Anton Eduard van ArkelandJan Hendrik de Boerwere the first to prepare metallic hafnium by passing hafnium tetra-iodide vapor over a heatedtungstenfilament in 1924.^[14][18]This process for differential purification of zirconium and hafnium is still in use today.^[2]

In 1923, four predicted elements were still missing from the periodic table: 43 (technetium) and 61 (promethium) are radioactive elements and are only present in trace amounts in the environment,^[36]thus making elements 75 (rhenium) and 72 (hafnium) the last two unknown non-radioactive elements. Since rhenium was discovered in 1925,^[37]hafnium was the next to last element with stable isotopes to be discovered.