Hypothetical element

A hypothetical element is a chemical element that has not been discovered or made by people.

Currently, all 118 elements in the first seven rows of the periodic table have been discovered or made. This makes all hypothetical elements part of the extended periodic table, beginning with ununennium.

Hypothetical elements are given systematic names based on the digits of their atomic number. For example, element 120 is unbinilium ("one-two-zero-ium").

All remaining hypothetical elements are expected to be very unstable, and are only of interest in research.

 History 
 Mendeleev's hypothetical elements 
When Dmitri Mendeleev made his periodic table, he left empty spaces for three elements that his theory predicted would exist, but had not been discovered. He named these elements using the Sanskrit words "eka-", "dvi-", and "tri-" depending on how far down it was from a known element in his table.

Mendeleev's table successfully predicted the chemical properties of four elements: scandium as "eka-boron", gallium as "eka-aluminium", germanium as "eka-silicon", and technetium as "eka-manganese". Mendeleev predicted several more elements, like hafnium, but chemists did not understand the lanthanides at the time, so his predictions were less accurate.

The eka- notation was used for hypothetical elements long after Mendeleev. For example, flerovium was known as "eka-lead" before the standard IUPAC system for naming hypothetical elements was made.

 Actinides 
At first, the actinides were believed to be transition metals. Uranium was placed on periodic tables as eka-tungsten, and neptunium and plutonium were thought to be eka-rhenium and eka-osmium. When these two elements were actually discovered, though, they had much more in common with uranium.

Glenn T. Seaborg suggested that these radioactive elements were not transition metals, but instead a series of metals like the lanthanides. This created a new series of hypothetical elements, the last of which (lawrencium) was discovered in 1961.

Seaborg also predicted the period 7 homologs of other elements, called transactinides, ending in eka-radon, now called oganesson. The last transactinide to be discovered was tennessine, made in 2009.

 Current hypothetical elements 
There are theoretically infinite hypothetical elements, one for each possible natural number of protons. It is harder to find hypothetical elements that people may be able to make. Nuclear fusion making superheavy elements is very difficult.

Most current efforts are focused on ununennium and unbinilium, elements 119 and 120. The heaviest element that people have attempted to make is unbiseptium.

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